#
# $Id: 6502_cpu.txt,v 1.1 2002/05/21 00:42:27 xodnizel Exp $
#
# This file is part of Commodore 64 emulator
#      and Program Development System.
#
# See README for copyright notice
#
# This file contains documentation for 6502/6510/8500/8502 instruction set.
#
#
# Written by
#   John West       (john@ucc.gu.uwa.edu.au)
#   Marko MŠkelŠ    (msmakela@kruuna.helsinki.fi)
#
#
# $Log: 6502_cpu.txt,v $
# Revision 1.1  2002/05/21 00:42:27  xodnizel
# updates
#
# Revision 1.8  1994/06/03  19:50:04  jopi
# Patchlevel 2
#
# Revision 1.7  1994/04/15  13:07:04  jopi
# 65xx Register descriptions added
#
# Revision 1.6  1994/02/18  16:09:36  jopi
#
# Revision 1.5  1994/01/26  16:08:37  jopi
# X64 version 0.2 PL 1
#
# Revision 1.4  1993/11/10  01:55:34  jopi
#
# Revision 1.3  93/06/21  13:37:18  jopi
#  X64 version 0.2 PL 0
#
# Revision 1.2  93/06/21  13:07:15  jopi
# *** empty log message ***
#
#

 Note: To extract the uuencoded ML programs in this article most
       easily you may use e.g. "uud" by Edwin Kremer ,
       which extracts them all at once.


Documentation for the NMOS 65xx/85xx Instruction Set

        6510 Instructions by Addressing Modes
        6502 Registers
        6510/8502 Undocumented Commands
        Register selection for load and store
        Decimal mode in NMOS 6500 series
        6510 features
        Different CPU types
        6510 Instruction Timing
        How Real Programmers Acknowledge Interrupts
        Memory Management
        Autostart Code
        Notes
        References


6510 Instructions by Addressing Modes

off- ++++++++++ Positive ++++++++++  ---------- Negative ----------
set  00      20      40      60      80      a0      c0      e0      mode

+00  BRK     JSR     RTI     RTS     NOP*    LDY     CPY     CPX     Impl/immed
+01  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     (indir,x)
+02   t       t       t       t      NOP*t   LDX     NOP*t   NOP*t     ? /immed
+03  SLO*    RLA*    SRE*    RRA*    SAX*    LAX*    DCP*    ISB*    (indir,x)
+04  NOP*    BIT     NOP*    NOP*    STY     LDY     CPY     CPX     Zeropage
+05  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     Zeropage
+06  ASL     ROL     LSR     ROR     STX     LDX     DEC     INC     Zeropage
+07  SLO*    RLA*    SRE*    RRA*    SAX*    LAX*    DCP*    ISB*    Zeropage

+08  PHP     PLP     PHA     PLA     DEY     TAY     INY     INX     Implied
+09  ORA     AND     EOR     ADC     NOP*    LDA     CMP     SBC     Immediate
+0a  ASL     ROL     LSR     ROR     TXA     TAX     DEX     NOP     Accu/impl
+0b  ANC**   ANC**   ASR**   ARR**   ANE**   LXA**   SBX**   SBC*    Immediate
+0c  NOP*    BIT     JMP     JMP ()  STY     LDY     CPY     CPX     Absolute
+0d  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     Absolute
+0e  ASL     ROL     LSR     ROR     STX     LDX     DEC     INC     Absolute
+0f  SLO*    RLA*    SRE*    RRA*    SAX*    LAX*    DCP*    ISB*    Absolute

+10  BPL     BMI     BVC     BVS     BCC     BCS     BNE     BEQ     Relative
+11  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     (indir),y
+12   t       t       t       t       t       t       t       t         ?
+13  SLO*    RLA*    SRE*    RRA*    SHA**   LAX*    DCP*    ISB*    (indir),y
+14  NOP*    NOP*    NOP*    NOP*    STY     LDY     NOP*    NOP*    Zeropage,x
+15  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     Zeropage,x
+16  ASL     ROL     LSR     ROR     STX  y) LDX  y) DEC     INC     Zeropage,x
+17  SLO*    RLA*    SRE*    RRA*    SAX* y) LAX* y) DCP*    ISB*    Zeropage,x

+18  CLC     SEC     CLI     SEI     TYA     CLV     CLD     SED     Implied
+19  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     Absolute,y
+1a  NOP*    NOP*    NOP*    NOP*    TXS     TSX     NOP*    NOP*    Implied
+1b  SLO*    RLA*    SRE*    RRA*    SHS**   LAS**   DCP*    ISB*    Absolute,y
+1c  NOP*    NOP*    NOP*    NOP*    SHY**   LDY     NOP*    NOP*    Absolute,x
+1d  ORA     AND     EOR     ADC     STA     LDA     CMP     SBC     Absolute,x
+1e  ASL     ROL     LSR     ROR     SHX**y) LDX  y) DEC     INC     Absolute,x
+1f  SLO*    RLA*    SRE*    RRA*    SHA**y) LAX* y) DCP*    ISB*    Absolute,x

        ROR intruction is available on MC650x microprocessors after
        June, 1976.

        Legend:

        t       Jams the machine
        *t      Jams very rarely
        *       Undocumented command
        **      Unusual operation
        y)      indexed using Y instead of X
        ()      indirect instead of absolute

Note that the NOP instructions do have other addressing modes than the
implied addressing. The NOP instruction is just like any other load
instruction, except it does not store the result anywhere nor affects the
flags.

6502 Registers

The NMOS 65xx processors are not ruined with too many registers. In addition
to that, the registers are mostly 8-bit. Here is a brief description of each
register:

     PC Program Counter
          This register points the address from which the next instruction
          byte (opcode or parameter) will be fetched. Unlike other
          registers, this one is 16 bits in length. The low and high 8-bit
          halves of the register are called PCL and PCH, respectively. The
          Program Counter may be read by pushing its value on the stack.
          This can be done either by jumping to a subroutine or by causing
          an interrupt.
     S Stack pointer
          The NMOS 65xx processors have 256 bytes of stack memory, ranging
          from $0100 to $01FF. The S register is a 8-bit offset to the stack
          page. In other words, whenever anything is being pushed on the
          stack, it will be stored to the address $0100+S.

          The Stack pointer can be read and written by transfering its value
          to or from the index register X (see below) with the TSX and TXS
          instructions.
     P Processor status
          This 8-bit register stores the state of the processor. The bits in
          this register are called flags. Most of the flags have something
          to do with arithmetic operations.

          The P register can be read by pushing it on the stack (with PHP or
          by causing an interrupt). If you only need to read one flag, you
          can use the branch instructions. Setting the flags is possible by
          pulling the P register from stack or by using the flag set or
          clear instructions.

          Following is a list of the flags, starting from the 8th bit of the
          P register (bit 7, value $80):
               N Negative flag
                    This flag will be set after any arithmetic operations
                    (when any of the registers A, X or Y is being loaded
                    with a value). Generally, the N flag will be copied from
                    the topmost bit of the register being loaded.

                    Note that TXS (Transfer X to S) is not an arithmetic
                    operation. Also note that the BIT instruction affects
                    the Negative flag just like arithmetic operations.
                    Finally, the Negative flag behaves differently in
                    Decimal operations (see description below).
               V oVerflow flag
                    Like the Negative flag, this flag is intended to be used
                    with 8-bit signed integer numbers. The flag will be
                    affected by addition and subtraction, the instructions
                    PLP, CLV and BIT, and the hardware signal -SO. Note that
                    there is no SEV instruction, even though the MOS
                    engineers loved to use East European abbreviations, like
                    DDR (Deutsche Demokratische Republik vs. Data Direction
                    Register). (The Russian abbreviation for their former
                    trade association COMECON is SEV.) The -SO (Set
                    Overflow) signal is available on some processors, at
                    least the 6502, to set the V flag. This enables response
                    to an I/O activity in equal or less than three clock
                    cycles when using a BVC instruction branching to itself
                    ($50 $FE).

                    The CLV instruction clears the V flag, and the PLP and
                    BIT instructions copy the flag value from the bit 6 of
                    the topmost stack entry or from memory.

                    After a binary addition or subtraction, the V flag will
                    be set on a sign overflow, cleared otherwise. What is a
                    sign overflow? For instance, if you are trying to add
                    123 and 45 together, the result (168) does not fit in a
                    8-bit signed integer (upper limit 127 and lower limit
                    -128). Similarly, adding -123 to -45 causes the
                    overflow, just like subtracting -45 from 123 or 123 from
                    -45 would do.

                    Like the N flag, the V flag will not be set as expected
                    in the Decimal mode. Later in this document is a precise
                    operation description.

                    A common misbelief is that the V flag could only be set
                    by arithmetic operations, not cleared.
               1 unused flag
                    To the current knowledge, this flag is always 1.
               B Break flag
                    This flag is used to distinguish software (BRK)
                    interrupts from hardware interrupts (IRQ or NMI). The B
                    flag is always set except when the P register is being
                    pushed on stack when jumping to an interrupt routine to
                    process only a hardware interrupt.

                    The official NMOS 65xx documentation claims that the BRK
                    instruction could only cause a jump to the IRQ vector
                    ($FFFE). However, if an NMI interrupt occurs while
                    executing a BRK instruction, the processor will jump to
                    the NMI vector ($FFFA), and the P register will be
                    pushed on the stack with the B flag set.
               D Decimal mode flag
                    This flag is used to select the (Binary Coded) Decimal
                    mode for addition and subtraction. In most applications,
                    the flag is zero.

                    The Decimal mode has many oddities, and it operates
                    differently on CMOS processors. See the description of
                    the ADC, SBC and ARR instructions below.
               I Interrupt disable flag
                    This flag can be used to prevent the processor from
                    jumping to the IRQ handler vector ($FFFE) whenever the
                    hardware line -IRQ is active. The flag will be
                    automatically set after taking an interrupt, so that the
                    processor would not keep jumping to the interrupt
                    routine if the -IRQ signal remains low for several clock
                    cycles.
               Z Zero flag
                    The Zero flag will be affected in the same cases than
                    the Negative flag. Generally, it will be set if an
                    arithmetic register is being loaded with the value zero,
                    and cleared otherwise. The flag will behave differently
                    in Decimal operations.
               C Carry flag
                    This flag is used in additions, subtractions,
                    comparisons and bit rotations. In additions and
                    subtractions, it acts as a 9th bit and lets you to chain
                    operations to calculate with bigger than 8-bit numbers.
                    When subtracting, the Carry flag is the negative of
                    Borrow: if an overflow occurs, the flag will be clear,
                    otherwise set. Comparisons are a special case of
                    subtraction: they assume Carry flag set and Decimal flag
                    clear, and do not store the result of the subtraction
                    anywhere.

                    There are four kinds of bit rotations. All of them store
                    the bit that is being rotated off to the Carry flag. The
                    left shifting instructions are ROL and ASL. ROL copies
                    the initial Carry flag to the lowmost bit of the byte;
                    ASL always clears it. Similarly, the ROR and LSR
                    instructions shift to the right.
     A Accumulator
          The accumulator is the main register for arithmetic and logic
          operations. Unlike the index registers X and Y, it has a direct
          connection to the Arithmetic and Logic Unit (ALU). This is why
          many operations are only available for the accumulator, not the
          index registers.
     X Index register X
          This is the main register for addressing data with indices. It has
          a special addressing mode, indexed indirect, which lets you to
          have a vector table on the zero page.
     Y Index register Y
          The Y register has the least operations available. On the other
          hand, only it has the indirect indexed addressing mode that
          enables access to any memory place without having to use
          self-modifying code.

6510/8502 Undocumented Commands

-- A brief explanation about what may happen while using don't care states.

        ANE $8B         A = (A | #$EE) & X & #byte
                        same as
                        A = ((A & #$11 & X) | ( #$EE & X)) & #byte

                        In real 6510/8502 the internal parameter #$11
                        may occasionally be #$10, #$01 or even #$00.
                        This occurs when the video chip starts DMA
                        between the opcode fetch and the parameter fetch
                        of the instruction.  The value probably depends
                        on the data that was left on the bus by the VIC-II.

        LXA $AB         C=Lehti:   A = X = ANE
                        Alternate: A = X = (A & #byte)

                        TXA and TAX have to be responsible for these.

        SHA $93,$9F     Store (A & X & (ADDR_HI + 1))
        SHX $9E         Store (X & (ADDR_HI + 1))
        SHY $9C         Store (Y & (ADDR_HI + 1))
        SHS $9B         SHA and TXS, where X is replaced by (A & X).

                        Note: The value to be stored is copied also
                        to ADDR_HI if page boundary is crossed.

        SBX $CB         Carry and Decimal flags are ignored but the
                        Carry flag will be set in substraction. This
                        is due to the CMP command, which is executed
                        instead of the real SBC.

        ARR $6B         This instruction first performs an AND
                        between the accumulator and the immediate
                        parameter, then it shifts the accumulator to
                        the right. However, this is not the whole
                        truth. See the description below.

Many undocumented commands do not use AND between registers, the CPU
just throws the bytes to a bus simultaneously and lets the
open-collector drivers perform the AND. I.e. the command called 'SAX',
which is in the STORE section (opcodes $A0...$BF), stores the result
of (A & X) by this way.

More fortunate is its opposite, 'LAX' which just loads a byte
simultaneously into both A and X.

        $6B  ARR

This instruction seems to be a harmless combination of AND and ROR at
first sight, but it turns out that it affects the V flag and also has
a special kind of decimal mode. This is because the instruction has
inherited some properties of the ADC instruction ($69) in addition to
the ROR ($6A).

In Binary mode (D flag clear), the instruction effectively does an AND
between the accumulator and the immediate parameter, and then shifts
the accumulator to the right, copying the C flag to the 8th bit. It
sets the Negative and Zero flags just like the ROR would. The ADC code
shows up in the Carry and oVerflow flags. The C flag will be copied
from the bit 6 of the result (which doesn't seem too logical), and the
V flag is the result of an Exclusive OR operation between the bit 6
and the bit 5 of the result.  This makes sense, since the V flag will
be normally set by an Exclusive OR, too.

In Decimal mode (D flag set), the ARR instruction first performs the
AND and ROR, just like in Binary mode. The N flag will be copied from
the initial C flag, and the Z flag will be set according to the ROR
result, as expected. The V flag will be set if the bit 6 of the
accumulator changed its state between the AND and the ROR, cleared
otherwise.

Now comes the funny part. If the low nybble of the AND result,
incremented by its lowmost bit, is greater than 5, the low nybble in
the ROR result will be incremented by 6. The low nybble may overflow
as a consequence of this BCD fixup, but the high nybble won't be
adjusted. The high nybble will be BCD fixed in a similar way. If the
high nybble of the AND result, incremented by its lowmost bit, is
greater than 5, the high nybble in the ROR result will be incremented
by 6, and the Carry flag will be set. Otherwise the C flag will be
cleared.

To help you understand this description, here is a C routine that
illustrates the ARR operation in Decimal mode:

        unsigned
           A,  /* Accumulator */
           AL, /* low nybble of accumulator */
           AH, /* high nybble of accumulator */

           C,  /* Carry flag */
           Z,  /* Zero flag */
           V,  /* oVerflow flag */
           N,  /* Negative flag */

           t,  /* temporary value */
           s;  /* value to be ARRed with Accumulator */

        t = A & s;                      /* Perform the AND. */

        AH = t >> 4;                    /* Separate the high */
        AL = t & 15;                    /* and low nybbles. */

        N = C;                          /* Set the N and */
        Z = !(A = (t >> 1) | (C << 7)); /* Z flags traditionally */
        V = (t ^ A) & 64;               /* and V flag in a weird way. */

        if (AL + (AL & 1) > 5)          /* BCD "fixup" for low nybble. */
          A = (A & 0xF0) | ((A + 6) & 0xF);

        if (C = AH + (AH & 1) > 5)      /* Set the Carry flag. */
          A = (A + 0x60) & 0xFF;        /* BCD "fixup" for high nybble. */

        $CB  SBX   X <- (A & X) - Immediate

The 'SBX' ($CB) may seem to be very complex operation, even though it
is a combination of the subtraction of accumulator and parameter, as
in the 'CMP' instruction, and the command 'DEX'. As a result, both A
and X are connected to ALU but only the subtraction takes place. Since
the comparison logic was used, the result of subtraction should be
normally ignored, but the 'DEX' now happily stores to X the value of
(A & X) - Immediate.  That is why this instruction does not have any
decimal mode, and it does not affect the V flag. Also Carry flag will
be ignored in the subtraction but set according to the result.

 Proof:

begin 644 vsbx
M`0@9$,D'GL(H-#,IJC(U-JS"*#0T*:HR-@```*D`H#V1*Z`_D2N@09$KJ0>%
M^QBE^VEZJ+$KH#F1*ZD`2"BI`*(`RP`(:-B@.5$K*4#P`E@`H#VQ*SAI`)$K
JD-Z@/[$K:0"1*Y#4J2X@TO\XH$&Q*VD`D2N0Q,;[$+188/_^]_:_OK>V
`
end

 and

begin 644 sbx
M`0@9$,D'GL(H-#,IJC(U-JS"*#0T*:HR-@```'BI`*!-D2N@3Y$KH%&1*ZD#
MA?L8I?M*2)`#J1@LJ3B@29$K:$J0`ZGX+*G8R)$K&/BXJ?2B8\L)AOP(:(7]
MV#B@3;$KH$\Q*Z!1\2L(1?SP`0!H1?TIM]#XH$VQ*SAI`)$KD,N@3[$K:0"1
9*Y#!J2X@TO\XH%&Q*VD`D2N0L<;[$))88-#X
`
end

These test programs show if your machine is compatible with ours
regarding the opcode $CB. The first test, vsbx, proves that SBX does
not affect the V flag. The latter one, sbx, proves the rest of our
theory. The vsbx test tests 33554432 SBX combinations (16777216
different A, X and Immediate combinations, and two different V flag
states), and the sbx test doubles that amount (16777216*4 D and C flag
combinations). Both tests have run successfully on a C64 and a Vic20.
They ought to run on C16, +4 and the PET series as well. The tests
stop with BRK, if the opcode $CB does not work as expected. Successful
operation ends in RTS. As the tests are very slow, they print dots on
the screen while running so that you know that the machine has not
jammed. On computers running at 1 MHz, the first test prints
approximately one dot every four seconds and a total of 2048 dots,
whereas the second one prints half that amount, one dot every seven
seconds.

If the tests fail on your machine, please let us know your processor's
part number and revision. If possible, save the executable (after it
has stopped with BRK) under another name and send it to us so that we
know at which stage the program stopped.

The following program is a Commodore 64 executable that Marko M"akel"a
developed when trying to find out how the V flag is affected by SBX.
(It was believed that the SBX affects the flag in a weird way, and
this program shows how SBX sets the flag differently from SBC.)  You
may find the subroutine at $C150 useful when researching other
undocumented instructions' flags. Run the program in a machine
language monitor, as it makes use of the BRK instruction. The result
tables will be written on pages $C2 and $C3.

begin 644 sbx-c100
M`,%XH`",#L&,$,&,$L&XJ8*B@LL7AOL(:(7\N#BM#L$M$,'M$L$(Q?OP`B@`
M:$7\\`,@4,'N#L'0U.X0P=#/SB#0[A+!T,<``````````````)BJ\!>M#L$M
L$,'=_\'0":T2P=W_PM`!8,K0Z:T.P2T0P9D`PID`!*T2P9D`PYD`!

Other undocumented instructions usually cause two preceding opcodes
being executed. However 'NOP' seems to completely disappear from 'SBC'
code $EB.

The most difficult to comprehend are the rest of the instructions
located on the '$0B' line.

All the instructions located at the positive (left) side of this line
should rotate either memory or the accumulator, but the addressing
mode turns out to be immediate! No problem. Just read the operand, let
it be ANDed with the accumulator and finally use accumulator
addressing mode for the instructions above them.

RELIGION_MODE_ON
/* This part of the document is not accurate.  You can
   read it as a fairy tale, but do not count on it when
   performing your own measurements. */

The rest two instructions on the same line, called 'ANE' and 'LXA'
($8B and $AB respectively) often give quite unpredictable results.
However, the most usual operation is to store ((A | #$ee) & X & #$nn)
to accumulator. Note that this does not work reliably in a real 64!
In the Commodore 128 the opcode $8B uses values 8C, CC, EE, and
occasionally 0C and 8E for the OR instead of EE,EF,FE and FF used in
the C64. With a C128 running at 2 MHz #$EE is always used.  Opcode $AB
does not cause this OR taking place on 8502 while 6510 always performs
it. Note that this behaviour depends on processor and/or video chip
revision.

Let's take a closer look at $8B (6510).

        A <- X & D & (A | VAL)

        where VAL comes from this table:

       X high   D high  D low   VAL
        even     even    ---    $EE (1)
        even     odd     ---    $EE
        odd      even    ---    $EE
        odd      odd      0     $EE
        odd      odd     not 0  $FE (2)

(1) If the bottom 2 bits of A are both 1, then the LSB of the result may
    be 0. The values of X and D are different every time I run the test.
    This appears to be very rare.
(2) VAL is $FE most of the time. Sometimes it is $EE - it seems to be random,
    not related to any of the data. This is much more common than (1).

  In decimal mode, VAL is usually $FE.

Two different functions have been discovered for LAX, opcode $AB. One
is A = X = ANE (see above) and the other, encountered with 6510 and
8502, is less complicated A = X = (A & #byte). However, according to
what is reported, the version altering only the lowest bits of each
nybble seems to be more common.

What happens, is that $AB loads a value into both A and X, ANDing the
low bit of each nybble with the corresponding bit of the old
A. However, there are exceptions. Sometimes the low bit is cleared
even when A contains a '1', and sometimes other bits are cleared. The
exceptions seem random (they change every time I run the test). Oops -
that was in decimal mode. Much the same with D=0.

What causes the randomness?  Probably it is that it is marginal logic
levels - when too much wired-anding goes on, some of the signals get
very close to the threshold. Perhaps we're seeing some of them step
over it. The low bit of each nybble is special, since it has to cope
with carry differently (remember decimal mode). We never see a '0'
turn into a '1'.

Since these instructions are unpredictable, they should not be used.

There is still very strange instruction left, the one named SHA/X/Y,
which is the only one with only indexed addressing modes. Actually,
the commands 'SHA', 'SHX' and 'SHY' are generated by the indexing
algorithm.

While using indexed addressing, effective address for page boundary
crossing is calculated as soon as possible so it does not slow down
operation. As a result, in the case of SHA/X/Y, the address and data
are processed at the same time making AND between them to take place.
Thus, the value to be stored by SAX, for example, is in fact (A & X &
(ADDR_HI + 1)).  On page boundary crossing the same value is copied
also to high byte of the effective address.

RELIGION_MODE_OFF


Register selection for load and store

   bit1 bit0     A  X  Y
    0    0             x
    0    1          x
    1    0       x
    1    1       x  x

So, A and X are selected by bits 1 and 0 respectively, while
 ~(bit1|bit0) enables Y.

Indexing is determined by bit4, even in relative addressing mode,
which is one kind of indexing.

Lines containing opcodes xxx000x1 (01 and 03) are treated as absolute
after the effective address has been loaded into CPU.

Zeropage,y and Absolute,y (codes 10x1 x11x) are distinquished by bit5.


Decimal mode in NMOS 6500 series

  Most sources claim that the NMOS 6500 series sets the N, V and Z
flags unpredictably when performing addition or subtraction in decimal
mode. Of course, this is not true. While testing how the flags are
set, I also wanted to see what happens if you use illegal BCD values.

  ADC works in Decimal mode in a quite complicated way. It is amazing
how it can do that all in a single cycle. Here's a C code version of
the instruction:

        unsigned
           A,  /* Accumulator */
           AL, /* low nybble of accumulator */
           AH, /* high nybble of accumulator */

           C,  /* Carry flag */
           Z,  /* Zero flag */
           V,  /* oVerflow flag */
           N,  /* Negative flag */

           s;  /* value to be added to Accumulator */

        AL = (A & 15) + (s & 15) + C;         /* Calculate the lower nybble. */

        AH = (A >> 4) + (s >> 4) + (AL > 15); /* Calculate the upper nybble. */

        if (AL > 9) AL += 6;                  /* BCD fixup for lower nybble. */

        Z = ((A + s + C) & 255 != 0);         /* Zero flag is set just
                                                 like in Binary mode. */

        /* Negative and Overflow flags are set with the same logic than in
           Binary mode, but after fixing the lower nybble. */

        N = (AH & 8 != 0);
        V = ((AH << 4) ^ A) & 128 && !((A ^ s) & 128);

        if (AH > 9) AH += 6;                  /* BCD fixup for upper nybble. */

        /* Carry is the only flag set after fixing the result. */

        C = (AH > 15);
        A = ((AH << 4) | (AL & 15)) & 255;

  The C flag is set as the quiche eaters expect, but the N and V flags
are set after fixing the lower nybble but before fixing the upper one.
They use the same logic than binary mode ADC. The Z flag is set before
any BCD fixup, so the D flag does not have any influence on it.

Proof: The following test program tests all 131072 ADC combinations in
       Decimal mode, and aborts with BRK if anything breaks this theory.
       If everything goes well, it ends in RTS.

begin 600 dadc
M 0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@   'BI&*  A/N$_$B@+)$KH(V1
M*Q@(I?PI#X7]I?LI#V7]R0J0 FD%J"D/A?VE^RGP9?PI\ C $) ":0^JL @H
ML ?)H) &""@X:5\X!?V%_0AH*3W@ ! ""8"HBD7[$ JE^T7\, 28"4"H**7[
M9?S0!)@) J@8N/BE^V7\V A%_= G:(3]1?W0(.;[T(?F_-"#:$D8\ )88*D=
0&&4KA?NI &4LA?RI.&S[  A%

end

  All programs in this chapter have been successfully tested on a Vic20
and a Commodore 64 and a Commodore 128D in C64 mode. They should run on
C16, +4 and on the PET series as well. If not, please report the problem
to Marko M"akel"a. Each test in this chapter should run in less than a
minute at 1 MHz.

SBC is much easier. Just like CMP, its flags are not affected by
the D flag.

Proof:

begin 600 dsbc-cmp-flags
M 0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@   'B@ (3[A/RB XH8:66HL2N@
M09$KH$R1*XII::BQ*Z!%D2N@4)$K^#BXI?OE_-@(:(7].+BE^^7\"&A%_? !
5 .;[T./F_-#?RA"_8!@X&#CEY<7%

end

  The only difference in SBC's operation in decimal mode from binary mode
is the result-fixup:

        unsigned
           A,  /* Accumulator */
           AL, /* low nybble of accumulator */
           AH, /* high nybble of accumulator */

           C,  /* Carry flag */
           Z,  /* Zero flag */
           V,  /* oVerflow flag */
           N,  /* Negative flag */

           s;  /* value to be added to Accumulator */

        AL = (A & 15) - (s & 15) - !C;        /* Calculate the lower nybble. */

        if (AL & 16) AL -= 6;                 /* BCD fixup for lower nybble. */

        AH = (A >> 4) - (s >> 4) - (AL & 16); /* Calculate the upper nybble. */

        if (AH & 16) AH -= 6;                 /* BCD fixup for upper nybble. */

        /* The flags are set just like in Binary mode. */

        C = (A - s - !C) & 256 != 0;
        Z = (A - s - !C) & 255 != 0;
        V = ((A - s - !C) ^ s) & 128 && (A ^ s) & 128;
        N = (A - s - !C) & 128 != 0;

        A = ((AH << 4) | (AL & 15)) & 255;

  Again Z flag is set before any BCD fixup. The N and V flags are set
at any time before fixing the high nybble. The C flag may be set in any
phase.

  Decimal subtraction is easier than decimal addition, as you have to
make the BCD fixup only when a nybble overflows. In decimal addition,
you had to verify if the nybble was greater than 9. The processor has
an internal "half carry" flag for the lower nybble, used to trigger
the BCD fixup. When calculating with legal BCD values, the lower nybble
cannot overflow again when fixing it.
So, the processor does not handle overflows while performing the fixup.
Similarly, the BCD fixup occurs in the high nybble only if the value
overflows, i.e. when the C flag will be cleared.

  Because SBC's flags are not affected by the Decimal mode flag, you
could guess that CMP uses the SBC logic, only setting the C flag
first. But the SBX instruction shows that CMP also temporarily clears
the D flag, although it is totally unnecessary.

  The following program, which tests SBC's result and flags,
contains the 6502 version of the pseudo code example above.

begin 600 dsbc
M 0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@   'BI&*  A/N$_$B@+)$KH':1
M*S@(I?PI#X7]I?LI#^7]L /I!1@I#ZBE_"GPA?VE^RGP"#CE_2GPL KI7RBP
M#ND/.+ )*+ &Z0^P NE?A/T%_87]*+BE^^7\"&BH.+CXI?OE_-@(1?W0FVB$
8_47]T)3F^]">YOS0FFA)&- $J3C0B%A@

end

  Obviously the undocumented instructions RRA (ROR+ADC) and ISB
(INC+SBC) have inherited also the decimal operation from the official
instructions ADC and SBC. The program droradc proves this statement
for ROR, and the dincsbc test proves this for ISB. Finally,
dincsbc-deccmp proves that ISB's and DCP's (DEC+CMP) flags are not
affected by the D flag.

begin 644 droradc
M`0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@```'BI&*``A/N$_$B@+)$KH(V1
M*S@(I?PI#X7]I?LI#V7]R0J0`FD%J"D/A?VE^RGP9?PI\`C`$)`":0^JL`@H
ML`?)H)`&""@X:5\X!?V%_0AH*3W@`!`""8"HBD7[$`JE^T7\,`28"4"H**7[
M9?S0!)@)`J@XN/BE^R;\9_S8"$7]T"=HA/U%_=`@YOO0A>;\T(%H21CP`EA@
2J1T892N%^ZD`92R%_*DX;/L`
`
end

begin 644 dincsbc
M`0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@```'BI&*``A/N$_$B@+)$KH':1
M*S@(I?PI#X7]I?LI#^7]L`/I!1@I#ZBE_"GPA?VE^RGP"#CE_2GPL`KI7RBP
M#ND/.+`)*+`&Z0^P`NE?A/T%_87]*+BE^^7\"&BH.+CXI?O&_.?\V`A%_="9
::(3]1?W0DN;[T)SF_-"8:$D8T`2I.-"&6&#\
`
end

begin 644 dincsbc-deccmp
M`0@9",D'GL(H-#,IJC(U-JS"*#0T*:HR-@```'B@`(3[A/RB`XH8:7>HL2N@
M3Y$KH%R1*XII>ZBQ*Z!3D2N@8)$KBFE_J+$KH%61*Z!BD2OX.+BE^^;\Q_S8
L"&B%_3BXI?OF_,?\"&A%_?`!`.;[T-_F_-#;RA"M8!@X&#CFYL;&Q\?GYP#8
`
end


6510 features

   o  PHP always pushes the Break (B) flag as a `1' to the stack.
      Jukka Tapanim"aki claimed in C=lehti issue 3/89, on page 27 that the
      processor makes a logical OR between the status register's bit 4
      and the bit 8 of the stack pointer register (which is always 1).
      He did not give any reasons for this argument, and has refused to clarify
      it afterwards. Well, this was not the only error in his article...

   o  Indirect addressing modes do not handle page boundary crossing at all.
      When the parameter's low byte is $FF, the effective address wraps
      around and the CPU fetches high byte from $xx00 instead of $xx00+$0100.
      E.g. JMP ($01FF) fetches PCL from $01FF and PCH from $0100,
      and LDA ($FF),Y fetches the base address from $FF and $00.

   o  Indexed zero page addressing modes never fix the page address on
      crossing the zero page boundary.
      E.g. LDX #$01 : LDA ($FF,X) loads the effective address from $00 and $01.

   o  The processor always fetches the byte following a relative branch
      instruction. If the branch is taken, the processor reads then the
      opcode from the destination address. If page boundary is crossed, it
      first reads a byte from the old page from a location that is bigger
      or smaller than the correct address by one page.

   o  If you cross a page boundary in any other indexed mode,
      the processor reads an incorrect location first, a location that is
      smaller by one page.

   o  Read-Modify-Write instructions write unmodified data, then modified
      (so INC effectively does LDX loc;STX loc;INX;STX loc)

   o  -RDY is ignored during writes
      (This is why you must wait 3 cycles before doing any DMA --
      the maximum number of consecutive writes is 3, which occurs
      during interrupts except -RESET.)

   o  Some undefined opcodes may give really unpredictable results.

   o  All registers except the Program Counter remain unmodified after -RESET.
      (This is why you must preset D and I flags in the RESET handler.)


Different CPU types

The Rockwell data booklet 29651N52 (technical information about R65C00
microprocessors, dated October 1984), lists the following differences between
NMOS R6502 microprocessor and CMOS R65C00 family:


     1. Indexed addressing across page boundary.
             NMOS: Extra read of invalid address.
             CMOS: Extra read of last instruction byte.


     2. Execution of invalid op codes.
             NMOS: Some terminate only by reset. Results are undefined.
             CMOS: All are NOPs (reserved for future use).


     3. Jump indirect, operand = XXFF.
             NMOS: Page address does not increment.
             CMOS: Page address increments and adds one additional cycle.


     4. Read/modify/write instructions at effective address.
             NMOS: One read and two write cycles.
             CMOS: Two read and one write cycle.


     5. Decimal flag.
             NMOS: Indeterminate after reset.
             CMOS: Initialized to binary mode (D=0) after reset and interrupts.


     6. Flags after decimal operation.
             NMOS: Invalid N, V and Z flags.
             CMOS: Valid flag adds one additional cycle.


     7. Interrupt after fetch of BRK instruction.
             NMOS: Interrupt vector is loaded, BRK vector is ignored.
             CMOS: BRK is executed, then interrupt is executed.


6510 Instruction Timing

  The NMOS 6500 series processors always perform at least two reads
for each instruction. In addition to the operation code (opcode), they
fetch the next byte. This is quite efficient, as most instructions are
two or three bytes long.

  The processors also use a sort of pipelining. If an instruction does
not store data in memory on its last cycle, the processor can fetch
the opcode of the next instruction while executing the last cycle. For
instance, the instruction EOR #$FF truly takes three cycles. On the
first cycle, the opcode $49 will be fetched. During the second cycle
the processor decodes the opcode and fetches the parameter #$FF. On
the third cycle, the processor will perform the operation and store
the result to accumulator, but simultaneously it fetches the opcode
for the next instruction. This is why the instruction effectively
takes only two cycles.

  The following tables show what happens on the bus while executing
different kinds of instructions.

  Interrupts

     NMI and IRQ both take 7 cycles. Their timing diagram is much like
     BRK's (see below). IRQ will be executed only when the I flag is
     clear. IRQ and BRK both set the I flag, whereas the NMI does not
     affect its state.

     The processor will usually wait for the current instruction to
     complete before executing the interrupt sequence. To process the
     interrupt before the next instruction, the interrupt must occur
     before the last cycle of the current instruction.

     There is one exception to this rule: the BRK instruction. If a
     hardware interrupt (NMI or IRQ) occurs before the fourth (flags
     saving) cycle of BRK, the BRK instruction will be skipped, and
     the processor will jump to the hardware interrupt vector. This
     sequence will always take 7 cycles.

     You do not completely lose the BRK interrupt, the B flag will be
     set in the pushed status register if a BRK instruction gets
     interrupted. When BRK and IRQ occur at the same time, this does
     not cause any problems, as your program will consider it as a
     BRK, and the IRQ would occur again after the processor returned
     from your BRK routine, unless you cleared the interrupt source in
     your BRK handler. But the simultaneous occurrence of NMI and BRK
     is far more fatal. If you do not check the B flag in the NMI
     routine and subtract two from the return address when needed, the
     BRK instruction will be skipped.

     If the NMI and IRQ interrupts overlap each other (one interrupt
     occurs before fetching the interrupt vector for the other
     interrupt), the processor will most probably jump to the NMI
     vector in every case, and then jump to the IRQ vector after
     processing the first instruction of the NMI handler. This has not
     been measured yet, but the IRQ is very similar to BRK, and many
     sources state that the NMI has higher priority than IRQ. However,
     it might be that the processor takes the interrupt that comes
     later, i.e. you could lose an NMI interrupt if an IRQ occurred in
     four cycles after it.

     After finishing the interrupt sequence, the processor will start
     to execute the first instruction of the interrupt routine. This
     proves that the processor uses a sort of pipelining: it finishes
     the current instruction (or interrupt sequence) while reading the
     opcode of the next instruction.

     RESET does not push program counter on stack, and it lasts
     probably 6 cycles after deactivating the signal. Like NMI, RESET
     preserves all registers except PC.

  Instructions accessing the stack

     BRK

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away),
                       increment PC
        3  $0100,S  W  push PCH on stack (with B flag set), decrement S
        4  $0100,S  W  push PCL on stack, decrement S
        5  $0100,S  W  push P on stack, decrement S
        6   $FFFE   R  fetch PCL
        7   $FFFF   R  fetch PCH

     RTI

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away)
        3  $0100,S  R  increment S
        4  $0100,S  R  pull P from stack, increment S
        5  $0100,S  R  pull PCL from stack, increment S
        6  $0100,S  R  pull PCH from stack

     RTS

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away)
        3  $0100,S  R  increment S
        4  $0100,S  R  pull PCL from stack, increment S
        5  $0100,S  R  pull PCH from stack
        6    PC     R  increment PC

     PHA, PHP

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away)
        3  $0100,S  W  push register on stack, decrement S

     PLA, PLP

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away)
        3  $0100,S  R  increment S
        4  $0100,S  R  pull register from stack

     JSR

        #  address R/W description
       --- ------- --- -------------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch low address byte, increment PC
        3  $0100,S  R  internal operation (predecrement S?)
        4  $0100,S  W  push PCH on stack, decrement S
        5  $0100,S  W  push PCL on stack, decrement S
        6    PC     R  copy low address byte to PCL, fetch high address
                       byte to PCH

  Accumulator or implied addressing

        #  address R/W description
       --- ------- --- -----------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  read next instruction byte (and throw it away)

  Immediate addressing

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch value, increment PC

  Absolute addressing

     JMP

        #  address R/W description
       --- ------- --- -------------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch low address byte, increment PC
        3    PC     R  copy low address byte to PCL, fetch high address
                       byte to PCH

     Read instructions (LDA, LDX, LDY, EOR, AND, ORA, ADC, SBC, CMP, BIT,
                        LAX, NOP)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch low byte of address, increment PC
        3    PC     R  fetch high byte of address, increment PC
        4  address  R  read from effective address

     Read-Modify-Write instructions (ASL, LSR, ROL, ROR, INC, DEC,
                                     SLO, SRE, RLA, RRA, ISB, DCP)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch low byte of address, increment PC
        3    PC     R  fetch high byte of address, increment PC
        4  address  R  read from effective address
        5  address  W  write the value back to effective address,
                       and do the operation on it
        6  address  W  write the new value to effective address

     Write instructions (STA, STX, STY, SAX)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch low byte of address, increment PC
        3    PC     R  fetch high byte of address, increment PC
        4  address  W  write register to effective address

  Zero page addressing

     Read instructions (LDA, LDX, LDY, EOR, AND, ORA, ADC, SBC, CMP, BIT,
                        LAX, NOP)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch address, increment PC
        3  address  R  read from effective address

     Read-Modify-Write instructions (ASL, LSR, ROL, ROR, INC, DEC,
                                     SLO, SRE, RLA, RRA, ISB, DCP)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch address, increment PC
        3  address  R  read from effective address
        4  address  W  write the value back to effective address,
                       and do the operation on it
        5  address  W  write the new value to effective address

     Write instructions (STA, STX, STY, SAX)

        #  address R/W description
       --- ------- --- ------------------------------------------
        1    PC     R  fetch opcode, increment PC
        2    PC     R  fetch address, increment PC
        3  address  W  write register to effective address

  Zero page indexed addressing

     Read instructions (LDA, LDX, LDY, EOR, AND, ORA, ADC, SBC, CMP, BIT,
                        LAX, NOP)

        #   address  R/W description
       --- --------- --- ------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch address, increment PC
        3   address   R  read from address, add index register to it
        4  address+I* R  read from effective address

       Notes: I denotes either index register (X or Y).

              * The high byte of the effective address is always zero,
                i.e. page boundary crossings are not handled.

     Read-Modify-Write instructions (ASL, LSR, ROL, ROR, INC, DEC,
                                     SLO, SRE, RLA, RRA, ISB, DCP)

        #   address  R/W description
       --- --------- --- ---------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch address, increment PC
        3   address   R  read from address, add index register X to it
        4  address+X* R  read from effective address
        5  address+X* W  write the value back to effective address,
                         and do the operation on it
        6  address+X* W  write the new value to effective address

       Note: * The high byte of the effective address is always zero,
               i.e. page boundary crossings are not handled.

     Write instructions (STA, STX, STY, SAX)

        #   address  R/W description
       --- --------- --- -------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch address, increment PC
        3   address   R  read from address, add index register to it
        4  address+I* W  write to effective address

       Notes: I denotes either index register (X or Y).

              * The high byte of the effective address is always zero,
                i.e. page boundary crossings are not handled.

  Absolute indexed addressing

     Read instructions (LDA, LDX, LDY, EOR, AND, ORA, ADC, SBC, CMP, BIT,
                        LAX, LAE, SHS, NOP)

        #   address  R/W description
       --- --------- --- ------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch low byte of address, increment PC
        3     PC      R  fetch high byte of address,
                         add index register to low address byte,
                         increment PC
        4  address+I* R  read from effective address,
                         fix the high byte of effective address
        5+ address+I  R  re-read from effective address

       Notes: I denotes either index register (X or Y).

              * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100.

              + This cycle will be executed only if the effective address
                was invalid during cycle #4, i.e. page boundary was crossed.

     Read-Modify-Write instructions (ASL, LSR, ROL, ROR, INC, DEC,
                                     SLO, SRE, RLA, RRA, ISB, DCP)

        #   address  R/W description
       --- --------- --- ------------------------------------------
        1    PC       R  fetch opcode, increment PC
        2    PC       R  fetch low byte of address, increment PC
        3    PC       R  fetch high byte of address,
                         add index register X to low address byte,
                         increment PC
        4  address+X* R  read from effective address,
                         fix the high byte of effective address
        5  address+X  R  re-read from effective address
        6  address+X  W  write the value back to effective address,
                         and do the operation on it
        7  address+X  W  write the new value to effective address

       Notes: * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100.

     Write instructions (STA, STX, STY, SHA, SHX, SHY)

        #   address  R/W description
       --- --------- --- ------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch low byte of address, increment PC
        3     PC      R  fetch high byte of address,
                         add index register to low address byte,
                         increment PC
        4  address+I* R  read from effective address,
                         fix the high byte of effective address
        5  address+I  W  write to effective address

       Notes: I denotes either index register (X or Y).

              * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100. Because
                the processor cannot undo a write to an invalid
                address, it always reads from the address first.

  Relative addressing (BCC, BCS, BNE, BEQ, BPL, BMI, BVC, BVS)

        #   address  R/W description
       --- --------- --- ---------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch operand, increment PC
        3     PC      R  Fetch opcode of next instruction,
                         If branch is taken, add operand to PCL.
                         Otherwise increment PC.
        4+    PC*     R  Fetch opcode of next instruction.
                         Fix PCH. If it did not change, increment PC.
        5!    PC      R  Fetch opcode of next instruction,
                         increment PC.

       Notes: The opcode fetch of the next instruction is included to
              this diagram for illustration purposes. When determining
              real execution times, remember to subtract the last
              cycle.

              * The high byte of Program Counter (PCH) may be invalid
                at this time, i.e. it may be smaller or bigger by $100.

              + If branch is taken, this cycle will be executed.

              ! If branch occurs to different page, this cycle will be
                executed.

  Indexed indirect addressing

     Read instructions (LDA, ORA, EOR, AND, ADC, CMP, SBC, LAX)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  read from the address, add X to it
        4   pointer+X   R  fetch effective address low
        5  pointer+X+1  R  fetch effective address high
        6    address    R  read from effective address

       Note: The effective address is always fetched from zero page,
             i.e. the zero page boundary crossing is not handled.

     Read-Modify-Write instructions (SLO, SRE, RLA, RRA, ISB, DCP)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  read from the address, add X to it
        4   pointer+X   R  fetch effective address low
        5  pointer+X+1  R  fetch effective address high
        6    address    R  read from effective address
        7    address    W  write the value back to effective address,
                           and do the operation on it
        8    address    W  write the new value to effective address

       Note: The effective address is always fetched from zero page,
             i.e. the zero page boundary crossing is not handled.

     Write instructions (STA, SAX)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  read from the address, add X to it
        4   pointer+X   R  fetch effective address low
        5  pointer+X+1  R  fetch effective address high
        6    address    W  write to effective address

       Note: The effective address is always fetched from zero page,
             i.e. the zero page boundary crossing is not handled.

  Indirect indexed addressing

     Read instructions (LDA, EOR, AND, ORA, ADC, SBC, CMP)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  fetch effective address low
        4   pointer+1   R  fetch effective address high,
                           add Y to low byte of effective address
        5   address+Y*  R  read from effective address,
                           fix high byte of effective address
        6+  address+Y   R  read from effective address

       Notes: The effective address is always fetched from zero page,
              i.e. the zero page boundary crossing is not handled.

              * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100.

              + This cycle will be executed only if the effective address
                was invalid during cycle #5, i.e. page boundary was crossed.

     Read-Modify-Write instructions (SLO, SRE, RLA, RRA, ISB, DCP)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  fetch effective address low
        4   pointer+1   R  fetch effective address high,
                           add Y to low byte of effective address
        5   address+Y*  R  read from effective address,
                           fix high byte of effective address
        6   address+Y   R  read from effective address
        7   address+Y   W  write the value back to effective address,
                           and do the operation on it
        8   address+Y   W  write the new value to effective address

       Notes: The effective address is always fetched from zero page,
              i.e. the zero page boundary crossing is not handled.

              * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100.

     Write instructions (STA, SHA)

        #    address   R/W description
       --- ----------- --- ------------------------------------------
        1      PC       R  fetch opcode, increment PC
        2      PC       R  fetch pointer address, increment PC
        3    pointer    R  fetch effective address low
        4   pointer+1   R  fetch effective address high,
                           add Y to low byte of effective address
        5   address+Y*  R  read from effective address,
                           fix high byte of effective address
        6   address+Y   W  write to effective address

       Notes: The effective address is always fetched from zero page,
              i.e. the zero page boundary crossing is not handled.

              * The high byte of the effective address may be invalid
                at this time, i.e. it may be smaller by $100.

  Absolute indirect addressing (JMP)

        #   address  R/W description
       --- --------- --- ------------------------------------------
        1     PC      R  fetch opcode, increment PC
        2     PC      R  fetch pointer address low, increment PC
        3     PC      R  fetch pointer address high, increment PC
        4   pointer   R  fetch low address to latch
        5  pointer+1* R  fetch PCH, copy latch to PCL

       Note: * The PCH will always be fetched from the same page
               than PCL, i.e. page boundary crossing is not handled.

                How Real Programmers Acknowledge Interrupts

  With RMW instructions:

        ; beginning of combined raster/timer interrupt routine
        LSR $D019       ; clear VIC interrupts, read raster interrupt flag to C
        BCS raster      ; jump if VIC caused an interrupt
        ...             ; timer interrupt routine

        Operational diagram of LSR $D019:

          #  data  address  R/W
         --- ----  -------  ---  ---------------------------------
          1   4E     PC      R   fetch opcode
          2   19    PC+1     R   fetch address low
          3   D0    PC+2     R   fetch address high
          4   xx    $D019    R   read memory
          5   xx    $D019    W   write the value back, rotate right
          6  xx/2   $D019    W   write the new value back

        The 5th cycle acknowledges the interrupt by writing the same
        value back. If only raster interrupts are used, the 6th cycle
        has no effect on the VIC. (It might acknowledge also some
        other interrupts.)

  With indexed addressing:

        ; acknowledge interrupts to both CIAs
        LDX #$10
        LDA $DCFD,X

        Operational diagram of LDA $DCFD,X:

          #  data  address  R/W  description
         --- ----  -------  ---  ---------------------------------
          1   BD     PC      R   fetch opcode
          2   FD    PC+1     R   fetch address low
          3   DC    PC+2     R   fetch address high, add X to address low
          4   xx    $DC0D    R   read from address, fix high byte of address
          5   yy    $DD0D    R   read from right address

        ; acknowledge interrupts to CIA 2
        LDX #$10
        STA $DDFD,X

        Operational diagram of STA $DDFD,X:

          #  data  address  R/W  description
         --- ----  -------  ---  ---------------------------------
          1   9D     PC      R   fetch opcode
          2   FD    PC+1     R   fetch address low
          3   DC    PC+2     R   fetch address high, add X to address low
          4   xx    $DD0D    R   read from address, fix high byte of address
          5   ac    $DE0D    W   write to right address

  With branch instructions:

        ; acknowledge interrupts to CIA 2
                LDA #$00  ; clear N flag
                JMP $DD0A
        DD0A    BPL $DC9D ; branch
        DC9D    BRK       ; return

        You need the following preparations to initialize the CIA registers:

                LDA #$91  ; argument of BPL
                STA $DD0B
                LDA #$10  ; BPL
                STA $DD0A
                STA $DD08 ; load the ToD values from the latches
                LDA $DD0B ; freeze the ToD display
                LDA #$7F
                STA $DC0D ; assure that $DC0D is $00

        Operational diagram of BPL $DC9D:

          #  data  address  R/W  description
         --- ----  -------  ---  ---------------------------------
          1   10    $DD0A    R   fetch opcode
          2   91    $DD0B    R   fetch argument
          3   xx    $DD0C    R   fetch opcode, add argument to PCL
          4   yy    $DD9D    R   fetch opcode, fix PCH
        ( 5   00    $DC9D    R   fetch opcode )

        ; acknowledge interrupts to CIA 1
                LSR       ; clear N flag
                JMP $DCFA
        DCFA    BPL $DD0D
        DD0D    BRK

        ; Again you need to set the ToD registers of CIA 1 and the
        ; Interrupt Control Register of CIA 2 first.

        Operational diagram of BPL $DD0D:

          #  data  address  R/W  description
         --- ----  -------  ---  ---------------------------------
          1   10    $DCFA    R   fetch opcode
          2   11    $DCFB    R   fetch argument
          3   xx    $DCFC    R   fetch opcode, add argument to PCL
          4   yy    $DC0D    R   fetch opcode, fix PCH
        ( 5   00    $DD0D    R   fetch opcode )

        ; acknowledge interrupts to CIA 2 automagically
                ; preparations
                LDA #$7F
                STA $DD0D       ; disable all interrupt sources of CIA2
                LDA $DD0E
                AND #$BE        ; ensure that $DD0C remains constant
                STA $DD0E       ; and stop the timer
                LDA #$FD
                STA $DD0C       ; parameter of BPL
                LDA #$10
                STA $DD0B       ; BPL
                LDA #$40
                STA $DD0A       ; RTI/parameter of LSR
                LDA #$46
                STA $DD09       ; LSR
                STA $DD08       ; load the ToD values from the latches
                LDA $DD0B       ; freeze the ToD display
                LDA #$09
                STA $0318
                LDA #$DD
                STA $0319       ; change NMI vector to $DD09
                LDA #$FF        ; Try changing this instruction's operand
                STA $DD05       ; (see comment below).
                LDA #$FF
                STA $DD04       ; set interrupt frequency to 1/65536 cycles
                LDA $DD0E
                AND #$80
                ORA #$11
                LDX #$81
                STX $DD0D       ; enable timer interrupt
                STA $DD0E       ; start timer

                LDA #$00        ; To see that the interrupts really occur,
                STA $D011       ; use something like this and see how
        LOOP    DEC $D020       ; changing the byte loaded to $DD05 from
                BNE LOOP        ; #$FF to #$0F changes the image.

        When an NMI occurs, the processor jumps to Kernal code, which jumps to
        ($0318), which points to the following routine:

        DD09    LSR $40         ; clear N flag
                BPL $DD0A       ; Note: $DD0A contains RTI.

        Operational diagram of BPL $DD0A:

          #  data  address  R/W  description
         --- ----  -------  ---  ---------------------------------
          1   10    $DD0B    R   fetch opcode
          2   11    $DD0C    R   fetch argument
          3   xx    $DD0D    R   fetch opcode, add argument to PCL
          4   40    $DD0A    R   fetch opcode, (fix PCH)

  With RTI:

        ; the fastest possible interrupt handler in the 6500 family
                ; preparations
                SEI
                LDA $01         ; disable ROM and enable I/O
                AND #$FD
                ORA #$05
                STA $01
                LDA #$7F
                STA $DD0D       ; disable CIA 2's all interrupt sources
                LDA $DD0E
                AND #$BE        ; ensure that $DD0C remains constant
                STA $DD0E       ; and stop the timer
                LDA #$40
                STA $DD0C       ; store RTI to $DD0C
                LDA #$0C
                STA $FFFA
                LDA #$DD
                STA $FFFB       ; change NMI vector to $DD0C
                LDA #$FF        ; Try changing this instruction's operand
                STA $DD05       ; (see comment below).
                LDA #$FF
                STA $DD04       ; set interrupt frequency to 1/65536 cycles
                LDA $DD0E
                AND #$80
                ORA #$11
                LDX #$81
                STX $DD0D       ; enable timer interrupt
                STA $DD0E       ; start timer

                LDA #$00        ; To see that the interrupts really occur,
                STA $D011       ; use something like this and see how
        LOOP    DEC $D020       ; changing the byte loaded to $DD05 from
                BNE LOOP        ; #$FF to #$0F changes the image.

        When an NMI occurs, the processor jumps to Kernal code, which
        jumps to ($0318), which points to the following routine:

        DD0C    RTI

        How on earth can this clear the interrupts? Remember, the
        processor always fetches two successive bytes for each
        instruction.

        A little more practical version of this is redirecting the NMI
        (or IRQ) to your own routine, whose last instruction is JMP
        $DD0C or JMP $DC0C.  If you want to confuse more, change the 0
        in the address to a hexadecimal digit different from the one
        you used when writing the RTI.

        Or you can combine the latter two methods:

        DD09    LSR $xx  ; xx is any appropriate BCD value 00-59.
                BPL $DCFC
        DCFC    RTI

        This example acknowledges interrupts to both CIAs.

  If you want to confuse the examiners of your code, you can use any
of these techniques. Although these examples use no undefined opcodes,
they do not necessarily run correctly on CMOS processors. However, the
RTI example should run on 65C02 and 65C816, and the latter branch
instruction example might work as well.

  The RMW instruction method has been used in some demos, others were
developed by Marko M"akel"a. His favourite is the automagical RTI
method, although it does not have any practical applications, except
for some time dependent data decryption routines for very complicated
copy protections.


