This isn't correct. ModR/M and SIB are not prefixes. They are suffixes and essentially part of the core instruction encoding for certain memory and register acc

This isn't correct. ModR/M and SIB are not prefixes. They are suffixes and essentially part of the core instruction encoding for certain memory and register access instruction. they are the primary means of encoding the myriad addressing modes of the x86. And their existence is not elided in any meaningful way, their value is explicitly derived from the instruction operands (SIB is scale, index, base), so when you see an instruction like: mov BYTE PTR [rdi+rbx*4],0x4 SIB is determined by the register indices of rdi, rbx, and 4, all right there in the instruction. Likewise, Mod R/M encodes the addressing mode, which is clear from the operands in the assembler listing. Though x86 is such as mess that there are cases where you can encode the same instruction in either a Mod R/M form or a shorter form, eg PUSH/POP. REX is a prefix, but it is a bit special as it must be the last one, and repeats are undefined. It is not elided because of commonality but because its presence and value is usually implied from the operands, it is therefore redundant to list it. For instance, PUSH R12 must use a REX prefix (REX.B with the one byte encoding).