Arithmetic and Logic (ALU) Operations

CSC-167 · Semester II · Microprocessor

beginner25 minAvailable

8085 ALU Operations

8-bit arithmetic and logic with flag register output.

Operand A (accumulator)Operand B

Execution result

1A + 0F = 00

binary: 00011010 ADD 00001111 = 00000000

Flag register

S = sign (MSB of result), Z = zero, AC = auxiliary carry from low nibble, P = parity (even = 1), CY = carry/borrow out of bit 7. The accumulator holds the result in A.

Accumulator (A)00

Lab brief

Objective

Write and step through 8085 programs that perform addition, subtraction, AND, OR and XOR, and read the resulting flag states.

Theory & concepts

The 8085 ALU performs 8-bit arithmetic and logic on the accumulator. ADD and SUB operate with an implicit operand in A; the result replaces A and the flag register records the outcome. Logic operations AND, OR and XOR work bit by bit, and every logic operation clears the carry flag.

Rendering diagram…

ALU operations on the 8085 accumulator

Instruction	Action	Flags set
ADD r	A = A + r	S Z AC P CY
SUB r	A = A - r	S Z AC P CY
ANA r	A = A AND r	S Z P (CY=0)
ORA r	A = A OR r	S Z P (CY=0)
XRA r	A = A XOR r	S Z P (CY=0)

MVI A, 5AH
MVI B, 3CH
ADD B   ; A = 96H, CY = 0
SUB B   ; A = 5AH, CY = 0
ANA B   ; A = 18H
ORA B   ; A = 7EH
XRA B   ; A = 66H

Prerequisites

Lesson: Registers and flag organization
Lesson: Data transfer and arithmetic instructions

Instructions

1Open the 8085 simulator and set the initial registers to A = 5AH, B = 3CH.
2Assemble and run MVI A,5AH; MVI B,3CH; ADD B.
3Record A and every flag after the ADD, then repeat with SUB, ANA, ORA and XRA.
4Redo the run in single-step mode and note which T-states update the accumulator.
5Try the same program with A = 80H, B = 80H and explain the carry behaviour.

Tasks

Fill the result table
For each operation, record the accumulator value and S, Z, AC, P, CY.
Verify a known answer
Confirm ADD with 25H + 4FH gives 74H and no carry.
Explain the flags
In your own words, explain why ANA clears the carry flag.

Self-check

All five operation results match the hand-computed values.
The flag states match the flag rules for each operation.
The student can explain when CY is set by an 8-bit addition.

Environment

8085 assemblerregister and flag viewsingle-step executionmemory and I/O panel