8085 Microprocessor Register Usage

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Questions and Answers

What is the primary function of the PUSH instruction?

  • To store the contents of a register pair onto the stack. (correct)
  • To load data into a register pair from memory.
  • To retrieve data from the stack.
  • To increment the Stack Pointer (SP).

How does the PUSH instruction affect the Stack Pointer (SP)?

  • It decrements the SP by 2. (correct)
  • It does not affect the SP.
  • It increments the SP by 2.
  • It resets the SP to 0.

What is the main purpose of the POP instruction?

  • To retrieve data from the stack into a register pair. (correct)
  • To store data from a register pair onto the stack.
  • To decrement the Stack Pointer (SP).
  • To clear the contents of the stack.

If SP = 3FFEH and POP H is executed and memory location 3FFEH contains 12H and 3FFFH contains 34H, what will be the new value of SP and register H and L?

<p>SP = 4000H, H = 12H, L = 34H (C)</p> Signup and view all the answers

What is the primary function of the Program Counter (PC) in the 8085 microprocessor?

<p>To hold the address of the next instruction to be fetched and executed. (B)</p> Signup and view all the answers

What operation does the Stack Pointer (SP) perform when data is pushed onto the stack?

<p>Decrements to point to the next available location (D)</p> Signup and view all the answers

During a CALL instruction, what value is typically pushed onto the stack?

<p>The address of the instruction following the <code>CALL</code> instruction (A)</p> Signup and view all the answers

What happens to the Stack Pointer (SP) after a RET instruction is executed?

<p>It is incremented by 2. (A)</p> Signup and view all the answers

If the PC is at 2000H and the SP is at 4000H before a CALL 3000H instruction, what will be the SP value immediately after the CALL instruction?

<p>3FFE H (B)</p> Signup and view all the answers

A RET instruction is executed, and the stack contains 2001H at locations 3FFFH and 3FFEH. If SP = 3FFEH and PC = 3000H prior to the RET, what will be the PC value after RET is executed?

<p>2001H (B)</p> Signup and view all the answers

What is the behavior of Program Counter (PC) after fetching an instruction?

<p>It is incremented to fetch next instruction. (A)</p> Signup and view all the answers

Flashcards

PUSH Instruction

The PUSH instruction copies the contents of a specified register pair onto the stack.

PUSH Instruction - Effect on SP

The PUSH instruction decreases the stack pointer (SP) value by 2. Each byte of the register pair is stored in consecutive memory locations pointed to by the SP.

POP Instruction

The POP instruction retrieves the top two bytes from the stack and places them into a specified register pair.

POP Instruction - Effect on SP

The POP instruction increases the stack pointer (SP) value by 2. Each byte is retrieved from the memory locations pointed to by the SP.

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PUSH/POP - Application

The PUSH and POP operations are used for temporary data storage, transferring data between subroutines, and managing function calls.

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Program Counter (PC)

A 16-bit register that holds the memory address of the next instruction to be executed. It automatically increments after each instruction is fetched to sequentially move through the program.

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What happens to the PC during a CALL instruction?

The PC is updated with the address of the subroutine when a CALL instruction is executed.

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What happens to the PC during a RET instruction?

The PC is updated with the return address stored on the stack when a RET instruction is executed.

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Stack Pointer (SP)

A 16-bit register that points to the top of the stack in memory. The stack is used for temporary storage during subroutine calls, interrupts, and operations like PUSH and POP.

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What happens to the SP when data is pushed onto the stack?

The SP is decremented by 2 when data is pushed onto the stack.

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What happens to the SP when data is popped off the stack?

The SP is incremented by 2 when data is popped off the stack.

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How does a CALL instruction affect the PC and SP?

The address of the instruction following the CALL instruction is saved to the stack, and the PC is loaded with the address of the subroutine.

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How does a RET instruction affect the PC and SP?

The return address is retrieved from the stack and loaded back into the PC. The SP is incremented to reflect the removal of the return address.

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Study Notes

8085 Microprocessor Register Usage

  • Program Counter (PC): A 16-bit register holding the address of the next instruction.
  • PC Operation: Increments automatically after fetching an instruction, unless a branch, CALL, or interrupt occurs.
  • PC Role in Instructions:
    • During CALL, the next instruction's address is pushed to the stack, and the PC is set to the subroutine's address.
    • During RET, the saved address is popped from the stack and loaded into the PC.

Stack Pointer (SP)

  • SP Purpose: A 16-bit register pointing to the top of the stack in memory, used for temporary storage.
  • SP Operation: Decremented for PUSH operations, incremented for POP operations.

CALL Instruction Effects

  • Purpose: Transferred control to a subroutine.
  • Operation: Pushes the return address to the stack; updates the PC with the subroutine's address.
  • Register Effects: PC updated with the subroutine address; SP decremented by 2.

RET Instruction Effects

  • Purpose: Returns control to the main program.
  • Operation: Pops the return address from stack; loads it to the PC.
  • Register Effects: PC updated with the return address; SP incremented by 2.

PUSH Instruction Effects

  • Purpose: Pushes the contents of a register pair onto the stack.
  • Operation: Decrements SP by 2; stores register pair content in memory locations addressed by SP..
  • Register Effects: SP descends; PC unaffected.

POP Instruction Effects

  • Purpose: Pops the top two bytes from the stack to a register pair.
  • Operation: Retrieves data, increments SP by 2.
  • Register Effects: SP ascends; PC remains unchanged.

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