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I'm writing a program in MIPS using Spim and I want to convert temperatures to/from fahrenheit and celsius. For example: 

Enter a number to convert: 100
Enter the Temperature: C
100 C is the same as 212 F

So I'm having trouble getting the program to recognize if the user input a "C" or an "F" and jump to the appropriate label. Here's the part of the code I'm having trouble with:

li $v0, 8           # Loads System service to read string
syscall
move $s1, $v0        # Move value to register to save for later use

beq $s1, 'C', Fahrenheit
beq $s1, 'F', Celsius

The program just passes by the 'beq' lines without doing anything. Any help would be much appreciated!

Peter Cordes
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Kyle2595
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  • The read-character system call is `$v0=12` https://courses.missouristate.edu/KenVollmar/MARS/Help/SyscallHelp.html. You could have used that. – Peter Cordes Mar 12 '20 at 02:23

1 Answers1

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Unlike reading integers, reading strings does not read the user input into register $v0. A register in MIPS is only 4 bytes, so it doesn't make sense to store a string in a single register. Instead, MIPS will read it into memory starting at a specified address.

The read string syscall in MIPS works like this:

  • $a0 is the start address of the buffer in memory where the string is to be kept (you don't need to choose this yourself--you just allocate the space with a label name and MIPS will choose an address for you)
  • $a1 is the length of the string buffer in bytes. For a single character you want this to be 2, because strings use their last byte as a null-terminator. If this is 1, the syscall won't even read anything from the user since that character automatically needs to be a null-terminator.

You can allocate 2 bytes of memory for the string by including this in your .data section

myString:
    .space 2

Then perform the syscall in your .text section:

li $v0, 8
la $a0, myString   # myString is a label for the memory address
la $a1, 2
syscall

Then read the character (a single byte) you want into a register using lb

lb $s1, ($a0)

Your beq instruction should work as you expect now.

eigenchris
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