8 bit data subtraction | 8085 Lab Program

 Section: Arithmetic & Logic Operations

Aim: To subtract two 8 bit numbers stored at consecutive memory locations and also to verify the result.

Apparatus Required: 8085 microprocessor kit, key board.

Algorithm:

1. Initialize memory pointer to data location.
2. Get the first number from memory in accumulator.
3. Get the second number and subtract from the accumulator.
4. If the result yields a borrow, the content of the acc. is complemented and 01H is added to it (2’s complement). A register is cleared and the content of that reg. is incremented in case there is a borrow. If there is no borrow the content of the acc. is directly taken as the result.
5. Store the answer at next memory location.

Program:

Address	Label	Mnemonics	Operand	Comment
4100	START	MVI	C, 00	Clear C reg.
4102		LXI	H, 4200	Initialize HL reg. to 4500
4105		MOV	A, M	Transfer first data to accumulator
4106		INX	H	Increment HL reg. to point next mem. Location.
4107		SUB	M	Subtract first number from acc. Content.
4108		JNC	L1	Jump to location if result does not yield borrow.
410B		INR	C	Increment C reg.
410C		CMA		Complement the Acc. Content
410D		ADI	01H	Add 01H to content of acc.
410F	L1	INX	H	Increment HL reg. to point next mem. Location.
4110		MOV	M, A	Transfer the result from acc. to memory.
4111		INX	H	Increment HL reg. to point next mem. Location.
4112		MOV	M, C	Move carry to mem.
4113		HLT		Stop the program

Observation:

• Input:
  • Address 4200: 04
  • Address 4201: 01
• Output:
  • Address 4202: 03
  • Address 4203: 00

Result: Thus the 8-bit numbers stored at 4200 & 4201 are subtracted and the result is stored at 4202 & 4203.

Flowchart:

               +-------------------+
               |       START       |
               +---------+---------+
                         |
                         v
               +-------------------+
               |    [C] <- 00H     |
               +---------+---------+
                         |
                         v
               +-------------------+
               |   [HL] <- 4200H   |
               +---------+---------+
                         |
                         v
               +-------------------+
               |    [A] <- [M]     |
               +---------+---------+
                         |
                         v
               +-------------------+
               |  [HL] <- [HL] + 1 |
               +---------+---------+
                         |
                         v
               +-------------------+
               |  [A] <- [A] - [M] |
               +---------+---------+
                         |
                         v
                       /   \
                     /  Is   \
                   /  there a  \
                  <   Borrow?  >
                   \          /
                     \      /
                       \  /
                        |
            +-----------+-----------+
            |                       |
           YES                      NO
            |                       |
            v                       |
  +-------------------+             |
  |  Complement [A]   |             |
  |  Add 01H to [A]   |             |
  +---------+---------+             |
            |                       |
            v                       |
  +-------------------+             |
  |   [C] <- [C] + 1  |             |
  +---------+---------+             |
            |                       |
            +-----------+-----------+
                        |
                        v
              +-------------------+
              |  [HL] <- [HL] + 1 |
              +---------+---------+
                        |
                        v
              +-------------------+
              |    [M] <- [A]     |
              +---------+---------+
                        |
                        v
              +-------------------+
              |  [HL] <- [HL] + 1 |
              +---------+---------+
                        |
                        v
              +-------------------+
              |    [M] <- [C]     |
              +---------+---------+
                        |
                        v
              +-------------------+
              |       STOP        |
              +-------------------+

Viva Questions:

1. What is meant by ADI instruction?
2. What is an instruction?
3. What is Mnemonic?
4. What is the purpose of CMA instruction?
5. What is the function of stack pointer?
6. Why ADI 01H is used in two’s complement of an 8-bit number?
7. How many memory locations can be addressed by a microprocessor with 14 address lines?

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