我目前正在尝试实现一个递归函数来计算MIPS中字符串的长度。这是我现在的代码。我正在努力弄清楚基本情况以及如何让它返回字符串长度。我觉得我很接近,因为当我单步执行程序时,它正确计算并存储输入字符串的长度。任何帮助表示赞赏:
.text
#.align 2
main:
la $a0, input #load string for user input
li $v0, 4 #command to print string in syscall
syscall
li $v0, 8 #command to take user input
la $a0, buffer #store string of user input in buffer
li $a1,256 #length of buffer for string
syscall
move $t0,$a0 #move string address to $t0
addi $t2,$t2,0 #initialize word length counter
addi $sp, $sp, -16 #allocate space in stack
sw $t0,16($sp) #store string address in stack, which will be the working string
sw $ra,12($sp) #save return address
sw $t2,8($sp) #save word length counter
jal reclength
# if we get here then the string is finished
reclength:
lw $t0,16($sp) #load word from stack and store address in $t0
lw $t1,8($sp) #load counter from stack and store in $t1
lbu $t2,0($t0) #get first bit from word in position 0 of address stored in $t0
## BASE CASE ##
beq $t2,$zero,finisher
## RECURSIVE CASE ##
addi $t1,$t1,1 #incremement length counter
addi $t0,$t0,1 #point to the next character
addi $sp, $sp, -16
sw $t1,8($sp) #store the counter in the stack
sw $ra,12($sp) #store the return address in the stack
sw $t0,16($sp) #store new address in stack
jal reclength
#jr $ra
#nop
finisher:
lw $t1,8($sp) #load counter from stack
addi $t1,$t1,-1 #decrement counter because null counter was counted
la $a0, ending #load string for user input
li $v0, 4 #command to print string in syscall
syscall
jr $ra
.data
#.align 2
input: .asciiz "Enter a string : "
buffer: .space 256
ending: .asciiz "Finished"
答案 0 :(得分:1)
您为每个函数调用继续向下移动堆栈指针,但在返回时不会向上调整它。
我发现整个设计在堆栈上的返回值有点奇怪。通常会使用$v0
来返回函数的值。所以我会改写这样的函数:
reclength:
lw $t0,12($sp) # load string address from stack and place it in $t0
lbu $t2,0($t0) # read one byte from the string and place it in $t2
bne $t2,$zero,recurse
li $v0, 0 # base case : length = 0
j return
recurse:
# recursive case : return reclength(addr+1) + 1
addi $t0,$t0,1 # point to the next character
addi $sp, $sp, -16
sw $ra,8($sp) # store the return address in the stack
sw $t0,12($sp) # store new address in stack
jal reclength
addi $v0,$v0,1
return:
lw $ra, 8($sp) # restore the return address
addiu $sp,$sp,16 # restore the stack pointer
jr $ra
你会这样称呼:
addi $sp, $sp, -16 #allocate space in stack
sw $t0,12($sp) #store string address in stack, which will be the working string
sw $ra,8($sp) #save return address
jal reclength
您也可以摆脱堆栈上的字符串地址,并使用$a0
代替。但是我会把它作为锻炼给你。
您可能还希望在reclength
返回main
后正确退出该计划:
li $v0,10 # syscall 10 = exit
syscall