8085 program to exchange content of HL register pair with DE register pair

Problem – Write an assembly language program in 8085 microprocessor to exchange content of HL register pair with DE register pair using PUSH and POP instructions. Example – Assumption – Content is already present in HL and DE register. Algorithm –

1. Initialize stack pointer (SP) by 3FFF.
2. Push the content of H and L register into the stack. Decrements SP by 2.
3. Push the content of D and E register into the stack. Decrements SP by 2.
4. Pop the upper two bytes from top of stack and place it in HL register. Increment SP by 2.
5. Pop the remaining two bytes from top of stack and place it in DE register. Increment SP by 2.

Program –

MEMORY ADDRESS	MNEMONICS	COMMENT
2000	LXI SP 3FFF	SP <- 3FFF
2003	PUSH H	SP <- SP – 1, M[SP] <- H, SP <- SP – 1, M[SP] <- L
2004	PUSH D	SP <- SP – 1, M[SP] <- D, SP <- SP – 1, M[SP] <- E
2005	POP H	L <- M[SP], SP <- SP + 1, H <- M[SP], SP <- SP + 1
2006	POP D	E <- M[SP], SP <- SP + 1, D <- M[SP], SP <- SP + 1
2007	HLT	ENDT

Explanation – Registers used H, L, D, E:

1. LXI SP 3FFF: initialize SP by 3FFF.
2. PUSH H: push the content of H and L register into the stack and decrements stack pointer by 2.
3. PUSH D: push the content of D and E register into the stack and decrements stack pointer by 2.
4. POP H: pop the upper two bytes from top of stack and place it in HL register pair and increment SP by 2.
5. POP D: pop the upper two bytes from top of stack and place it in DE register pair and increment SP by 2.
6. HLT: stops executing the program and halts any further execution.

Advantages:

• The program is simple and easy to understand, making it useful for teaching purposes.
  
• It is a useful utility program for exchanging the contents of two register pairs.
  
• It can be customized to exchange other register pairs as well, by modifying the instructions to load and store the register values.
  

Disadvantages:

• The program is not optimized for speed, as it uses multiple instructions to move each byte of data.
  
• It can only exchange the contents of two specific register pairs, and cannot be used to exchange the contents of individual registers.
  
• The program does not check for errors or boundary conditions, such as when the register contents exceed the valid range.