Mastering Assembly Programming

Shim code

The code we are about to write will look a tiny bit different from the code we are used to as we are not expecting an executable file to be generated out of it; instead, we will generate a binary file containing a 32-bit procedure assumed to be loaded at a specific address, and that is what we are going to tell the compiler in the first two lines of our patch.asm source file:

; Tell the assembler we are writing 32-bit codeuse32; Then specify the address where the procedure; is expected to be loaded atorg 0x414d98

Then we will define two labels pointing at addresses outside our procedure. Fortunately, Flat Assembler allows us to define a label at an arbitrary address, like this:

; Assign label to the code where jump ; to fgets is ...