为什么在循环中使用 $v0 寄存器 return 错误输出？
Why does using $v0 register in a loop return wrong output?
以下节目
- 给定用户输入的下限和上限，确定该范围内的最小值和最小索引
对于测试用例（下限：2 上限：4），我尝试了两种不同的代码，区别如下。
下面的代码没有return预期的输出
findMin:
addi $t0, $a0, 0 # initialise $t0 (current pointer) to lower bound
addi $t1, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($t1) # initialise min (value) to the lower bound
Loop: slt $t4, $a1, $t0
bne $t4, $zero, End # branch to end if upper < lower
lw, $t3, 0($t0) # store the content of the current pointer
slt $t4, $t3, $t2 # if current ($t3) < min ($t2), store 1 in $t4
beq $t4, $zero, LoopEnd # if it is 0, go to LoopEnd
addi $t2, $t3, 0 # store content ($t3) as minimum ($t2)
addi $v0, $t0, 0 # store the address of min (DIFFERENCE)
LoopEnd: addi $t0, $t0, 4 # increments current pointer lower bound
j Loop # Jump to loop
End: jr $ra # return from this function
但是，以下代码 return 预期值：
findMin:
addi $t0, $a0, 0 # $t0 is the pointer to the current item
addi $t1, $a0, 0 # $t1 is the pointer to the minimum item
lw $t2, 0($t1) # $t2 stores the value of minimum item
loop:
slt $t4, $a1, $t0 # check if last pointer < current pointer
bne $t4, $zero, exit # if current pointer > last pointer, exit
lw $t3, 0($t0) # $t3 stores the value of current item
slt $t4, $t3, $t2 # if the current value is lesser than minimum value
beq $t4, $zero, skip # if current value is not lesser, then skip
addi $t1, $t0, 0 # minimum pointer = current pointer (DIFFERENCE)
lw $t2, 0($t1) # $t2 stores the value of minimum item
skip:
addi $t0, $t0, 4 # move to the next item
j loop
exit:
addi $v0, $t1, 0 # $v0 stores the address of the minimum item (DIFFERENCE)
jr $ra # return from this function
这背后的原理是什么？
以下为完整代码（可选）
# arrayFunction.asm
.data
array: .word 8, 2, 1, 6, 9, 7, 3, 5, 0, 4
newl: .asciiz "\n"
.text
main:
# Print the original content of array
# setup the parameter(s)
# call the printArray function
la $a0, array # base address of array
la $a1, 10 # number of elements in array
jal printArray # call function
# Ask the user for two indices
li $v0, 5 # System call code for read_int
syscall
add $t0, $v0, $zero # store input in $t0
li $v0, 5 # System call code for read_int
syscall
add $t1, $v0, $zero # store input in $t1
# Call the findMin function
# setup the parameter(s)
la $a0, array # load address of array into $a0
la $a1, array # load address of array into $a1
sll $t0, $t0, 2 # calculate offset of lower bound
sll $t1, $t1, 2 # calculate offset of upper bound
add $a0, $a0, $t0 # set $a0 to the lower bound
add $a1, $a1, $t1 # set $a1 to the upper bound
# call the function
jal findMin # call function
# Print the min item
# place the min item in $t3 for printing
addi $t3, $t2, 0 # placing min item in $t3
addi $t4, $v0, 0 # saving the pointer to the min element
# Print an integer followed by a newline
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # print $t3
syscall # make system call
li $v0, 4 # system call code for print_string
la $a0, newl
syscall # print newline
#Calculate and print the index of min item
la $a0, array
sub $t3, $t4, $a0
srl $t3, $t3, 2
# Place the min index in $t3 for printing
# Print the min index
# Print an integer followed by a newline
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # print $t3
syscall # make system call
li $v0, 4 # system call code for print_string
la $a0, newl
syscall # print newline
# End of main, make a syscall to "exit"
li $v0, 10 # system call code for exit
syscall # terminate program
#######################################################################
### Function printArray ###
#Input: Array Address in $a0, Number of elements in $a1
#Output: None
#Purpose: Print array elements
#Registers used: $t0, $t1, $t2, $t3
#Assumption: Array element is word size (4-byte)
printArray:
addi $t1, $a0, 0 #$t1 is the pointer to the item
sll $t2, $a1, 2 #$t2 is the offset beyond the last item
add $t2, $a0, $t2 #$t2 is pointing beyond the last item
l1:
beq $t1, $t2, e1
lw $t3, 0($t1) # $t3 is the current item
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # integer to print
syscall # print it
addi $t1, $t1, 4
j l1 # Another iteration
e1:
li $v0, 4 # system call code for print_string
la $a0, newl #
syscall # print newline
jr $ra # return from this function
#######################################################################
### Student Function findMin ###
#Input: Lower Array Pointer in $a0, Higher Array Pointer in $a1
#Output: $v0 contains the address of min item
#Purpose: Find and return the minimum item
# between $a0 and $a1 (inclusive)
#Registers used: $t0 (counter), $t1 (max add), $t2 (min), $v0 (min pos), $t3 (current item)
#Assumption: Array element is word size (4-byte), $a0 <= $a1
findMin:
addi $t0, $a0, 0 # initialise $t0 (current pointer) to lower bound
addi $t1, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($t1) # initialise min (value) to the lower bound
Loop: slt $t4, $a1, $t0
bne $t4, $zero, End # branch to end if upper < lower
lw, $t3, 0($t0) # store the content of the current pointer
slt $t4, $t3, $t2 # if current ($t3) < min ($t2), store 1 in $t4
beq $t4, $zero, LoopEnd # if it is 0, go to LoopEnd
addi $t2, $t3, 0 # store content ($t3) as minimum ($t2)
addi $t1, $t0, 0 # store the address of min
LoopEnd: addi $t0, $t0, 4 # increments current pointer lower bound
j Loop # Jump to loop
End: addi $v0, $t1, 0
jr $ra # return from this function
在第一种情况下，问题是您最初将最小指针保存到 t1 寄存器，但在 return 上您希望它在 v0 上。现在，在最小值不完全在下限索引上的情况下，这不会成为问题，因为在循环中，您将新找到的最小值指针保存到 v0，因此在 return 一切会如预期的那样。但是如果像 2,4 这样的下限的最小值在索引 2 中，因此在循环中，因为没有找到新的最小点，因此 v0 中不会写入任何内容，因此在 return 中它会有一些垃圾价值。
将开头部分改成这样可以正常工作：
addi $v0, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($v0) # initialise min (value) to the lower bound
以下节目
- 给定用户输入的下限和上限，确定该范围内的最小值和最小索引
对于测试用例（下限：2 上限：4），我尝试了两种不同的代码，区别如下。
下面的代码没有return预期的输出
findMin:
addi $t0, $a0, 0 # initialise $t0 (current pointer) to lower bound
addi $t1, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($t1) # initialise min (value) to the lower bound
Loop: slt $t4, $a1, $t0
bne $t4, $zero, End # branch to end if upper < lower
lw, $t3, 0($t0) # store the content of the current pointer
slt $t4, $t3, $t2 # if current ($t3) < min ($t2), store 1 in $t4
beq $t4, $zero, LoopEnd # if it is 0, go to LoopEnd
addi $t2, $t3, 0 # store content ($t3) as minimum ($t2)
addi $v0, $t0, 0 # store the address of min (DIFFERENCE)
LoopEnd: addi $t0, $t0, 4 # increments current pointer lower bound
j Loop # Jump to loop
End: jr $ra # return from this function
但是，以下代码 return 预期值：
findMin:
addi $t0, $a0, 0 # $t0 is the pointer to the current item
addi $t1, $a0, 0 # $t1 is the pointer to the minimum item
lw $t2, 0($t1) # $t2 stores the value of minimum item
loop:
slt $t4, $a1, $t0 # check if last pointer < current pointer
bne $t4, $zero, exit # if current pointer > last pointer, exit
lw $t3, 0($t0) # $t3 stores the value of current item
slt $t4, $t3, $t2 # if the current value is lesser than minimum value
beq $t4, $zero, skip # if current value is not lesser, then skip
addi $t1, $t0, 0 # minimum pointer = current pointer (DIFFERENCE)
lw $t2, 0($t1) # $t2 stores the value of minimum item
skip:
addi $t0, $t0, 4 # move to the next item
j loop
exit:
addi $v0, $t1, 0 # $v0 stores the address of the minimum item (DIFFERENCE)
jr $ra # return from this function
这背后的原理是什么？
以下为完整代码（可选）
# arrayFunction.asm
.data
array: .word 8, 2, 1, 6, 9, 7, 3, 5, 0, 4
newl: .asciiz "\n"
.text
main:
# Print the original content of array
# setup the parameter(s)
# call the printArray function
la $a0, array # base address of array
la $a1, 10 # number of elements in array
jal printArray # call function
# Ask the user for two indices
li $v0, 5 # System call code for read_int
syscall
add $t0, $v0, $zero # store input in $t0
li $v0, 5 # System call code for read_int
syscall
add $t1, $v0, $zero # store input in $t1
# Call the findMin function
# setup the parameter(s)
la $a0, array # load address of array into $a0
la $a1, array # load address of array into $a1
sll $t0, $t0, 2 # calculate offset of lower bound
sll $t1, $t1, 2 # calculate offset of upper bound
add $a0, $a0, $t0 # set $a0 to the lower bound
add $a1, $a1, $t1 # set $a1 to the upper bound
# call the function
jal findMin # call function
# Print the min item
# place the min item in $t3 for printing
addi $t3, $t2, 0 # placing min item in $t3
addi $t4, $v0, 0 # saving the pointer to the min element
# Print an integer followed by a newline
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # print $t3
syscall # make system call
li $v0, 4 # system call code for print_string
la $a0, newl
syscall # print newline
#Calculate and print the index of min item
la $a0, array
sub $t3, $t4, $a0
srl $t3, $t3, 2
# Place the min index in $t3 for printing
# Print the min index
# Print an integer followed by a newline
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # print $t3
syscall # make system call
li $v0, 4 # system call code for print_string
la $a0, newl
syscall # print newline
# End of main, make a syscall to "exit"
li $v0, 10 # system call code for exit
syscall # terminate program
#######################################################################
### Function printArray ###
#Input: Array Address in $a0, Number of elements in $a1
#Output: None
#Purpose: Print array elements
#Registers used: $t0, $t1, $t2, $t3
#Assumption: Array element is word size (4-byte)
printArray:
addi $t1, $a0, 0 #$t1 is the pointer to the item
sll $t2, $a1, 2 #$t2 is the offset beyond the last item
add $t2, $a0, $t2 #$t2 is pointing beyond the last item
l1:
beq $t1, $t2, e1
lw $t3, 0($t1) # $t3 is the current item
li $v0, 1 # system call code for print_int
addi $a0, $t3, 0 # integer to print
syscall # print it
addi $t1, $t1, 4
j l1 # Another iteration
e1:
li $v0, 4 # system call code for print_string
la $a0, newl #
syscall # print newline
jr $ra # return from this function
#######################################################################
### Student Function findMin ###
#Input: Lower Array Pointer in $a0, Higher Array Pointer in $a1
#Output: $v0 contains the address of min item
#Purpose: Find and return the minimum item
# between $a0 and $a1 (inclusive)
#Registers used: $t0 (counter), $t1 (max add), $t2 (min), $v0 (min pos), $t3 (current item)
#Assumption: Array element is word size (4-byte), $a0 <= $a1
findMin:
addi $t0, $a0, 0 # initialise $t0 (current pointer) to lower bound
addi $t1, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($t1) # initialise min (value) to the lower bound
Loop: slt $t4, $a1, $t0
bne $t4, $zero, End # branch to end if upper < lower
lw, $t3, 0($t0) # store the content of the current pointer
slt $t4, $t3, $t2 # if current ($t3) < min ($t2), store 1 in $t4
beq $t4, $zero, LoopEnd # if it is 0, go to LoopEnd
addi $t2, $t3, 0 # store content ($t3) as minimum ($t2)
addi $t1, $t0, 0 # store the address of min
LoopEnd: addi $t0, $t0, 4 # increments current pointer lower bound
j Loop # Jump to loop
End: addi $v0, $t1, 0
jr $ra # return from this function
在第一种情况下，问题是您最初将最小指针保存到 t1 寄存器，但在 return 上您希望它在 v0 上。现在，在最小值不完全在下限索引上的情况下，这不会成为问题，因为在循环中，您将新找到的最小值指针保存到 v0，因此在 return 一切会如预期的那样。但是如果像 2,4 这样的下限的最小值在索引 2 中，因此在循环中，因为没有找到新的最小点，因此 v0 中不会写入任何内容，因此在 return 中它会有一些垃圾价值。
将开头部分改成这样可以正常工作：
addi $v0, $a0, 0 # initialise minimum pointer to upper bound
lw $t2, 0($v0) # initialise min (value) to the lower bound