8085 program to subtract two 8-bit numbers with or without borrow

Problem – Write a program to subtract two 8-bit numbers with or without borrow where first number is at 2500 memory address and second number is at 2501 memory address and store the result into 2502 and borrow into 2503 memory address.

Example –

Algorithm –

1. Load 00 in a register C (for borrow)
2. Load two 8-bit number from memory into registers
3. Move one number to accumulator
4. Subtract the second number with accumulator
5. If borrow is not equal to 1, go to step 7
6. Increment register for borrow by 1
7. Store accumulator content in memory
8. Move content of register into accumulator
9. Store content of accumulator in other memory location
10. Stop

Program –

Memory Mnemonics Operands Comment
2000 MVI C, 00 [C] <- 00
2002 LHLD 2500 [H-L] <- [2500]
2005 MOV A, H [A] <- [H]
2006 SUB L [A] <- [A] – [L]
2007 JNC 200B Jump If no borrow
200A INR C [C] <- [C] + 1
200B STA 2502 [A] -> [2502], Result
200E MOV A, C [A] <- [C]
2010 STA 2503 [A] -> [2503], Borrow
2013 HLT Stop

Explanation – Registers A, H, L, C are used for general purpose:

1. MOV is used to transfer the data from memory to accumulator (1 Byte)
2. LHLD is used to load register pair directly using 16-bit address (3 Byte instruction)
3. MVI is used to move data immediately into any of registers (2 Byte)
4. STA is used to store the content of accumulator into memory(3 Byte instruction)
5. INR is used to increase register by 1 (1 Byte instruction)
6. JNC is used to jump if no borrow (3 Byte instruction)
7. SUB is used to subtract two numbers where one number is in accumulator(1 Byte)
8. HLT is used to halt the program