* **S**-bit : An 8-bit 2's complement number. It can be extended to a 16-bit 2’s complement number depending on the W-bit by making all of the bits in the higher-order byte equal the most significant bit in the low order byte. This is known as sign extension.
| 1 | 1 | 16bit operation with a sign extended 8bit immediate operand
* **V**-bit : V-bit decides the number of shifts for rotate and shift instructions. If V = 0, then count = 1; if V = 1, the count is in CL register. For example, if V = 1 and CL = 2 then shift or rotate instruction shifts or rotates 2-bits
* **Z**-bit : Used as a compare bit with the zero flag in conditional repeat and loop instructions. ex branch if zero is set or clear.
| Register ID / REG | Register Name |
|:-------------------:|:-------------:|
| 0 0 0 | AL AX |
| 0 0 1 | CL CX |
| 0 1 0 | DL DX |
| 0 1 1 | BL BX |
| 1 0 0 | AH SP |
| 1 0 1 | CH BP |
| 1 1 0 | DH SI |
| 1 1 1 | BH DI |
The second byte of the instruction usually identifies the instruction's operands. The **MOD** (mode) field weather on of the operands is in memory or if both are registers. In some instructions like the immediate-to-memory type the **REG** field is used as an extension of the opcode. The encoding of **R/M** depends on how MOD is set. if MOD=11 (register-register mode) then **R/M** specifies the second Register using the Register ID. otherwise it specifies how the effective address in memory is calculated
|R/M | Memory Mode with no displacement [ 0 0 ] | Memory mode with 8 bit displacement [ 0 1 ] | Memory Mode with 16 bit displacement [ 1 0 ] | Register Mode [ 1 1 ] W = 0| Register Mode [ 1 1 ] W = 1 |
