Question

I’ve been working on a very basic MIPS program that computes some stuff on ticket sales. I have all the functionality implemented, but when I do my IF statements (blt –> branch if less than) I am running into some errors. I have the following data stored in temporary registers:

- $t0 = # of VIP tickets.
- $t1 = # of General Admission tickets.
- $t2 = # of Box Office tickets.
- $t4 = Average ticket sales.

I have checked that the values are stored accordingly and the code does work if the condition is FALSE, but if the condition is met it will branch and still print the code before the branch. Here is part of my code:

  blt $t0, $t4, VIP_LESS  # if $t0 < $t4 then VIP
  b VIP_GREAT
  VIP_LESS:
    li $v0, 4
    la $a0, less_avg_vip
    syscall 
  VIP_GREAT:
    li $v0, 4
    la $a0, great_avg_vip
    syscall 
  blt $t1, $t4, GEN_LESS # if $t1 < $t4 then GEN
  b GEN_GREAT
  GEN_LESS:
    li $v0, 4
    la $a0, less_avg_general
    syscall
  GEN_GREAT:
    li $v0, 4
    la $a0, great_avg_general
    syscall 
  blt $t2, $t4, BOX_LESS  # if $t2 < $t4 then BOX_LESS
  b BOX_GREAT
  BOX_LESS:
    li $v0, 4
    la $a0, less_avg_box
    syscall 
  BOX_GREAT:
    li $v0, 4
    la $a0, great_avg_box
    syscall 
  li $v0, 4
  la $a0, endl
  syscall

Can anyone see why I am getting a problem with this? The output of my program looks something like this:

VIP: 1349 tickets
General: 5278 tickets
Box: 4367 tickets

Average: 3215 tickets

VIP: Less than average.
General: Greater than average.
General: Less than average.
Box: Greater than average.
Box: Less than average.

EDIT: working code.

 blt $t0, $t4, VIP_LESS  # if $t0 < $t4 then VIP
  li $v0, 4
  la $a0, great_avg_vip
  syscall
  j GENERAL_IF
  VIP_LESS:
    li $v0, 4
    la $a0, less_avg_vip
    syscall 
  GENERAL_IF: 
    blt $t1, $t4, GEN_LESS # if $t1 < $t4 then GEN
    li $v0, 4
    la $a0, great_avg_general
    syscall 
    j BOX_IF
    GEN_LESS:
      li $v0, 4
      la $a0, less_avg_general
      syscall
  BOX_IF:
    blt $t2, $t4, BOX_LESS  # if $t2 < $t4 then BOX_LESS
    li $v0, 4
    la $a0, great_avg_box
    syscall 
    j END_IF
    BOX_LESS:
      li $v0, 4
      la $a0, less_avg_box
      syscall 
  END_IF:
    li $v0, 4
    la $a0, endl
    syscall 

Answer 1

You just need some unconditional branches so that you don’t execute both the “greater than” and “less than” code paths, e.g.

  blt $t0, $t4, VIP_LESS  # if $t0 < $t4 then VIP
  li $v0, 4
  la $a0, great_avg_vip
  syscall 
                <<< you need an unconditional branch here to VIP_GE
  VIP_LESS:
    li $v0, 4
    la $a0, less_avg_vip
    syscall 
    xor $a0, $a0, $a0
  VIP_GE:       <<< label here so that you can skip previous block
  ...

You need to do this for each of your three if/else blocks.