9/22/2023 0 Comments Opcode sheet for 8086 pdfIntel's own PDF manuals document this in detail see vol.2 of the SDM, specifically the intro chapters before the entries for each instruction. What is an indexing mode? What is a register number? How is a register represented? etc. The r/m field, which occupies the three least significant bits of the byte, can specify a register as the location of an operand, or can form part of the addressing-mode encoding in combination with the field as described above The meaning of the reg field is determined by the first (opcode) byte of the instruction. The reg field, which occupies the next three bits following the mod field, specifies either a register number or three more bits of opcode information. The mod field, which occupies the two most significant bits of the byte, combines with the r/m field to form 32 possible values: eight registers and 24 indexing modes Some documentation I've found: The ModR/M byte contains three fields of information: What are all the bits used for in the MODR/M byte and what are the possible options? The 8086 documentation sites seem a bit vague when the MODR/M byte is mentioned and it's really difficult to comprehend what it is and does.
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