Reputation: 6448
I'd like to pass a character as an argument to a function in MIPS. Do I do this by storing the character into register $a0, use jal to move to the function, then extract what's in $a0 into a separate register using lw?
If someone could give me an example of passing an argument or two in MIPS, I'd really appreciate it. I've found a lot of info on the MIPS calling conventions, but not any simple and succinct examples.
Upvotes: 7
Views: 26002
Reputation: 1
a very easy way to go about it would be to load the argument in a temp register and then just use:
move $a0,$t0
this way the argument stored in the temp register will be given as an argument
Upvotes: 0
Reputation: 4961
No need to use lw
which is for extracting words from memory. You can simply use $a0
in the sub-routine.
Take a look at this example of a "print-char" function:
.text
main:
#save $ra on stack
addi $sp $sp -4
sw $fp 0($sp)
move $fp $sp
addi $sp $sp -4
sw $ra -4($fp)
#call sub-routine
addi $a0 $zero 'A'
jal printchar
#restore and shrink stack
lw $ra -4($fp)
lw $fp 0($fp)
addi $sp $sp 8
jr $ra
#prints a char and then a new line
printchar:
#call print-char syscall
addi $v0 $zero 11
syscall
addi $a0 $zero 10
syscall
jr $ra
As demonstrated, you the value of the $a0
register is just used in the sub-routine as it returns the value that it was given before the jal
.
Also demonstrated is proper expansion and shrinking of the stack as is necessary for calling a sub-routing. As you will notice, the sub-routine does not perform this operation as it does not call a sub-routine and there-fore does not need to save the $ra
. Stack manipulations would also be required in the sub-routine if it were to use an $s
register as the MIPS calling convention specifies these as callee saved.
Upvotes: 4