Undocumented Instruction Support

To view disassembly with 6502 "illegal" instructions, use the Extended Disassembler Tool:

.E [from] [to]

where from and to are 16-bit hexadecimal addresses. Other than the display of undocumented instructions, the Extended Disassember Tool behaves exactly like the Disassembler Tool.

See Appendix C: Supported Illegal Instructions

These instructions may be entered into the assembler alongside the official 6502 instructions, with no special syntax:

.A 1800 LAX $EC81 ; LOAD A AND X
.A 1803 BRK
.A 1804
SYS 6144

BRK
  A--X--Y--P--S--PC--
.;42 42 00 30 F4 1808

See here for information about these instructions (PDF)