Practical No. 9
Title: To Study the General Purpose Registers of the 8086 Microprocessor

Aim
To understand the structure, function, and purpose of the General Purpose Registers within the 8086 microprocessor architecture.

Introduction
The 8086 microprocessor, a foundational component in computing history, processes data using internal high-speed storage units called registers. Registers temporarily hold data, instructions, and memory addresses necessary for CPU operations.

Among these, the General Purpose Registers (GPRs) are directly accessible to programmers and are used for arithmetic, logical, and data transfer operations. Each 16-bit register can be split into two 8-bit registers, providing flexibility for manipulating both 16-bit and 8-bit data.

While the 8085 microprocessor (an 8-bit processor) had general-purpose registers B, C, D, E, H, L, the 8086 (16-bit) introduced AX, BX, CX, and DX to match its architecture. This practical focuses on concepts from Chapter 4.2.2: 8086 Internal Architecture.

Procedure / Example

General Purpose Registers of 8086:

• AX (Accumulator Register): Used for arithmetic operations (add, subtract, multiply, divide) and I/O operations. Can be accessed as AH (high 8 bits) and AL (low 8 bits).
• BX (Base Register): Used for memory addressing. Can hold the starting address of a data segment. Can also be used for general-purpose calculations.
• CX (Count Register): Primarily used as a counter for loops. The LOOP instruction decrements CX automatically. Can also be used for general calculations.
• DX (Data Register): Used for I/O port addressing and higher-order bits in multiplication/division operations.

Each 16-bit register can be accessed as two separate 8-bit registers:

• AX → AH (high), AL (low)
• BX → BH, BL
• CX → CH, CL
• DX → DH, DL

Example: Using General Purpose Registers in 8086 Assembly

; Move the value 25 into AL

MOV AL, 25

 

; Move the value 10 into BL

MOV BL, 10

 

; Add BL to AL, result stored in AL

ADD AL, BL  ; AL now holds 35

 

; Move 16-bit value 5000H into AX

MOV AX, 5000H

 

; Move 16-bit value 2000H into BX

MOV BX, 2000H

 

; Add BX to AX, result stored in AX

ADD AX, BX  ; AX now holds 7000H

 

; Use CX as a loop counter

MOV CX, 0005H

LOOP_START:

    ; … instructions inside loop …

    DEC CX

    JNZ LOOP_START

Exercises

1.     Register Identification: For each register pair, state its full name and primary purpose.

Register	Full Name	Primary Purpose
AX
BX
CX
DX
AL
BH
CL
DH

2.     Scenario Application: Decide the most suitable 8086 register for each task and justify.

• Performing a 16-bit multiplication where one operand is in memory.
• Storing the starting address of a block of data in memory.
• Controlling the number of repetitions in a program loop.
• Holding the higher 16 bits of a 32-bit division dividend.

Result / Conclusion
This practical provided a comprehensive overview of the General Purpose Registers of the 8086 microprocessor. Understanding their 16-bit structure, 8-bit accessibility, and specialized/general-purpose roles in arithmetic, memory addressing, loop control, and I/O operations is essential for effective assembly programming and grasping the 8086 architecture.