X86 Instruction Prefix

 X86 instructions support data size of 8, 16, 32 and 64 bits.  The default size (either 16 or 32 which is considered more commonly used) is set in bit in code segment register. To override the default, we be can use an instruction prefix. 

There are 3 other prefix originated from the 8086 that modify the behaviour of instructions. They are used see to

1. Overdue the default segment register

2. Lock the bus to support synchronzation

3. Repay the instruction until ECR counts for ti 0.  This prefix is commonly used to move a number of bytes  ironically, this method is slower comparing to software routine using load (to register) and store (to memory). If we use floating point register, the performance is even higher in this way.