2-2 Vol. 2A
INSTRUCTION FORMAT
• F2H—REPNE/REPNZ (used only with string instructions; when used with
the escape opcode 0FH, this prefix is treated as a mandatory prefix for
some SIMD instructions)
• F3H—REP or REPE/REPZ (used only with string instructions; when used
with the escape opcode 0FH, this prefix is treated as an mandatory prefix
for some SIMD instructions)
• Group 2
— Segment override prefixes:
• 2EH—CS segment override (use with any branch instruction is reserved)
• 36H—SS segment override prefix (use with any branch instruction is
reserved)
• 3EH—DS segment override prefix (use with any branch instruction is
reserved)
• 26H—ES segment override prefix (use with any branch instruction is
reserved)
• 64H—FS segment override prefix (use with any branch instruction is
reserved)
• 65H—GS segment override prefix (use with any branch instruction is
reserved)
— Branch hints:
• 2EH—Branch not taken (used only with Jcc instructions)
• 3EH—Branch taken (used only with Jcc instructions)
• Group 3
• 66H—Operand-size override prefix (when used with the escape opcode
0FH, this is treated as a mandatory prefix for some SIMD instructions)
• Group 4
• 67H—Address-size override prefix
The LOCK prefix (F0H) forces an operation that ensures exclusive use of shared
memory in a multiprocessor environment. See “LOCK—Assert LOCK# Signal Prefix”
in Chapter 3, “Instruction Set Reference, A-M,” for a description of this prefix.
Repeat prefixes (F2H, F3H) cause an instruction to be repeated for each element of a
string. Use these prefixes only with string instructions (MOVS, CMPS, SCAS, LODS,
STOS, INS, and OUTS). Their use, followed by 0FH, is treated as a mandatory prefix
by a number of SSE/SSE2/SSE3 instructions. Use of repeat prefixes and/or unde-
fined opcodes with other Intel 64 or IA-32 instructions is reserved; such use may
cause unpredictable behavior.
Branch hint prefixes (2EH, 3EH) allow a program to give a hint to the processor about
the most likely code path for a branch. Use these prefixes only with conditional
branch instructions (Jcc). Other use of branch hint prefixes and/or other undefined
