关于MIPS上手应知应会-如何把C语言改写为MIPS！

寄存器

Name	Reg. Num	Usage
$zero$	0	constant value =0(恒为0)
$at$	1	reserved for assembler(为汇编程序保留)
$v0–v1$	2 – 3	values for results(过程调用返回值)
$a0–a3$	4 – 7	Arguments(过程调用参数)
$t0–t7$	8 – 15	Temporaries(临时变量)
$s0–s7$	16 – 23	Saved(保存)
$t8–t9$	24 – 25	more temporaries(其他临时变量)
$k0–k1$	26 – 27	reserved for kernel(为OS保留)
$gp$	28	global pointer(全局指针)
$sp$	29	stack pointer (栈指针)
$fp$	30	frame pointer (帧指针)
$ra$	31	return address (过程调用返回地址)

指令

使用技巧

翻译C/C++

if/else语句

if(a >= b) //Do something...
else if(a < c+b) //Do something...
else //Do something...

# $s0 = a, $s1 = b, $s2 = c
if_begin:
bgt $s1, $s0, if_else1
	# Do something
if_else1:
add $t0, $s1, $s2
bngt $t0, $s0, if_else2
	# Do something
if_else2:
	# Do something
if_end:

switch语句

switch(a) {
    case 2:
        // Do something
       	break;
    case 4:
        // Do something
        break;
    default:
        // Do something
}

# $s0 = a
switch_begin:
	case_2:
	bne $s0, 2, case_4
    	# Do something
    j switch_end
    
    case_4:
    bne $s0, 4, default
    	# Do something
    j switch_end
    
    default:
    	# Do something
switch_end:

for循环

for(int i = 0; i < n; i++) // Do something

# $s0 = n
li $t0, 0
for_begin:
bne $t0, $s0, end_for 
	# Do something
addi $t0, $t0, 1
j for_begin
end_for:

while 循环

int i = a;
while(i < n) {
    // Do something...
    i++;
}

# $s0 = n, $s1 = a, $t0 = i
move $t0, $s1
while_begin:
bne $t0, $s0, end_while
	# Do something
	addi $t0, $t0, 1
j while_begin
end_while:

do…while循环

do {
    i++;
    //Do something
} while(i < n);

# $t0 = i, $s0 = n
li $t0, 1
dowhile:
	addi $t0, $t0, 1
	# Do something
beq $s0, $t0, end_dowhile
j dowhile
end_dowhile:

一维数组定义与使用

int arr[100];
for(int i = 0; i < 100; i++) arr[i] = i;

.data
	arr: .space 400				#长度100的int型数组，总共使用400字节
.text
	# $t0 = i
	li $t0, 0;
	for_begin:
	beq $t0, 100, end_for
		move $t1, $t0
		sll $t1, $t1, 2			# i*4得到偏移的字节数，MIPS按照字节寻址，地址从x00000000, 0x00000004...以此类推
								# 此外MIPS还是小端地址，如果输入0x12345678，那么0x00000002存的是0x56
		sw $t0, arr($t1)		# 这跟直接访问还挺像的
								# 实际上，arr是指一系列空间的首地址，加上偏移量$t1，得到arr[i]的地址
	addiu $t0, $t0, 1
	j for_begin
	end_for:
	
li $v0, 10						# 类似于C/C++中的return 0
syscall

二维数组定义与使用

#include 
using namespace std;
int arr[64][64];

int main() {
    int m, n;
    cin >> n >> m;
    for(int i = 0; i < n; i++)
    	for(int j = 0; j < m; j++) cin >> arr[i][j];
    for(int i = n-1; i >= 0; i--)
        for(int j = m-1; j >= 0; j--) cout << i << ' ' << j << ' ' << arr[i][j] << endl; 
    return 0;
}

.data
	arr: .space 16384

# 下面两个宏定义与数组大小密切相关，64*64大小的数组是这么做的
# 我们约定$t7, $t8, $t9只在宏定义中使用
.macro setarr(%d, %i, %j)		# 把arr[i][j]设置为d
	sll $t8, $t8, 6
	add $t9, $t8, %j
	sll $t9, $t9, 2
	sw %d, arr($t9)
.end_macro
.macro getarr(%d, %i, %j)		# 把d赋值为arr[i][j]
	sll $t8, $t8, 6
	add $t9, $t8, %j
	sll $t9, $t9, 2
	lw %d, arr($t9)
.end_macro

.text
	# $s0 = n, $s1 = m
	li $v0, 1
	syscall
	move $s0, $v0
	li $v0, 1
	syscall
	move $s0, $v0
	
	# $t0 = i, $t1 = j
	li $t0, 0
	li $t1, 0
	for_in_i:
	beq $t0, $s0, end_for_in_i
		for_in_j:
		beq $t1, $s1, end_for_in_j
			li $v0, 1
			syscall
			setarr($v0, $t0, $t1)
		addi $t1, $t1, 1
		j for_in_j
		end_for_in_j:
	addi $t0, $t0, 1
	j for_in_i
	end_for_in_i:
	
	# $t0 = i, $t1 = j
	subi $t0, $s0, 1 
	subi $t1, $s1, 1
	for_out_i:
	blt $t0, 0, end_for_out_i
		for_out_j:
		blt $t1, 0, end_for_out_j
			getarr($t3, $t0, $t1)
			# 输出$t0, $t1, $t2这里省略了
		subi $t1, $t1, 1
		j for_out_j
		end_for_out_j:
	subi $t0, $t0, 1
	j for_out_i
	end_for_out_j:
	
li $v0, 10						# 类似于C/C++中的return 0
syscall

例 ：哈密顿回路

#include 
using namespace std;

const int MAXN = 10;

int G[MAXN][MAXN];
bool vis[MAXN];
int n, m;
int ans;

int dfs(int i, int start) {
    bool flag = false;
    vis[i] = true;
    cout << i;
    for(int j = 1; j <= n; j++) if(!vis[j] && G[i][j]) {
        dfs(j, start);
    }
    for(int j = 1; j <= n; j++) if(!vis[j]) {
        flag = true;
        break;
    }
    if(G[i][start] && !flag) ans = 1;
    vis[i] = false;
    return 0;
}

int main() {
    cin >> n >> m;
    for(int i = 0; i < m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        G[u][v] = G[v][u] = 1;
    }

    for(int i = 1; i <= n; i++) {
        dfs(i, i);
        if(ans == 1) {
            cout << 1 << endl;
            return 0;
        }
    }
    cout << 0 << endl;
    return 0;
}

.data
	graph: .space 1024
	vis: .space 512
	endl: .word '\n'

.macro setGraph(%data, %i, %j)
	sll $t8, %i, 4
	add $t8, $t8, %j
	sll $t8, $t8, 2
	sw %data, graph($t8)
.end_macro
.macro getGraph(%ans, %i, %j)
	sll $t8, %i, 4
	add $t8, $t8, %j
	sll $t8, $t8, 2
	lw %ans, graph($t8)
.end_macro

.text
main:
	# $s0 = n
	li $v0, 5
	syscall
	move $s0, $v0
	# $s1 = m
	li $v0, 5
	syscall
	move $s1, $v0
	
	move $t0, $zero
	for_input:
	# for(i = 0; i != m; i++)
	beq $t0, $s1, end_for_input
		li $v0, 5
		syscall
		move $t1, $v0
		li $v0, 5
		syscall
		move $t2, $v0
		li $t3, 1
		setGraph($t3, $t1, $t2)
		setGraph($t3, $t2, $t1)
	addi $t0, $t0, 1
	j for_input
	end_for_input:
	
	li $t0, 1
	for_iter:
	# for(i = 1; i <= n; i++)
	bgt $t0, $s0, end_for_iter
		move $a0, $t0	# $a0 = p
		move $a1, $t0	# $a1 = start
		jal dfs
		beq $s3, 1, print_ans
	addi $t0, $t0, 1
	j for_iter
	end_for_iter:
	
	print_ans:	
	li $v0, 1
	move $a0, $s3
	syscall
li $v0, 10
syscall

dfs:	# $a0 = p, $a1 = start
addi $sp, $sp, -24
sw $ra, 20($sp)
sw $t0, 16($sp)
sw $t1, 12($sp)
sw $t2, 8($sp)
sw $t3, 4($sp)
sw $t4, 0($sp)
	sll $t0, $a0, 2
	li $t1, 1
	sw $t1, vis($t0)
	li $t0, 1
	for_nextpoint:
	# for(i = 1; i <= n; i++) if(graph[p][i] == 1 && !vis[i])
	bgt $t0, $s0, end_for_nextpoint
		getGraph($t1, $a0, $t0)
		bne $t1, 1, nextpoint_continue
		sll $t2, $t0, 2
		lw $t3, vis($t2)
		bne $t3, $zero, nextpoint_continue
			move $t4, $a0
			move $a0, $t0
			jal dfs
			move $a0, $t4
	nextpoint_continue:
	addi $t0, $t0, 1
	j for_nextpoint
	end_for_nextpoint:

	li $t0, 1	
	for_check:
	bgt $t0, $s0, end_for_check
		sll $t1, $t0, 2
		lw $t2, vis($t1)
		beq $t2, $zero, end_function
	addi $t0, $t0, 1
	j for_check
	end_for_check:

	getGraph($t0, $a0, $a1)
	bne $t0, 1, end_function
	li $s3, 1
	
	end_function:
	sll $t0, $a0, 2
	sw $zero, vis($t0)
lw $ra, 20($sp)
lw $t0, 16($sp)
lw $t1, 12($sp)
lw $t2, 8($sp)
lw $t3, 4($sp)
lw $t4, 0($sp)
addi $sp, $sp, 24
jr $ra

注意

立即数被符号位扩展

• 算术指令 ：add addi sub 总是将立即数做符号位扩展即便指令是无符号的;
  乘 、 除 指令 任何情况下都不进行扩展，总是当成 unsigned
• 逻辑指令：（andi, ori通常处理无符号数）不对立即 数做符号位扩展
• load / store指令： 地址计算时总是扩展立即数

参考链接

1. 谈一谈 MIPS 汇编 Challenge 题：找哈密顿回路:(https://flyinglandlord.github.io/2021/09/30/BUAA-CO-2021/Pre/%E8%B0%88%E4%B8%80%E8%B0%88MIPS%E6%B1%87%E7%BC%96Challenge%E9%A2%98/)

2. 流水线 MIPS 处理器的设计:https://zobinhuang.github.io/sec_learning/Tech_Computer_Architerture/Architecture_6_MIPS_Pipeline/#1_intro_1

3. 《初学计算机组成原理之MIPS指令集及汇编》: https://blog.csdn.net/weixin_51599896/article/details/123865620

4. MIPS指令详解:https://blog.csdn.net/weixin_46308081/article/details/115798605