How do I call a function from another file in RISC-V?

Calling a function from another file in RISC-V assembly involves several steps, primarily related to linking and proper function calling conventions. Here's a breakdown of how to achieve this:

1. Separate Assembly Files:

- You'll have at least two assembly files: one containing the main program (e.g., `main.s`) and another containing the function you want to call (e.g., `function.s`).

2. Function Definition in `function.s`:

- In `function.s`, define your function using a label. Ensure you follow the RISC-V calling convention, which typically involves using registers `a0`-`a7` for arguments and `a0` for the return value. Also, save and restore any callee-saved registers (like `s0`-`s11`) that your function modifies.

- Example `function.s`:

.global my_function
my_function:
  addi sp, sp, -16 # Allocate stack space
  sw ra, 8(sp) # Save return address
  sw s0, 0(sp) # Save s0
  mv s0, a0 # Copy argument to s0
  addi a0, s0, 5 # Add 5 to the argument
  lw s0, 0(sp) # Restore s0
  lw ra, 8(sp) # Restore return address
  addi sp, sp, 16 # Deallocate stack space
  ret # Return

3. Calling the Function in `main.s`:

- In `main.s`, you need to declare the function as external using the `.extern` directive. Then, you can use `jal` (jump and link) to call the function. Before calling, load the arguments into the appropriate registers.

- Example `main.s`:

.extern my_function
.global _start
_start:
  li a0, 10 # Load argument into a0
  jal my_function # Call the function
   # Result is now in a0
   # ... (rest of your program)
  li a7, 93 # Exit syscall
  ecall

4. Assembly and Linking:

- You'll need to assemble both files and then link them together. The exact commands depend on your RISC-V toolchain. Here's a general example using `riscv64-unknown-elf-as` and `riscv64-unknown-elf-ld`:

riscv64-unknown-elf-as function.s -o function.o
riscv64-unknown-elf-as main.s -o main.o
riscv64-unknown-elf-ld main.o function.o -o program

5. Explanation:

- The `.global my_function` directive in `function.s` makes the `my_function` label visible to the linker. The `.extern my_function` directive in `main.s` tells the assembler that `my_function` is defined elsewhere. The `jal` instruction in `main.s` jumps to the address of `my_function` and saves the return address in the `ra` register. The `ret` instruction in `function.s` returns to the address stored in `ra`.

- The stack is used to save the return address and any callee-saved registers to ensure proper function execution and return.

By following these steps, you can successfully call a function from another file in RISC-V assembly. Remember to adhere to the RISC-V calling conventions to avoid unexpected behavior.

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