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SCASB

Compares the value in the al register with the byte at the memory location pointed to by rdi. After the comparison, the rdi register is incremented or decremented by 1, depending on the Direction Flag (DF).

The table below covers what the source and destinations can be.

DO NOT support LOCK

The instruction is available in 64-bit mode, 32-bit mode, and 16-bit mode. It operates specifically on 8-bit operands regardless of the current processor mode.

The behavior of the pointer rdi depends on the state of the DF flag: if DF=0, rdi is incremented; if DF=1, rdi is decremented. Users MUST ensure the DF flag is set correctly via CLD or STD to avoid unintended memory access patterns. When used with the REP prefix, the instruction will continue until al equals the value at [rdi] or rcx reaches 0. Failure to initialize rcx correctly when using REP MAY result in a memory access violation (General Protection Fault) as the pointer increments through the address space.