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MOD-REG-R/M byte follows one or two opcode bytes of the instruction
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It provides addressing mode information for one or two operands.
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MOD-REG-R/M Byte:
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If operand is in memory, or operand is a register:
MOD field (bits [7:6]), combined with the R/M field (bits [2:0]), specify memory/register operand, as well as its addressing mode.
REG field (bits [5:3]) specifies another register operand in of the two-operand instruction.
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