## ISA
Instructions vary from 1 to 6 bytes.

### Instructions format
|       6bit       | 1bit      | 1bit  | 2bit | 3bit | 3bit |
| ---------------- | --------- | ----  | ---- | ---- | ---- |
|      Opcode      | D bit     | W bit | MOD  | REG  | R/M  |

* **D**-bit : The register specified in REG field is a source register (D = 0) or destination register (D =1).

* **W**-bit : Specifies whether the instruction operates on bytes (W = 0) or words (W = 1).

On some instructions:

* **S**-bit : An 8-bit 2's complement number. It can be extended to a 16-bit 2’s complement number internally depending on the W-bit 

| S   | W   |                      Operation                         |
| --- | --- |  ----------------------------------------------------- |
| 0   | 0   | 8bit operation                                         |
| 0   | 1   | 16bit operation with 16bit immediate operand           |
| 1   | 0   |  invalid?                                              |
| 1   | 1   | 8bit immediate operand extended to 16 signed internally|

* **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.

No instruction has parts of its opcode past the first 2 bytes I.e. all bytes after the first two are additional data bytes

The second byte of the instruction usually identifies the instruction's operands. The **MOD** (mode) field weather or not 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 function of **R/M** depends on how MOD. if MOD=11 (register-register mode) then **R/M** specifies the second Register, otherwise it specifies how the effective address in memory is calculated

|R/M  | Memory indirect with no displacement [ 0 0 ] | Memory indirect with 8 bit displacement [ 0 1 ] | Memory indirect with 16 bit displacement [ 1 0 ] | Register Mode [ 1 1 ]  W = 0| Register Mode [ 1 1 ] W = 1 |
|---- | ---------------------------------------- | ------------------------------------------- | -------------------------------------------- | --------------------------- | --------------------------- | 
|000  | [BX] + [SI]                              | [BX] + [SI] + d8                            | [BX] + [SI] + d16                            |    AL                       | AX                          |
|001  | [BX] + [DI]                              | [BX] + [DI] + d8                            | [BX] + [DI] + d16                            |    CL                       | CX                          |
|010  | [BP] + [SI]                              | [BP] + [SI] + d8                            | [BP] + [SI] + d16                            |    DL                       | DX                          |
|011  | [BP] + [DI]                              | [BP] + [DI] + d8                            | [BP] + [DI] + d16                            |    BL                       | BX                          |
|100  | [SI]                                     | [SI] + d8                                   | [SI] + d16                                   |    AH                       | SP                          |
|101  | [DI]                                     | [DI] + d8                                   | [DI] + d16                                   |    CH                       | BP                          |
|110  | d16 (direct)                             | [BP] + d8                                   | [BP] + d16                                   |    DH                       | SI                          |
|111  | [BX]                                     | [BX] + d8                                   | [BX] + d16                                   |    BH                       | DI                          |

Example instructions:

|  Bytecode  |   AT&T Syntax   |                         meaning                            |
| ---------- | --------------- | ---------------------------------------------------------- |
|81 c0 aa 55 | add $0x55aa,%ax | add 0x55aa to register ax                                  |
|03 06 aa 55 | add 0x55aa,%ax  | add the contents of memory locaton 0x55aa to register ax   |
|fe c0       | inc %al         | increment register al                                      |
|ff c0       | inc %ax         | increment register ax                                      |
|40          | inc %ax         | increment register ax                                      |


### Flags
Flag register:
|.. |.. |.. |.. | O | D | I | T | S | Z |.. | A |.. | P |.. | C |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

* C - Carry flag : carry out or borrow into the high order bit (8bit/16bit)

* P - Parity flag : set if result has even parity

* A - Auxiliary flag : carry out from the low nibble to the high nibble or an equiv borrow. Used by decimal arithmetic instructions

* Z - Zero flag : Set when result of Operation is zero

* S - Sign flag : set if the high order bit of the result is 1. ie the sign of the result

* T - Trap flag : Set the CPU into single step mode where it generates an interrupt after each instruction

* I - Interrupt flag : 0: interrupts are masked

* D - Direction flag : 1: string instructions decrement 0: they increment

* O - Overflow flag : set on arithmetic overflow