Exploring the Operation of CALL Instruction in 8085 Microprocessor

In the realm of microprocessor architecture, the CALL instruction is utilized to initiate the execution of a subroutine. Subroutines are defined as sequences of instructions designed to perform specific tasks, and they can be invoked from various parts of a program. The CALL instruction enables the seamless execution of these subroutines by temporarily storing the address of the next instruction and redirecting control flow to the subroutine. This article delves into the intricate workings of the CALL instruction in the context of the 8085 microprocessor.

Overview of the CALL Instruction in 8085 Microprocessor

The CALL instruction is a fundamental part of the 8085 microprocessor architecture. Its primary function is to allow the execution of a subroutine, which in turn aids in modular and organized programming. This instruction provides efficient means for code reuse and the structuring of complex tasks into manageable subparts. Understanding the mechanics of the CALL instruction is crucial for proficient microprocessor programming.

Operation of the CALL Instruction

When the CALL instruction is executed, a series of operations are carried out to ensure proper execution and return to the main program. These operations are centered around the use of the stack and the manipulation of the Program Counter (PC) and Stack Pointer (SP).

Stack Mechanism

The 8085 microprocessor employs a stack to temporarily store the return address. The stack operates using a Last-In, First-Out (LIFO) principle, allowing for the preservation of the sequence of subroutine calls.

Operation Flow

The address of the next instruction (i.e., the return address) is pushed onto the stack. This process involves two main steps:

The high-order byte of the return address is pushed onto the stack. The low-order byte of the return address is subsequently pushed onto the stack.

The Stack Pointer (SP) is decremented by 2 to adjust for the two bytes that have been pushed onto the stack.

The Program Counter (PC) is loaded with the address specified in the CALL instruction. This effectively transfers control to the subroutine.

Return from Subroutine

Upon completion of the subroutine, the RET instruction is typically executed to return the control flow to the original calling point. The RET instruction performs the following operations:

The low-order byte of the return address is popped from the stack first. The high-order byte of the return address is popped subsequent to the low-order byte. The Program Counter (PC) is set to the newly retrieved return address, allowing the main program to continue execution from the point following the original CALL instruction.

Example of CALL Instruction Usage

A simple example of the CALL instruction in action demonstrates its practical application and the sequence of operations involved. Consider the following program snippet:

START:   MVI A 05H            ; Load 5 into accumulator         CALL SUBR            ; Call subroutine SUBR         MVI A 06H            ; Assume further operations         HLT                  ; Halt the programSUBR:    INX H                ; Increment the HL register pair         ; Other subroutine instructions         RET                  ; Return from subroutine

In this example, when the program counter reaches the CALL instruction at the address of START, control is transferred to the subroutine SUBR. After the subroutine SUBR is executed and the RET instruction is encountered, control is returned to the main program, resuming execution right after the CALL instruction.

Summary of CALL Instruction in 8085 Microprocessor

Opcode: The opcode for the CALL instruction is CD, followed by the address of the subroutine.

Stack Usage: The stack plays a critical role in storing the return address, ensuring the program can resume correctly after subroutine execution.

Control Transfer: The CALL instruction changes the flow of control to the specified address, while the RET instruction returns control to the original calling point.

This mechanism of CALL and RET instructions is essential for organizing and managing the execution flow in 8085 microprocessor-based systems. By leveraging these instructions, developers can create modular and efficient code structures, making complex programming tasks more manageable.