[PDF] 6502 opcodes illegal

Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the Zilog Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined.
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  • How many opcodes in 6502?

    Of the 256 possible opcodes available using an 8-bit pattern, the original 6502 uses 151 of them, organized into 56 instructions with (possibly) multiple addressing modes. Depending on the instruction and addressing mode, the opcode may require zero, one or two additional bytes for operands.

  • Which is an illegal assembly instruction?

    Illegal instruction means that there's a byte in the assembled file which doesn't represent a valid CPU instruction. E.g. 0x90 is NOP, but 0xff probably doesn't map to a valid instruction. When the CPU comes to a byte it can't understand, it throws an illegal instruction exception.
  • The opcode is a byte value in the range 0 to 255 inclusive. The 6502 supports 151 opcodes and they are grouped into 56 operations. Each opcode in a group performs the same basic operation, such as adding two numbers together. They differ by the addressing mode used to specify the data to operate on.
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'Back in the days' so called 'illegal' opcodes were researched independently by opcodes are known to work slightly different on 6502 equipped machines.



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'Back in the days' so called 'illegal' opcodes were researched independently by The instructions of the 6502 are compressed into a 130-entry decode ROM.



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The 6502 also comes from a time when assembly programming was illegal opcodes while lib6502 prints a message and continues. Why Lisp?



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'Back in the days' so called 'illegal' opcodes were researched independently by opcodes are known to work slightly different on 6502 equipped machines.



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Appendix C: Atari "Sally" 6502 Undocumented Opcodes………………….. 20 ... directive enabling illegal opcodes fixed a bug with data commands emiting code.



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illegal opcodes: as mentioned before the unused opcodes are not tied to something “neutral” in the original MOS 6502 microprocessor variant (later.



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7 Nis 2022 Every microprocessor has one opcode defined as a “no ... chip does with illegal opcodes (although they see no need to document it!)



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When the 6502 retrieves an opcode of $EA it simply pauses for 2 cycles