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 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,
BX --> BH,BL 
CX --> CH,CL 
DX --> DH,DL 
 
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