acme/docs/Illegals.txt
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ACME
...the ACME Crossassembler for Multiple Environments
--- Undocumented ("illegal") opcodes ---
Since release 0.87, ACME contains support for some of the undocumented
opcodes of the 6502 processor. Here they are:
| addressing mode |
Mnemo | 8 8,x 8,y 16 16,x 16,y (8,x) (8),y | performs:
------+--------------------------------------------------+-----------
slo | 07 17 -- 0f 1f 1b 03 13 | asl + ora
rla | 27 37 -- 2f 3f 3b 23 33 | rol + and
sre | 47 57 -- 4f 5f 5b 43 53 | lsr + eor
rra | 67 77 -- 6f 7f 7b 63 73 | ror + adc
sax | 87 -- 97 8f -- -- 83 -- | stx + sta
lax | a7 -- b7 af -- bf a3 b3 | ldx + lda
dcp | c7 d7 -- cf df db c3 d3 | dec + cmp
isc | e7 f7 -- ef ff fb e3 f3 | inc + sbc
In release 0.89, further ones were added:
| addressing mode |
Mnemo | implied #8 8 8,x 16 16,x | performs:
------+-------------------------------------+------------------------
anc | -- 2b -- -- -- -- | A = A & arg, then C=N
asr | -- 4b -- -- -- -- | A = A & arg, then lsr
arr | -- 6b -- -- -- -- | A = A & arg, then ror
sbx | -- cb -- -- -- -- | X = (A & X) - arg
dop | 80* 80 04 14 -- -- | skips next byte
top | 0c* -- -- -- 0c 1c | skips next two bytes
jam | 02 -- -- -- -- -- | crash (wait for reset)
Example:
!cpu 6510 ; activate additional mnemonics...
lax (some_zp_label,x) ; ...and use them. No, this
dcp (other_zp_label),y ; example does not make sense.
*) Note that "dop" and "top" can be used with implied addressing, but
the generated opcodes are those for immediate and 16-bit absolute
addressing, respectively. Using dop/top with x-indexed addressing
might have its uses when timing is critical (crossing a page border
adds a penalty cycle).
There is no guarantee that these opcodes actually work on a given 6502
(or 6510, or 8500, or 8502) CPU. But as far as I know, nobody ever
found an unmodified C64/C128 where these illegals didn't work. That's
why I used "6510" as the CPU keyword instead of "6502illegal" or
something like that.
These illegals will definitely *not* work on 65c02 and 65816 CPUs. But
I really should not have to tell you that ;)
Because there are no official mnemonics for these opcodes, different
people use different names for them. I hope my choices are not too
exotic for your taste.
Just for the sake of completeness: Here are all the remaining opcodes
(the ones ACME won't generate):
Opcode| Description
------+--------------------------------------------------------------
0b | same as 2b anc #8
12 | same as 02 and others jam CRASH
1a | same as (*legal*) ea nop
22 | same as 02 and others jam CRASH
32 | same as 02 and others jam CRASH
34 | same as 14 and others dop 8,x
3a | same as (*legal*) ea nop
3c | same as 1c and others top 16,x
42 | same as 02 and others jam CRASH
44 | same as 04 dop 8
52 | same as 02 and others jam CRASH
54 | same as 14 and others dop 8,x
5a | same as (*legal*) ea nop
5c | same as 1c and others top 16,x
62 | same as 02 and others jam CRASH
64 | same as 04 dop 8
72 | same as 02 and others jam CRASH
74 | same as 14 and others dop 8,x
7a | same as (*legal*) ea nop
7c | same as 1c and others top 16,x
82 | same as c2/e2 dop #8, but said to CRASH sometimes
89 | same as 80 dop #8
8b | see notes below
92 | same as 02 and others jam CRASH
93 | see notes below
9b | see notes below
9c | see notes below
9e | see notes below
9f | see notes below
ab | see notes below
b2 | same as 02 and others jam CRASH
bb | see notes below
c2 | same as 82/e2 dop #8, but said to CRASH sometimes
d2 | same as 02 and others jam CRASH
d4 | same as 14 and others dop 8,x
da | same as (*legal*) ea nop
dc | same as 1c and others top 16,x
e2 | same as 82/c2 dop #8, but said to CRASH sometimes
eb | same as (*legal*) e9 sbc #8
f2 | same as 02 and others jam CRASH
f4 | same as 14 and others dop 8,x
fa | same as (*legal*) ea nop
fc | same as 1c and others top 16,x
Concerning opcodes 8b, 93, 9b, 9c, 9e, 9f, ab, bb:
These opcodes are said to be unstable. For more information about what
they do, see these documents:
John West, Marko Mäkelä. '64doc' file, 1994/06/03.
Extra Instructions Of The 65XX Series CPU, Adam Vardy, 27 Sept. 1996
6502 Undocumented Opcodes, by Freddy Offenga, 5/17/1997
AAY64 (All About Your 64)
I did not see much point in assigning mnemonics for these opcodes. The
reference documents above call them:
8b: ane, xaa
93: sha, axa, ahx
9b: shs, tas, xas
9c: shy, say, sya
9e: shx, xas, sxa
9f: sha, axa, ahx
ab: lxa, oal, atx
bb: las, lar, lae