---
tags: CA2020
---
# Assignment2: RISC-V Toolchain-2
contributed by < `kevin30292` >

## GCD

The following assembly program was implemented by `洪邵澤`.

.data
num1: .word 120    # number 1 (u)
num2: .word 78    # number 2 (v)
str1: .string "\n"
nums: .word 30 10

.text
main:
	    lw s0, num1 # u
	    lw s1, num2 # v
    	# check u or v is 0
    	beqz s1, print
    	beqz s0, assign
# use Euclidean algorithm
loop:
    	mv s2, s1
    	rem s1, s0, s1
	    mv s0, s2
    	bnez s1, loop
    	j print
        
assign:
	    mv s0, s1
	    j print

print:
	    addi a7, x0, 1	# print_int ecall
	    add a0, s0, x0	# integer
	    ecall
exit:

## Rewrite into C implementation

`gcd.c`

#include <stdio.h>
int _start() {
    int s0 = 120;
    int s1 = 78;
    if(s1 == 0){
        ;
    } else if(s0 == 0){
        s0 = s1;
    } else {
        while(s1) {
            int s2 = s1;
            s1 = s0 % s1;
            s0 = s2;
        }
    }
    volatile char *tx = (volatile char *)0x40002000;
    *tx = s0;
}


#include <stdio.h>
int mod(int a, int b)
{
    int r = a;
    while(r >= b) {
        r = r - b;
    }
    return r;
}
int _start() {
    int s0 = 120;
    int s1 = 78;
    if(s1 == 0){
        ;
    } else if(s0 == 0){
        s0 = s1;
    } else {
        int s2;
        while(s1) {
            s2 = s1;
            s1 = mod(s0, s1);
            s0 = s2;
        }
    }
    volatile char *tx = (volatile char *)0x40002000;
    *tx = (char)(s0 + '0');
    return 0;
}

## Execute with `rv32emu`

$ riscv-none-embed-gcc -march=rv32i -mabi=ilp32 -O3 -nostdlib -o gcd gcd.c


## ELF files generated by C compiler


#### Compare the assembly

1. The ELF files generated by C compiler didn't branch back to mod.
->It merge the mod function in to _start
2. It's use stack to store the array temporary.
->It doesn't need to access memory so many times.