我目前在MIPS中编写这个递归阶乘汇编代码时遇到了麻烦。我不想改变这个代码的格式，因为它是一个项目，但我想了解如何存储或调用$ra，以便它将返回到“分支：“，以输出计算的阶乘。

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.data
prompt1:    .asciiz "\nPlease enter an integer: \n"
negativePrompt: .asciiz "\nYour input must be greater than or equal to 0 in order to function.\n"
factorialIs:    .asciiz "\nThe factorial of your input number is: \n"

.globl main
.text
main:

    subu $sp, $sp, 32   # push memory for 8 values
    sw $ra, 20($sp)     # save return address
    sw $fp, 16($sp)     # save old frame pointer 
    addiu $fp, $sp, 28  # set up frame pointer

branch:

    li $v0, 4           # load macro for print_str 
    la $a0, prompt1     # pass argument to string
    syscall             # print string prompt1

    li $v0, 5           # load macro for read_int
    syscall             # get N value from user 

    move $a0, $v0               # store N as arg
    blt $a0, 0, negBranch       # if N is negative, handles case    
    jal factorial               # calls factorial function

    li $v0, 4               # macro for print string
    la $a0, factorialIs     # print string
    syscall

    li $v0, 1               # macro for print int
    move $a0, $v0           # print output int in v0
    syscall 

    lw $ra, 20($sp)         # restore return address of caller
    lw $fp, 16($sp)         # restore frame pointer
    addiu $sp, $sp, 32      # pop stack
    jr $ra                  # return to caller 
#exit:
#   "would you like to calculate again?" 
#   li $v0, 10              # load macro to exit
#   syscall                 # execute exit
    
negBranch: 
    li $v0, 4                   # load macro for print_str 
    la $a0, negativePrompt              # pass argument to string
    syscall                     # print string prompt1  
    li $t0, 0                   # initialize input value to 0
    j branch                        # ask user to input new number
    
factorial: 
    subu $sp, $sp, 32       # pushes memory for variables
    sw $ra, 12($sp)         # stores return address of recursion
    sw $fp, 8($sp)          # save frame pointer
    addiu $fp, $sp, 28      # set up frame pointer
    sw $a0, 0($fp)          # stores n in frame pointer

    lw $v0, 0($fp)
    bgtz $v0, zeroBranch        # if N is greater than 0, recurse
    li $v0, 1                   # return 1
    jr end

zeroBranch:
    lw $v1, 0($fp)          # load n value
    subu $v0, $v1, 1        # v0 = n - 1 
    move $a0, $v0           # a0 = v0 = n - 1 
    jal factorial           # recursive function call
    lw $v1, 0($fp)          # load original n value to v1
    mul $v0, $v0, $v1       # v0 = n * fact(n-1)    

end: 
    lw $ra, 12($sp)         # loads return address of main call
## this ^ line does not access the address from "jal factorial"
    lw $fp, 16($sp)         # loads initial n value
    addiu $sp, $sp, 32      # collapses stack frame
    jr $ra                  # return to main then exit

当我输入factorial（1）时，它如预期的那样递归地运行;然而，当我到达我的“end：”分支时，返回地址没有返回到我的“branch：”分支，它将输出阶乘输入的结果。