Putchar
Description: Put one character on the screen.
Gets: Character on the stack.
Returns: In R0: 0 on success, -1 on error.
Example:
.text
main: .word 0
calls $0, .getchar # Get char from the keyboard
pushl r0 # Push the char to the stack
calls $1, .putchar
pushl $0
calls $1, .exit