Intel 8086

During the age of the Intel 8086, a register was a physical storage on the processor. Registers were 16 bits in size and you would move memory across a bus into the register, execute some instructions, and then move that data back into memory.

Instruction Decoding

Instruction decoding is the translation of an instruction stream to the electro-mechanical operations that the processor needs to perform. These instructions are represented as a mnemonic, human friendly translation of the operation. For example, MOV is an instruction that copies data to a register.

MOV AX BX

In this particular example, we are copying data from register BX into register AX. This instruction is decoded into two bytes

|1|0|0|0|1|1|D W|mod| REG | RM |
|    8 bit      |    8 bit     |

• 100011 - The MOV instruction
• DW - Size of destination register; 8 bit or 16 bit
• MOD - Memory operation or a register operation
• REG - Register
• RM - Register or memory

Register Naming

• AX is targeting all 16 bits
• AH is targeting the 8 high bits
• AL is targeting the 8 low bits