Experiment-3: Write a program to perform: i. Subtraction of two 8 bit numbers without borrows. ii. Subtraction of two 8 bit numbers with borrows.

Objective:

To understand the assembly program

To draw flowchart for subtraction of two numbers

To develop the assembly program for subtraction

To analyse the program and interpret errors and result

Requirement:

Operating System – Windows, Linux

• 8085 Simulator

Example-1. Subtraction of two 8 bit numbers without borrows.

Algorithmic Steps:

1. Input two numbers say one in Accumulator and another in register B

2. Subtract B from A ; A= A-B

3. Check carry ( if A <B, carry flag will be set and difference will be in 2’s complement, Carry=’1’ means difference is -ve. Do correction (take 2’s complement to find magnitude of -ve number)

4. Save the result in variable ‘x’ and Borrow if any in Borrow variable

Program:

OUTPUT

Example-2: 16-bit subtraction (uses SUB and SBB )

ALGORITHM

Data variable

Algorithm:

• Load source (X) address in HL pair
• Load source (Y) address in DE pair
• Load Accumulator (A) from memory i.e [HL]
• Swap HL and DE pair HL ßàDE
• Load register B from memory [HL]
• Subtract A-B
• Save result in register C
• Increment HL
• Swap HL with DE
• Increment HL
• Load accumulator from memory address 4201 [HL]
• Swap HL and DE
• Load register B from memory
• Subtract with borrow
• Increment HL to save the result
• Save register C (least significant result) at 4205 [HL] <-  C
• Increment HL now 4206
• Save most significant digit of result in memory 4206 [HL] <- A
• stop

Program in assembly with OPCODEs

Result:

Viva Questions:

Que-1: How is subtraction performed in computer.

Ans:

Subtraction is performed by using 2’s complement arithmetic.

Que-2: What are the commands available in 8085 microprocessor for performing subtraction

Ans:

1. SUB    –  Subtraction
2. SBB   – subtraction with borrow