Question

Rewrite the code for fact to use fewer instructions.

Short answer

The following code for fact is rewritten to use fewer instructions.

addi $sp, $sp, -8

sw $ra, 4($sp)

sw $a0, 0($sp)

slti $t0, $a0, 1

beq $t0, $zero, L1

addi $v0, $zero, 1

addi $sp, $sp, 8

jr $ra

L1:

addi $a0, $a0,-1

jal fact

lw $a0, 0($sp)

lw $ra, 4($sp)

addi $sp, $sp, 8

mul $v0, $a0, $v0

jr $ra

Step-by-step solution

fact code

The following is the given code for fact which has more instructions.

subu $sp,$sp,32 # Stack frame is 32 bytes long

sw $ra,20($sp) # Save return address

sw $fp,16($sp) # Save frame pointer

addiu $fp,$sp,28 # Set up frame pointer

sw $a0,0($fp) # Save argument (n)

Rewritten code.

The following code is rewritten so as the code for fact uses fewer instructions.

addi $sp, $sp, -8 #adjust stack for 2 items

sw $ra, 4($sp) #save return address

sw $a0, 0($sp) #save argument

slti $t0, $a0, 1 #test for n<1

beq $t0, $zero, L1

addi $v0, $zero, 1 #yes, result is 1

addi $sp, $sp, 8 #pop 2 items from stack

jr $ra #stack and return

L1:

addi $a0, $a0,-1 #else decrement n

jal fact #recursive call

lw $a0, 0($sp) # restore original n

lw $ra, 4($sp) #and return address

addi $sp, $sp, 8 #pop 2 items from stack

mul $v0, $a0, $v0 #multiply to get

jr $ra #result and return

Explanation:

Instruction	Description
addi $sp, $sp, -8	The stack is adjusted for two items.
sw $ra, 4($sp)	Return address is saved.
sw $a0, 0($sp)	Argument is saved.
slti $t0, $a0, 1	Tested if n is less than 1.
addi $v0, $zero, 1	Result is 1.
addi $sp, $sp, 8	Two items are popped from the stack.
jr $ra	Stack and return.
addi $a0, $a0, -1	If result is no, decrement of 1.
jal fact	Recursive call.
lw $a0, 0($sp)	Restore original value of n
lw $ra, 4($sp)	Return address
addi $sp, $sp, 8	Two items are popped from the stack.
mul $v0, $a0, $v0	Multiplied to get result
jr $ra	Result and returns.