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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.
The Motorola 6800 microprocessor was the first for which an undocumented assembly mnemonic HCF became widely known. The operation codes (opcodes—the portions of the machine language instructions that specify an operation to be performed) hexadecimal 9D and DD were reported and given the unofficial mnemonic HCF in a December 1977 article by Gerry Wheeler in BYTE magazine on undocumented ...
The 6502 instruction set includes BRK (opcode $00), which is technically a software interrupt (similar in spirit to the SWI mnemonic of the Motorola 6800 and ARM processors). BRK is most often used to interrupt program execution and start a machine language monitor for testing and debugging during software development.
Undocumented instructions, known as illegal opcodes, on the MOS Technology 6502 and its variants are sometimes used by programmers. These were removed in the WDC 65C02. A Commodore 64 demo using undocumented features to bypass the machine's usual screen border
Internals of BRK/IRQ/NMI/RESET on a MOS 6502; Brad Taylor. "6502 'B' flag & BRK Opcode". 6502 Family Microprocessor Resources and Forum; 65xx Interrupt Primer – An extensive discussion of 65xx family interrupt processing. Investigating 65C816 Interrupts – An extensive discussion of interrupt processing that is specific to 65C816 native mode ...
NMOS AND-by-default logic can produce unusual glitches or buggy behavior in NMOS components, such as the 6502 "illegal opcodes" which are absent in CMOS 6502s. In some cases such as Commodore's VIC-II chip, the bugs present in the chip's logic were extensively exploited by programmers for graphics effects.
This is the opcode table for the MOS Technology 6502 microprocessor from 1975. The 6502 uses 8-bit opcodes. The 6502 uses 8-bit opcodes. Of the 256 possible opcodes available using an 8-bit pattern, the original 6502 uses only 151 of them, organized into 56 instructions with (possibly) multiple addressing modes . [ 1 ]
Technically the Apple IIc was an Apple IIe in a smaller case, more portable and easier to use but also less expandable. The IIc used the CMOS-based 65C02 microprocessor which added 27 new instructions to the 6502, but was incompatible with programs that used deprecated illegal opcodes of the 6502.