movb

movb is the mnemonic for the move byte instruction in x86 assembly. It performs an 8-bit data transfer between a source and a destination, making it the byte-sized member of the MOV family (the others being movw, movl, movq for 16, 32, and 64 bits respectively). In practice, movb copies a single byte from a source operand to a destination operand.

Operands can be 8-bit registers or memory locations. In 64-bit mode, the 8-bit registers include al, bl, cl, dl (and the corresponding high-byte aliases such as ah, bh, etc.). The destination can be an 8-bit register or an 8-bit memory location. When the destination is a full-width register (e.g., ax, eax, rax), the instruction writes only the low 8 bits of that register, leaving the upper bits unchanged unless a later instruction modifies them. This means movb does not automatically zero-extend or sign-extend to the full register width; if full-register value is required, additional instructions such as movzx or a prior zeroing step are needed.

Flags are unaffected by movb; the instruction does not modify the processor’s status flags.

Common usage includes copying a byte from memory to a register, transferring a byte between registers, or writing a single byte to memory. In AT&T syntax the mnemonic is movb with the source and destination operands written as movb source, destination; in Intel syntax the form is mov r/m8, r8 or mov r8, r/m8, reflecting the same byte-sized operation.

See also: mov (the generic form), movw/movl/movq (larger sizes), movzx/movsx (zero- or sign-extension).