JUMP vs. CALL Instructions in Computer Architecture: Control Flow Explained
Understand the differences between JUMP and CALL instructions in computer programming. This guide clarifies their functionalities, impact on program flow, and the use of the stack for managing subroutine returns with CALL instructions.
JUMP vs. CALL Instructions in Computer Architecture
Jump Instructions
Jump instructions alter a program's execution sequence by changing the program counter (PC) to a new address. The PC holds the address of the next instruction to be executed. A jump instruction causes the program to jump to a new location in the program. The jump instruction itself doesn't save any information about where to return after the jump.
CALL Instructions
CALL instructions also change the program counter, but they're used to call subroutines—self-contained blocks of code designed to perform specific tasks. The CALL instruction saves the return address (the address of the instruction following the CALL) onto a stack before jumping to the subroutine. After the subroutine finishes, a RET (return) instruction retrieves the return address from the stack, resuming execution where it left off.
Key Differences: JUMP vs. CALL
| Feature | JUMP Instruction | CALL Instruction |
|---|---|---|
| Execution | Simple branch to a new location. | Branch to a subroutine; stores return address on stack; uses RET to return. |
| Stack Pointer Initialization | Not required | Required |
| Stack Pointer Change | No change | Decremented (usually by 2 bytes to store the return address) |
| Return Address | Not stored; no return | Stored on the stack; RET instruction used to return. |
| Addressing Modes | Typically supports immediate, direct, and relative addressing modes | Typically supports immediate, direct, register, and relative addressing modes. |
| Program Counter Modification | Permanently changes the PC | Temporarily changes the PC; return address allows resuming original sequence |
| Jump Target | Can be any instruction in the program. | Typically points to the start of a subroutine. |
| Types of Jumps | Long, short, absolute | Long, absolute (Short jumps are not typically used with CALL) |
| Execution Time (T-states) | 10 T-states | 18 T-states |
| Execution Time (machine cycles) | 3 | 5 |
| Jump Type | Conditional or unconditional | Unconditional |
Conclusion
JUMP and CALL are fundamental control transfer instructions. JUMP provides simple branching, while CALL enables structured subroutine calls, managing return addresses through the stack. CALL instructions are usually slower than JUMP instructions due to the stack operations involved.