Segment Override prefix
To override the association for one instruction of one of the registers BX, BP, SI or DI, we use the segment override prefix. For example “mov ax, [cs:bx]” associates BX with CS for this one instruction. The processor places a special byte before the instruction called a prefix.No prefix is needed or placed for default association. Opcode has not changed, but the prefix byte has modified the default association to association with the desired segment register for this one instruction.