Fibonacci series in MIPS,SPIM

fib.s

-- Copy right (c) Manu dot Datta @ gmail dot com
.data

msg1: .asciiz "Enter a positive index for Fibonacci Calculation : "
msg2: .asciiz "The Finonacci number is : "
msg3: .asciiz "Recusive calls made to Fibonacci : "
msgerr1: .asciiz "The Finonacci number is : 1 \n"
msgerr2: .asciiz "Recusive calls made to Fibonacci : 0 \n"

nl: .asciiz "\n";

.text
.globl main

main:   move	$a1, $zero
        move 	$a2, $zero
        li 	$a3, 1
get:    li 	$v0, 4
        la 	$a0,msg1           # User prompt
        syscall
        li 	$v0, 5
        syscall                   # read input
	   bltz 	$v0,loopg
        slt 	$t0, $v0, $zero          
        beq 	$t0, 1, done            
	   move 	$a1, $v0            # moving the user value into $a1
	   jal	calfib
        move 	$v1, $a0            # Temp storage of answer now in $a0 to $v1
        li 	$v0, 4
        la 	$a0, msg2           # The Fib number is...
        syscall
        li 	$v0, 1              # ... the answer
        move 	$a0, $v1            # Bring back temp Storage of answer from $v1
        syscall
        j 	done
loopg:
	   li 	$v0, 4
        la 	$a0,msgerr1              
        syscall
	   li 	$v0, 4
        la 	$a0,msgerr2             
        syscall
	   j		 get	
calfib:
	   subu	$sp,$sp,32		#set up standard activation record 
	   sw	$ra,20($sp)
	   sw	$fp,16($sp)
	   addiu	$fp,$sp,28
	   sw	$a1,0($fp)
	   addiu	$a2,$a2,1			#increment number of recursive calls
	   beq	$a1,1,ret1
	   beq	$a1,0,ret1
	   lw	$a1,0($fp)
	   subu	$a1,$a1,1			#subtract 1
	   jal	calfib			#calculate fib(n-1)
	   move	$a3,$a0			#save it in a3
	   sw	$a3,4($fp)
	   lw	$a1,0($fp)
	   subu	$a1,$a1,2			#subtract 2
	   jal	calfib			#calculate fib(n-2)
	   lw	$a3,4($fp)
	   add	$a0,$a3,$a0
	   j		donec
ret1:
	   li	$a0,1
donec:	
	   lw	$ra,20($sp)		#remove activation record of fibonacci
	   lw	$fp,16($sp)
	   addiu	$sp,$sp,32
	   jr	$ra
done:   
	   li $v0, 4                   
        la $a0, nl                  
        syscall                     
	   la $a0, msg3                
        syscall
	   subu	$a2,$a2,1
	   li	$v0, 1            
        move	$a0, $a2        
        syscall
	   li	$v0, 4                   
	   la	$a0, nl                 
        syscall
endit:                    
	   li	$v0, 10
	   syscall