MIPS递归函数工作不正常

Question

我正在MIPS中执行一个阶乘递归函数。这是代码：

fact: #a1 = n   a2 = res
addi $t1,$zero,1
beq $a1,$t1,end  # if n == 1 return res
#else
mul $a2,$a2,$a1
subi $a1,$a1,1
jal fact

end:
sb $a2,res
jr $ra

问题是，这个代码只有在我写j fact而不是jal fact时才有效，因为jal fact的原因，代码会无限运行。

Answer 1

fact: #a1 = n   a2 = res
addi $t1,$zero,1
beq $a1,$t1,end  # if n == 1 return res
#else
addi $sp, $sp, 8
sw $a1, 0($sp) #  in the top of the stack store the argument you called fact
sw $ra, 4($sp) # ra holds the return adress ( I do not know whether you know it)
mul $a2,$a2,$a1
subi $a1,$a1,1
jr $ra
jal fact
lw $a1, 0($sp) # load the value from memory adress that the top stack pointer shows to back to the register a1
lw $ra, 4($sp) 
addi $sp, $sp, 8 # delete current top , top-1 from the stack
jr $ra
end:
sb $a2,res
jr $ra