Final answer:
JAL and JALR are instructions used in RISC architectures. JAL computes the target by adding an offset to the PC and JALR adds a register and an immediate value. Both save the return address (PC + 4) in a designated register.
Step-by-step explanation:
The JAL (Jump and Link) and JALR (Jump and Link Register) instructions are fundamental components of RISC (Reduced Instruction Set Computing) architectures such as MIPS and RISC-V. These instructions are used for function calls and returns in assembly language programming.
Execution of JAL Instruction
The JAL instruction causes the program to jump to a specified target address and simultaneously saves the address of the next instruction by placing the program counter (PC) plus four (PC + 4) into a designated register. The target address for JAL is calculated by adding a signed immediate value to the PC. This immediate value is often a compile-time constant representing the offset from the current position to the target.
Execution of JALR Instruction
Unlike JAL, the JALR instruction computes the target address by adding a signed immediate value to the value in a specific register, usually a base address. This allows for more dynamic branching, as the base address can change during program execution. After calculating the target address, the instruction jumps to this address and stores PC + 4 in the specified register, thus facilitating the return from a subroutine call.
Both instructions 'save' PC4 by writing it into the designated link register (often register x1 in RISC-V or $ra in MIPS). This saved address is then used to return control to the calling code after the subroutine has completed its execution.