[SOLVED] MARS MIPS CLI does not accept newline character when read char syscall is called and instead returns an error

MARS MIPS CLI does not accept newline character when read char syscall is called and instead returns an error

I have the following program that reads a character and prints it:

.macro print_char
    li $v0, 11
    lbu $a0, ($s0)
    syscall
.end_macro
 
.macro read_char
    li $v0, 12
    syscall
    sb $v0, ($s0)
.end_macro
 
.text
    # Initialize registers.
    li $s0, 0x10010000
    li $t0, 0x0
 
    read_char
    print_char

It works in the MARS MIPS IDE, I can press the Enter key and the simulator will print a newline. But when I try to do the same thing using the MARS MIPS CLI, it returns an invalid char input (syscall 12) error. Why is this?

Solution

The following code contains an exception handler, that handles the exception on the bad input for syscall #12, then changes the 0xc to 0xd; advances the exception pc (epc) and resumes the user code:

.macro print_char
    li $v0, 11
    lbu $a0, ($s0)
    syscall
.end_macro
 
.macro read_char
    li $v0, 12
    syscall
    sb $v0, ($s0)
.end_macro
 
.text
    # Initialize registers.
    li $s0, 0x10010000
    li $t0, 0x0
 
    read_char
    print_char
    
    li $v0, 10
    syscall             # terminate program
    


.ktext 0x80000180       # install this next code as kernel exception handler
    li $v0, 0xd         # change 0xc in $v0 to 0xd
    mfc0 $k1, $14       # fetch epc (address of the offending syscall)
    addi $k1, $k1, 4    # advance to next instruction
    mtc0 $k1, $14       # update epc
    eret                # resume user code

Note that this handler has absolutely no error checking — in particular, it assumes that the only reason it might be invoked is to handle that syscall #12 error.