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98 lines
2.4 KiB
98 lines
2.4 KiB
5 years ago
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# Simple routine to demo a loop
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# Compute the sum of N integers: 1 + 2 + 3 + ... + N
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# Same result as example4, but here a function performs the
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# addition operation: int add(int num1, int num2)
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# ------------------------------------------------------------------
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.text
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.globl main
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main:
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# Register assignments
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# $s0 = N
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# $s1 = counter (i)
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# $s2 = sum
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# Print msg1
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li $v0,4 # print_string syscall code = 4
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la $a0, msg1
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syscall
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# Get N from user and save
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li $v0,5 # read_int syscall code = 5
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syscall
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move $s0,$v0 # syscall results returned in $v0
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# Initialize registers
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li $s1, 0 # Reg $s1 = counter (i)
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li $s2, 0 # Reg $s2 = sum
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# Main loop body
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loop: addi $s1, $s1, 1 # i = i + 1
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# Call add function
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move $a0, $s2 # Argument 1: sum ($s2)
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move $a1, $s1 # Argument 2: i ($s1)
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jal add2 # Save current PC in $ra, and jump to add2
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move $s2,$v0 # Return value saved in $v0. This is sum ($s2)
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beq $s0, $s1, exit # if i = N, continue
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j loop
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# Exit routine - print msg2
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exit: li $v0, 4 # print_string syscall code = 4
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la $a0, msg2
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syscall
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# Print sum
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li $v0,1 # print_string syscall code = 4
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move $a0, $s2
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syscall
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# Print newline
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li $v0,4 # print_string syscall code = 4
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la $a0, lf
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syscall
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li $v0,10 # exit
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syscall
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# ------------------------------------------------------------------
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# FUNCTION: int add(int num1, int num2)
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# Arguments are stored in $a0 and $a1
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# Return value is stored in $v0
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# Return address is stored in $ra (put there by jal instruction)
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# Typical function operation is:
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# 1.) Store registers on the stack that we will overwrite
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# 2.) Run the function
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# 3.) Save the return value
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# 4.) Restore registers from the stack
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# 5.) Return (jump) to previous location
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# Note: This function is longer than it needs to be,
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# in order to demonstrate the usual 5 step function process...
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add2: # Store registers on the stack that we will overwrite (just $s0)
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addi $sp,$sp, -4 # Adjust stack pointer
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sw $s0,0($sp) # Save $s0 on the stack
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# Run the function
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add $s0,$a0,$a1 # Sum = sum + i
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# Save the return value in $v0
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move $v0,$s0
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# Restore overwritten registers from the stack
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lw $s0,0($sp)
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addi $sp,$sp,4 # Adjust stack pointer
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# Return from function
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jr $ra # Jump to addr stored in $ra
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# ------------------------------------------------------------------
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# Start .data segment (data!)
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.data
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msg1: .asciiz "Number of integers (N)? "
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msg2: .asciiz "Sum = "
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lf: .asciiz "\n"
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