Which statement about the 8085 stack is correct?

It is a LIFO area in RAM, and SP holds the address of the top of the stack

It is a FIFO area in ROM addressed by the program counter

It is a set of internal registers inside the CPU

It is a LIFO area whose top is pointed to by the HL pair

SP = FFFFH, B = 12H, C = 34H. After PUSH B, what is the state?

FFFEH = 12H, FFFDH = 34H, SP = FFFDH

FFFFH = 34H, 0000H = 12H, SP = 0001H

FFFEH = 34H, FFFDH = 12H, SP = FFFDH

FFFEH = 12H, FFFDH = 34H, SP = FFFFH

How does the stack allow a subroutine to return to the right place?

CALL saves the return address on the stack, and RET pops it back into the PC

RET reloads PC from the HL pair

CALL stores the return address in the W and Z registers only

The stack stores the subroutine's code so it can be re-run

2068 Bhadra · 4 marks

Short note: stack memory

Short answer

Stack memory is a LIFO RAM area addressed by the stack pointer (SP) that grows downward. PUSH decrements SP twice and stores a register pair; POP does the reverse. It handles CALL/RET, interrupts, and temporary register saving.

Write a short note on stack memory.

A stack is a block of read/write memory used as a last-in first-out (LIFO) store: the last item pushed is the first one popped. In the 8085 the stack lies in RAM and is addressed by the 16-bit stack pointer (SP), which always holds the address of the top of stack.

Operation

The programmer loads SP at the start, e.g. LXI SP,0FFFFH.
The stack grows downward (towards lower addresses).
PUSH rp: SP ← SP−1, store high byte of the register pair; SP ← SP−1, store low byte.
POP rp: low byte ← [SP], SP ← SP+1; high byte ← [SP], SP ← SP+1.

 PUSH B  (B=12H, C=34H, SP=FFFFH)
 Address   Before   After
 FFFEH      --       12H   (B)
 FFFDH      --       34H   (C)  <- SP = FFFDH

Uses

Saving register contents temporarily (PUSH/POP) when a subroutine needs the registers.
CALL saves the return address on the stack; RET pops it into the PC.
Interrupts and RST instructions store the PC on the stack automatically.
Passing parameters to subroutines.

Points to remember: PUSH and POP work on register pairs (B, D, H, PSW) only; PUSH/POP must be balanced, and the stack must not overlap program or data area. Nested subroutines are possible because the stack stores return addresses in order. Stack depth is limited only by available memory.

✓ Answer checked by Claude Opus 5.5 (high effort)

All Microprocessor chapter 2 questions →