This was a computer organization class assignment. I implemented a factorial using the following algorithm:
In MIPS, I use $a0 as the parameter n of F(n). For example, letting $a0 equal to 5 and jumping to the fact label means F(5), and at the end of the recursion, the result will be stored in $v0.
main:
addi $a0, $zero, 5 # let the parameter n be 5
jal fact # jump to fact label, i.e. calling F(5)
j exit
fact:
addi $sp, $sp, -8 # allocate 8 bytes to this stack
sw $ra, 0($sp) # save the address of the instruction that calls fact label (instruction address)
sw $a0, 4($sp) # save the value of n
slti $t0, $a0, 1 # $t0 is used for conditions. If n < 1 then $t0 = 1, else $t0 = 0
beq $t0, $zero, L1 # if $t0 == 0 then jump to branch L1
addi $v0, $zero, 1 # let $v0 be 1
addi $sp, $sp, 8 # let $sp point to upper stack
jr $ra # jump to the next instruction of the instruction calling fact
L1:
addi $a0, $a0, -1 # n = n - 1
jal fact # jump to fact label again, like as calling F(n - 1)
lw $a0, 4($sp) # recover the value of n
mul $v0, $a0, $v0 # $v0 *= $a0, like as F(n) = n * F(n - 1)
lw $ra, 0($sp) # recover instruction address
addi, $sp, $sp, 8 # let $sp point to the upper stack
jr $ra # jump to the next instruction of the instruction calling L1
exit:
The execution result of the MARS simulator looks like this:
You can see that the value of $v0 is 0x78, 120 in decimal, which is the result of F(5).
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