Intel 16-bit Registers

8086 Architecture

• The 8086 has two parts, the Bus Interface Unit (BIU) and the

Execution Unit (EU).

• The BIU fetches instructions, reads and writes data, and computes the

20-bit address.

• The EU decodes and executes the instructions using the 16-bit ALU.

• The BIU contains the following registers:

IP - the Instruction Pointer

CS - the Code Segment Register

DS - the Data Segment Register

SS - the Stack Segment Register

ES - the Extra Segment Register

The BIU fetches instructions using the CS and IP, written CS:IP, to contract the 20-bit address.

Data is fetched using a segment register (usually the DS)

and an effective address (EA) computed by the EU depending on the

addressing mode.

The EU contains the following 16-bit registers:

AX - the Accumulator

BX - the Base Register

CX - the Count Register

DX - the Data Register

SP - the Stack Pointer \ defaults to stack segment

BP - the Base Pointer /

SI - the Source Index Register

DI - the Destination Register

These are referred to as general-purpose registers, although, as seen by

their names, they often have a special-purpose use for some instructions.

The AX, BX, CX, and DX registers can be considers as two 8-bit registers, a

High byte and a Low byte.

This allows byte operations and compatibility with

the previous generation of 8-bit processors, the 8080 and 8085. 8085 source

code could be translated in 8086 code and assembled.

The 8-bit registers are:

AX --> AH,AL

BX --> BH,BL

CX --> CH,CL

DX --> DH,DL