3 ms·

The ModR/M and SIB prefixes are probably the most common prefixes in instructions. They are so common that assemblers elide their existence when you read code.

The ModR/M and SIB prefixes are probably the most common prefixes in instructions. They are so common that assemblers elide their existence when you read code. REX is in the same boat: so common that it's usually elided. The VEX prefix is also really common (all of the V* AVX instructions, like VMOVDQ), and then the LOCK prefix (all atomics). After all of those, REP is not that uncommon of a prefix to run into, although many people prefer SIMD memcpy/memset to REP MOVSB/REP STOSB. It is slightly unusual.

bonzini 3y ago
ModRM and SIB are not a prefix, they're part of the opcode (second and third byte after all the prefixes and the 0Fh/0F38h/0F3Ah opcode map selectors)
- EarlKing 3y ago
  More specifically, they're affixed to certain opcodes that require them. There are a number of byte-sized opcodes that do not require a ModRM or SIB byte (although a number of those got gobbled up to make the REX prefix, but that's another story). TL;DR Weeee! Intel machine language is crazy!
EarlKing 3y ago
There's a good reason for using vector instructions over REP: Until relatively recently that was how you got maximum performance in small, tight loops. REP is making a comeback precisely because of ERMS and FSRM, so unfortunately this will become a bigger problem going forward.
epcoa 3y ago
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).

bonzini 3y ago

ModRM and SIB are not a prefix, they're part of the opcode (second and third byte after all the prefixes and the 0Fh/0F38h/0F3Ah opcode map selectors)

EarlKing 3y ago
More specifically, they're affixed to certain opcodes that require them. There are a number of byte-sized opcodes that do not require a ModRM or SIB byte (although a number of those got gobbled up to make the REX prefix, but that's another story). TL;DR Weeee! Intel machine language is crazy!

EarlKing 3y ago

There's a good reason for using vector instructions over REP: Until relatively recently that was how you got maximum performance in small, tight loops. REP is making a comeback precisely because of ERMS and FSRM, so unfortunately this will become a bigger problem going forward.

epcoa 3y ago

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).