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6502 instruction operation codes (opcodes) are 8 bits long and have the general form AAABBBCC, where AAA and CC define the opcode, and BBB defines the addressing mode. [78] For example, the ORA instruction performs a bitwise OR on the bits in the accumulator with another value.
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 ...
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 original 6502 has 56 instructions, which, when combined with different addressing modes, produce a total of 151 opcodes of the possible 256 8-bit opcode patterns. The remaining 105 unused opcodes are undefined, with the set of codes with low-order 4-bits with 3, 7, B or F left entirely unused, the code with low-order 2 having only a single ...
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 ...
An opcode table (also called an opcode matrix) is a visual representation of all opcodes in an instruction set. ... 6502/6510/8500/8502 opcode table
For instance, the MOS 6502 has eight variations of the addition instruction, ADC, which differ only in where they look to find the two operands. [ 5 ] Using the variation of the instruction, or " opcode ", that most closely matches the ultimate operation can reduce the number of instructions to one, saving memory used by the program code and ...
In the 6502, which has only a single arithmetic register A, this instruction can be represented entirely by its 8-bit opcode $06. [1] On processors with more registers, all that is needed is the opcode and register number, another 4 or 5 bits. On MIPS, for instance, the instruction needs only a 6-bit opcode and a 5-bit register number.