Which pair of statements about PC and SP in the 8085 is correct?

PC holds the address of the next instruction to fetch; SP holds the top-of-stack address and is decremented on PUSH

PC holds the current instruction's opcode; SP is incremented on PUSH

PC is an 8-bit register; SP is a 16-bit register that grows upward

PC holds the address of the top of the stack; SP holds the next instruction address

CALL 4000H is at address 2000H and SP = 3FFFH. What are PC and SP after it executes, and what is stored at 3FFEH and 3FFDH?

PC = 4000H, SP = 3FFDH; 3FFEH = 20H (PCH), 3FFDH = 03H (PCL)

PC = 4000H, SP = 4001H; 3FFEH = 20H, 3FFDH = 00H

PC = 2003H, SP = 3FFDH; 3FFEH = 40H, 3FFDH = 00H

PC = 4000H, SP = 3FFDH; 3FFEH = 03H (PCL), 3FFDH = 20H (PCH)

After the CALL above, the subroutine executes RET with SP = 3FFDH. What happens?

PCL <- M[3FFDH] = 03H, PCH <- M[3FFEH] = 20H, SP = 3FFFH, PC = 2003H

PCH <- M[3FFDH], PCL <- M[3FFEH], SP = 3FFBH

PC <- 4000H and SP is unchanged

PCL <- M[3FFDH], PCH <- M[3FFEH], SP = 3FFEH

How do PUSH B and POP D differ from CALL and RET in their effect on the PC?

PUSH/POP only increment PC normally (1-byte opcode fetch); CALL/RET load PC with a new value

PUSH/POP load PC from the stack, just like RET

PUSH/POP do not change SP; only CALL/RET change SP

PUSH increments PC by 2 and POP decrements PC by 2

2074 Bhadra · 4+4 marks

PC and SP in CALL, RET, PUSH, POP

Short answer

PC holds the address of the next instruction; SP points to the top of stack. CALL pushes the return address and loads the subroutine address, decreasing SP by 2; RET pops it, increasing SP by 2. PUSH decrements SP and stores a register pair; POP loads a pair and increments SP.

What is the use of Program Counter and Stack Pointer registers of 8085 microprocessor? How these registers get affected during CALL, RET, PUSH and POP instructions explain with suitable examples.

Program Counter (PC)

A 16-bit register that holds the address of the next instruction to be fetched. It is incremented automatically after each byte fetched, so instructions execute in sequence. Jump, call and return load a new address into it.

Stack Pointer (SP)

A 16-bit register that holds the address of the top of the stack, a LIFO region of RAM used to save return addresses and register contents. In the 8085 the stack grows downward: SP is decremented on a push and incremented on a pop. It is initialised with LXI SP,addr.

Effect on PC and SP

CALL addr (3 bytes, 18T): Suppose CALL 4000H is at 2000H, SP = 3FFFH.

Before: PC = 2000H   SP = 3FFFH
Fetch bytes:  PC = 2003H (next instruction address)
SP-1 = 3FFEH  <- PCH = 20H
SP-2 = 3FFDH  <- PCL = 03H
After:  SP = 3FFDH     PC = 4000H

The return address is pushed on the stack, SP decreases by 2, and PC gets the subroutine address.

RET (1 byte, 10T):

PCL <- M[SP] = 03H ;  PCH <- M[SP+1] = 20H
SP  <- SP + 2 = 3FFFH
After:  PC = 2003H

SP increases by 2 and PC returns to the instruction after the CALL.

PUSH B (stores BC; 12T): PC increases only by 1 (fetching the opcode); SP changes.

Before: B = 12H, C = 34H, SP = 3FFFH
M[SP-1] = 3FFEH <- B = 12H
M[SP-2] = 3FFDH <- C = 34H
After:  SP = 3FFDH

POP D (loads DE; 10T):

E <- M[SP] ;  D <- M[SP+1] ;  SP <- SP + 2

If M[3FFDH] = 34H and M[3FFEH] = 12H: E = 34H, D = 12H, SP = 3FFFH.

PUSH/POP do not change the PC except for its normal increment, while CALL and RET load PC with a new value as well as using the stack.

✓ Answer checked by Claude Opus 5.5 (high effort)

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PC and SP in CALL, RET, PUSH, POP

Program Counter (PC)

Stack Pointer (SP)

Effect on PC and SP

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