关于MIPS上手应知应会-如何把C语言改写为MIPS！

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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 somethingbreak;case 4:// Do somethingbreak;default:// Do something
}

# $s0 = a
switch_begin:case_2:bne $s0, 2, case_4# Do somethingj switch_endcase_4:bne $s0, 4, default# Do somethingj switch_enddefault:# 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 somethingaddi $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;

.dataarr: .space 400				#长度100的int型数组，总共使用400字节
.text# $t0 = ili $t0, 0;for_begin:beq $t0, 100, end_formove $t1, $t0sll $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0x56sw $t0, arr($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, 1j for_beginend_for:li $v0, 10						# 类似于C/C++中的return 0
syscall

二维数组定义与使用

#include <iostream>
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;
}

.dataarr: .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dsll $t8, $t8, 6add $t9, $t8, %jsll $t9, $t9, 2sw %d, arr($t9)
.end_macro
.macro getarr(%d, %i, %j)		# 把d赋值为arr[i][j]sll $t8, $t8, 6add $t9, $t8, %jsll $t9, $t9, 2lw %d, arr($t9)
.end_macro.text# $s0 = n, $s1 = mli $v0, 1syscallmove $s0, $v0li $v0, 1syscallmove $s0, $v0# $t0 = i, $t1 = jli $t0, 0li $t1, 0for_in_i:beq $t0, $s0, end_for_in_ifor_in_j:beq $t1, $s1, end_for_in_jli $v0, 1syscallsetarr($v0, $t0, $t1)addi $t1, $t1, 1j for_in_jend_for_in_j:addi $t0, $t0, 1j for_in_iend_for_in_i:# $t0 = i, $t1 = jsubi $t0, $s0, 1 subi $t1, $s1, 1for_out_i:blt $t0, 0, end_for_out_ifor_out_j:blt $t1, 0, end_for_out_jgetarr($t3, $t0, $t1)# 输出$t0, $t1, $t2这里省略了subi $t1, $t1, 1j for_out_jend_for_out_j:subi $t0, $t0, 1j for_out_iend_for_out_j:li $v0, 10						# 类似于C/C++中的return 0
syscall

例 ：哈密顿回路

#include <iostream>
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;
}

.datagraph: .space 1024vis: .space 512endl: .word '\n'.macro setGraph(%data, %i, %j)sll $t8, %i, 4add $t8, $t8, %jsll $t8, $t8, 2sw %data, graph($t8)
.end_macro
.macro getGraph(%ans, %i, %j)sll $t8, %i, 4add $t8, $t8, %jsll $t8, $t8, 2lw %ans, graph($t8)
.end_macro.text
main:# $s0 = nli $v0, 5syscallmove $s0, $v0# $s1 = mli $v0, 5syscallmove $s1, $v0move $t0, $zerofor_input:# for(i = 0; i != m; i++)beq $t0, $s1, end_for_inputli $v0, 5syscallmove $t1, $v0li $v0, 5syscallmove $t2, $v0li $t3, 1setGraph($t3, $t1, $t2)setGraph($t3, $t2, $t1)addi $t0, $t0, 1j for_inputend_for_input:li $t0, 1for_iter:# for(i = 1; i <= n; i++)bgt $t0, $s0, end_for_itermove $a0, $t0	# $a0 = pmove $a1, $t0	# $a1 = startjal dfsbeq $s3, 1, print_ansaddi $t0, $t0, 1j for_iterend_for_iter:print_ans:	li $v0, 1move $a0, $s3syscall
li $v0, 10
syscalldfs:	# $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, 2li $t1, 1sw $t1, vis($t0)li $t0, 1for_nextpoint:# for(i = 1; i <= n; i++) if(graph[p][i] == 1 && !vis[i])bgt $t0, $s0, end_for_nextpointgetGraph($t1, $a0, $t0)bne $t1, 1, nextpoint_continuesll $t2, $t0, 2lw $t3, vis($t2)bne $t3, $zero, nextpoint_continuemove $t4, $a0move $a0, $t0jal dfsmove $a0, $t4nextpoint_continue:addi $t0, $t0, 1j for_nextpointend_for_nextpoint:li $t0, 1	for_check:bgt $t0, $s0, end_for_checksll $t1, $t0, 2lw $t2, vis($t1)beq $t2, $zero, end_functionaddi $t0, $t0, 1j for_checkend_for_check:getGraph($t0, $a0, $a1)bne $t0, 1, end_functionli $s3, 1end_function:sll $t0, $a0, 2sw $zero, vis($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

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