How to write self-modifying code in x86 assembly

Question

I'm looking at writing a JIT compiler for a hobby virtual machine I've been working on recently. I know a bit of assembly, (I'm mainly a C programmer. I can read most assembly with reference for opcodes I don't understand, and write some simple programs.) but I'm having a hard time understanding the few examples of self-modifying code I've found online.

This is one such example: http://asm.sourceforge.net/articles/smc.html

The example program provided does about four different modifications when run, none of which are clearly explained. Linux kernel interrupts are used several times, and aren't explained or detailed. (The author moved data into several registers before calling the interrupts. I assume he was passing arguments, but these arguments aren't explained at all, leaving the reader to guess.)

What I'm looking for is the simplest, most straightforward example in code of a self-modifying program. Something that I can look at, and use to understand how self-modifying code in x86 assembly has to be written, and how it works. Are there any resources you can point me to, or any examples you can give that would adequately demonstrate this?

I'm using NASM as my assembler.

EDIT: I'm also running this code on Linux.

Answer 1

I've never written self-modifying code, although I have a basic understanding about how it works. Basically you write on memory the instructions you want to execute then jump there. The processor interpret those bytes you've written an instructions and (tries) to execute them. For example, viruses and anti-copy programs may use this technique.
Regarding the system calls, you were right, arguments are passed via registers. For a reference of linux system calls and their argument just check here.