mirror of https://github.com/uffejakobsen/acme.git
146 lines
7.0 KiB
Plaintext
146 lines
7.0 KiB
Plaintext
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ACME
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...the ACME Crossassembler for Multiple Environments
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--- Undocumented ("illegal") opcodes ---
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In release 0.87, support for some of the undocumented opcodes of the
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NMOS 6502 processor was added.
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In release 0.89, some more were added.
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In release 0.94.8, another one was added (lxa).
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In release 0.95.3, C64DTV2 support was added, which includes these
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opcodes as well.
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In release 0.95.4, the remaining seven were added.
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In release 0.95.6, "ANC" was removed from C64DTV2 mode.
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Here are the new mnemonics, possible addressing modes and generated
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opcodes (mnemonics in parentheses are used by other sources):
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| addressing mode |
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mnemonic | 8 8,x 8,y 16 16,x 16,y (8,x) (8),y | performs:
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----------------+--------------------------------------+-----------
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slo (aso) | 07 17 0f 1f 1b 03 13 | asl + ora
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rla (rln) | 27 37 2f 3f 3b 23 33 | rol + and
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sre (lse) | 47 57 4f 5f 5b 43 53 | lsr + eor
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rra (rrd) | 67 77 6f 7f 7b 63 73 | ror + adc
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sax (axs, aax) | 87 97 8f 83 | stx + sta
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lax | a7 b7 af bf a3 b3 | ldx + lda
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dcp (dcm) | c7 d7 cf df db c3 d3 | dec + cmp
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isc (isb, ins) | e7 f7 ef ff fb e3 f3 | inc + sbc
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las (lar, lae) | bb | A,X,S = {addr} & S
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tas (shs, xas) | 9b | S = A & X {addr} = A&X& {H+1}
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sha (axa, ahx) | 9f 93 | {addr} = A & X & {H+1}
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shx (xas, sxa) | 9e | {addr} = X & {H+1}
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shy (say, sya) | 9c | {addr} = Y & {H+1}
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| addressing mode |
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mnemonic | implied #8 8 8,x 16 16,x | performs:
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----------------+---------------------------------+-----------------------
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anc (ana, anb) | 0b* | A = A & arg, then C=N
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alr/asr | 4b | A = A & arg, then lsr
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arr | 6b | A = A & arg, then ror
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sbx (axs, sax) | cb | X = (A & X) - arg
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dop (nop, skb) | 80** 80 04 14 | skips next byte
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top (nop, skw) | 0c** 0c 1c | skips next two bytes
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nop (skb, skw) | ea 80 04 14 0c 1c | see the two lines above
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jam (kil, hlt) | 02 | crash (wait for reset)
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These two are somewhat unstable, because they involve an arbitrary value:
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ane (xaa, axm) | 8b*** | A = (A | ??) & X & arg
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lxa (lax, atx) | ab*** | A,X = (A | ??) & arg
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Example:
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!cpu nmos6502 ; activate additional mnemonics...
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lax (some_zp_label,x) ; ...and use them. No, this
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dcp (other_zp_label),y ; example does not make sense.
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*) Up until ACME version 0.95.1, anc#8 generated opcode 0x2b. Since
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ACME version 0.95.2, anc#8 generates opcode 0x0b. Both opcodes work
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the same way on a real NMOS 6502 CPU, but they do not work on the
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C64DTV2.
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Using the "--dialect" CLI switch does not change the generated opcode!
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**) Note that DOP ("double nop") and TOP ("triple nop") can be used
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with implied addressing, but the generated opcodes are those for
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immediate and 16-bit absolute addressing, respectively, This way DOP
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and TOP can be used to skip the following one- or two-byte
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instruction.
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Using DOP/TOP with x-indexed addressing might have its uses when
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timing is critical (crossing a page border adds a penalty cycle).
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Unless using implied addressing, DOP/TOP can also be written as NOP.
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***) ANE and LXA first perform an ORA with an arbitrary(!) value and
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then perform an AND with the given argument. So they are unstable and
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therefore useless - unless the given argument is zero:
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ANE #0 reliably clears A - which is still useless; just use LDA #0.
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LXA #0 reliably clears both A and X.
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ACME will output a warning if these opcodes get assembled with a
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nonzero argument.
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There is no guarantee that these opcodes actually work on a given 6502
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(or 6510, or 8500, or 8501, or 8502) CPU. But as far as I know, nobody
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ever found an unmodified C64/C128 where these illegals didn't work.
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These illegals will definitely *not* work on 65c02 and 65816 CPUs. But
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I really should not have to tell you that ;)
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Because there are no official mnemonics for these opcodes, different
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people use different names for them. I hope my choices are not too
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exotic for your taste.
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Just for the sake of completeness: Here are all the remaining opcodes
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(the ones ACME won't generate even with "nmos6502" cpu chosen):
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Opcode| Description C64DTV2
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------+--------------------------------------------------------------
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12 | same as 02 and others jam CRASH bra rel
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1a | same as (*legal*) ea nop
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22 | same as 02 and others jam CRASH
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2b | same as 0b anc #8 dop
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32 | same as 02 and others jam CRASH sac #8
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34 | same as 14 and others dop 8,x
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3a | same as (*legal*) ea nop
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3c | same as 1c and others top 16,x
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42 | same as 02 and others jam CRASH sir #8
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44 | same as 04 dop 8
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52 | same as 02 and others jam CRASH
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54 | same as 14 and others dop 8,x
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5a | same as (*legal*) ea nop
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5c | same as 1c and others top 16,x
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62 | same as 02 and others jam CRASH
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64 | same as 04 dop 8
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72 | same as 02 and others jam CRASH
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74 | same as 14 and others dop 8,x
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7a | same as (*legal*) ea nop
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7c | same as 1c and others top 16,x
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82 | same as c2/e2 dop #8, but said to CRASH sometimes
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89 | same as 80 dop #8
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92 | same as 02 and others jam CRASH
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b2 | same as 02 and others jam CRASH
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c2 | same as 82/e2 dop #8, but said to CRASH sometimes
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d2 | same as 02 and others jam CRASH
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d4 | same as 14 and others dop 8,x
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da | same as (*legal*) ea nop
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dc | same as 1c and others top 16,x
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e2 | same as 82/c2 dop #8, but said to CRASH sometimes
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eb | same as (*legal*) e9 sbc #8
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f2 | same as 02 and others jam CRASH
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f4 | same as 14 and others dop 8,x
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fa | same as (*legal*) ea nop
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fc | same as 1c and others top 16,x
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For more information about what these opcodes do, see these documents:
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John West, Marko Mäkelä. '64doc' file, 1994/06/03.
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Extra Instructions Of The 65XX Series CPU, Adam Vardy, 27 Sept. 1996
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6502 Undocumented Opcodes, by Freddy Offenga, 5/17/1997
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AAY64 (All About Your 64)
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...but the most comprehensive work is:
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"No More Secrets - NMOS 6510 Unintended Opcodes"
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Download it from https://csdb.dk/release/?id=185341
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or ask google for the latest version.
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