Question

I am writing a high-level MIPS assembler. I’m having trouble off the bat trying to get fwrite to output correctly. An example assembly file looks like this:

.text
    addi    $t0,$0,7 
    addi    $t1,$0,1 
L1: 
    beq $t0,$0,DONE 
    mult    $t1,$t0 
    mflo    $t1  
    addi    $t0,$t0,-1 
    j   L1 
DONE:   
    add $a0,$0,$t1 
    addi    $v0,$0,1 
    syscall 
    addi    $v0,$0,10 
    syscall 

My code should convert these into 32-bit binary instructions and fwrite them to a file to be read by the faux-CPU.

I have a problem off the bat where the 32-bit delimiter at the beginning of the file should be 0xf0f0f0f0. I know you can’t “read” a binary file, but looking at the example file that I have, the delimiter is represented by /360/360/360/360 after being written by fwrite. Why is this and how would I replicate that behavior when I’m writing my own file (as in how do I use fwrite to achieve this to create other binary files)?

Answer 1

The fwrite(3) for your delimiter would look like this:

unsigned int delim = 0xF0F0F0F0;

fwrite(&delim, sizeof delim, 1, output);

This writes one object of delim‘s size to the standard C stream output.

You can use od(1) or xxd(1) to “read” binary files:

$ od -bc /etc/passwd
0000000 162 157 157 164 072 170 072 060 072 060 072 162 157 157 164 072
          r   o   o   t   :   x   :   0   :   0   :   r   o   o   t   :
$ xxd /etc/passwd
0000000: 726f 6f74 3a78 3a30 3a30 3a72 6f6f 743a  root:x:0:0:root:
0000010: 2f72 6f6f 743a 2f62 696e 2f62 6173 680a  /root:/bin/bash.

(The hexer(1) program (in the hexer package on Ubuntu) is a lot like an interactive xxd(1), you might find it useful some day.)