What is the main difference between CALL/RET and PUSH/POP?

CALL/RET change program flow and save/restore the return address; PUSH/POP only save/restore register pair data

CALL/RET use the stack but PUSH/POP use general purpose registers

PUSH/POP change the PC, but CALL/RET do not

CALL/RET increase SP and PUSH/POP decrease it

What does PUSH PSW save on the stack?

The accumulator and the flag register

A subroutine does PUSH B at the start but forgets POP B before RET. What happens?

RET pops the saved BC contents into PC, so the program returns to a wrong address

Nothing; RET always finds the return address, whatever was pushed

BC is restored automatically by RET

2069 Bhadra · 4+4 marks

Compare CALL/RET with PUSH/POP

Short answer

CALL/RET transfer control by pushing/popping the return address to/from the stack; PUSH/POP only save/restore register pair data. CALL is 3 bytes/18 T-states, RET 1 byte/10 T-states; PUSH and POP are 1 byte, 12 and 10 T-states.

Compare CALL and RET instructions with PUSH and POP instructions for 8085. Illustrate their usages with appropriate examples.

Both pairs use the stack (LIFO area in RAM pointed to by SP), but CALL/RET transfer program control and save/restore the return address, while PUSH/POP only save/restore data.

Point	CALL / RET	PUSH / POP
Purpose	Go to a subroutine and come back	Save and restore register pair contents
Data stored	Return address (PC)	Contents of BC, DE, HL or PSW (A + flags)
Effect on PC	CALL loads PC with the subroutine address; RET loads PC from the stack	PC only advances normally
Instruction length	CALL 3 bytes, RET 1 byte	1 byte each
T-states	CALL 18 (conditional 18/9), RET 10	PUSH 12, POP 10
SP change	CALL: SP-2, RET: SP+2	PUSH: SP-2, POP: SP+2
Who does the saving	Done automatically by CPU	Done by programmer explicitly
Pairing	Every CALL needs a RET	Every PUSH needs a matching POP in reverse order

Illustration

Delay subroutine that must keep the caller's BC:

        LXI SP,27FFH    ; initialise stack
        MVI B,05H
        CALL DELAY      ; return address (next instruction) -> stack
        MOV A,B         ; B is still 05H
        HLT
DELAY:  PUSH B          ; save BC
        LXI B,0FFFFH
LP:     DCX B
        MOV A,B
        ORA C
        JNZ LP
        POP B           ; restore BC
        RET             ; PC <- return address from stack

CALL DELAY: PC (address of MOV A,B) is pushed, SP = 27FDH, PC = address of DELAY.
PUSH B: B and C go to the stack, SP = 27FBH.
POP B: B and C come back, SP = 27FDH.
RET: return address popped to PC, SP = 27FFH, execution continues at MOV A,B.

Note the order: the last item pushed must be the first popped, otherwise RET gets a wrong address.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Compare CALL/RET with PUSH/POP

Illustration

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