Writing a function in MIPS with global variables?

Question

frogger:
    jal teechar
    jal update_frog
    lw $t0,game_indicator
    li $t0, 0

Okay, this is the function. I have teechar and update_frog functions completely written and working. I understand the first two bullets but not sure on the last two. How would i write the last two bullets in MIPS?

HERE IS FUNCTION Instructions:

Write a MIPS function frogger that takes no arguments, and returns a character (to make testing easier). It does the following:

• Call your teechar to read-and-echo a character c.
• Call your update_frog(c).
• If c is non-zero, store c in a global variable to indicate that the game is over, and how it ended. (You must initialize that global variable to zero, of course.)
• Return c.

Here is my code:

frogger:
    jal teechar
    jal update_frog

Sorry I am just having trouble thinking about how to do this :(

Answer 1

For the sake of a global variable, either you have to pass the arguement or use a special purpose register. Since you have specified the function takes no args, you will have to reserve a register to hold the value of c whenever frogger is called, which would then read the value from that register.

Its not a clean solution from the high level programming language, but you are in assembly right now and might as well get your hands dirty!