Example Program

Write a program that calculates the total amount given the amount for the meal, plus 8% tax, plus 15% tip. The meal amount is input from the user. The tax and tip rates are fixed. Here is a run of the program:

Enter food cost: 25
   Tax plus tip:  5
     Total cost: 30

## rest.asm
##
## Total restaurant bill calculator
##
## Register Use:
##
## $s0 tip+tax 
## $s1 tip rate
## $s2 tax rate
## $s3 meal cost
## $a0 tax+tip dollars

        .globl  main

        # Get meal cost
main:   li      $v0,4       # print a prompt (code 4 put in $v0)
        la      $a0,prompt  # address of prompt put in $a0
        syscall

        li      $v0,5       # input an integer  (code 5 put in $v0)
        syscall             # 32-bit result returned in $v0
        move    $s3,$v0     # save it in $s3 
        
        . . . . .

        .data
tip:    .word   15          # tip rate in percent
tax:    .word    8          # tax rate in percent

prompt: .asciiz "Enter food cost: "
head1 : .asciiz "   Tax plus tip: "
head2 : .asciiz "     Total cost: "
# end of file

The only information needed from the user is the cost of the food. This is prompted for and then read in (as text) and converted (by part of the trap handler) into a 32-bit two's complement integer. The SPIM trap handler expects the service code to be in register $v0. Arguments are passed into and returned from the services using various other registers. See Appendix G.