Mailing List Articles Atom Feed Comments Atom Feed Twitter Reddit Facebook

Tag Cloud

3d 3d printing account algorithms android announcement architecture archives arduino artificial intelligence artix assembly async audio automation backups bash batch blender blog bookmarklet booting bug hunting c sharp c++ challenge chrome os cluster code codepen coding conundrums coding conundrums evolved command line compilers compiling compression containerisation css dailyprogrammer data analysis debugging demystification distributed computing dns docker documentation downtime electronics email embedded systems encryption es6 features ethics event experiment external first impressions freeside future game github github gist gitlab graphics hardware hardware meetup holiday holidays html html5 html5 canvas infrastructure interfaces internet interoperability io.js jabber jam javascript js bin labs learning library linux lora low level lua maintenance manjaro minetest network networking nibriboard node.js open source operating systems optimisation own your code pepperminty wiki performance phd photos php pixelbot portable privacy problem solving programming problems project projects prolog protocol protocols pseudo 3d python reddit redis reference releases rendering resource review rust searching secrets security series list server software sorting source code control statistics storage svg systemquery talks technical terminal textures thoughts three thing game three.js tool tutorial tutorials twitter ubuntu university update updates upgrade version control virtual reality virtualisation visual web website windows windows 10 worldeditadditions xmpp xslt

Stacks of Assembly

This morning, before I got to continuing my revision for my upcoming Systems Analysis exam I thought that I would write another quick blog post. This time (again under request), I'm posting about the stack in assembly. I thought that I'd make a short animated gif in order to demonstrate it more clearly, but I under estimate the amount of time that it took to make and ended up working on it all morning...!

Anyway, here's the animated gif that I created. I also uploaded it to youtube if you'd prefer to watch it there instead.

Assembly Stack Demo

I omitted the base pointer in order to simplify the animation. I also omitted many of the setup and cleanup commands, because including them would have taken literally all day, and they would also have made the stack really large and hard to read.

Here's the code that was demonstrated in the animation:

#include <iostream>
using namespace std;

void swap(int& a, int& b)
    int temp = a;
    a = b;
    b = temp;

void main(int, char**) {
    int number1 = 10;
    int number2 = 20;
    cout << "number1=" << number1 << ", number2=" << number2 << endl;

    // The animation starts here...
    swap(number1, number2);
    // ...and ends here.

    cout << "number1=" << number1 << ", b=" << number2 << endl;

If you spot any mistakes, please let me know! I'll fix them as soon as I can.

This animation was made thanks to the following software:

If you're interested, you can find the source files for the animation here (Yes, there's a mistake in frame 5 but it didn't make it through to the final product).

On CPU Registers in Assembly

I've been given some directed reading from the Intel Software Developer's Manual recently, and I found it complicated enough that I had to make about 2 1/2 pages of notes on what I read. This post is an attempt to explain to you what I learnt, in the hopes that I don't forget it! There will probably be mistakes in here - please point them out in the comments below!

The x86 instruction set contains a number of registers, each of which can be accessed in a number of different ways depending on the number of bits you wish to get or set.

Description 8 bit (byte) 16 bit (word) 32 bit (dword) 64 bit (qword)
General purpose AL AX EAX RAX +
General purpose BL BX EBX RBX +
General purpose CL CX ECX RCX +
General purpose DL DX EDX RDX +
General purpose (high byte) AH * - - -
General purpose (high byte) BH * - - -
General purpose (high byte) CH * - - -
General purpose (high byte) DH * - - -
Base Pointer BPL + SP ESP RSP +
Stack Pointer SPL + BP EBP RBP +
General Purpose R8L + R8W + R8D + R8 +
General Purpose R9L + R9W + R9D + R9 +
General Purpose R10L + R10W + R10D + R10 +
General Purpose R11L + R11W + R11D + R11 +
General Purpose R12L + R12W + R12D + R12 +
General Purpose R13L + R13W + R13D + R13 +
General Purpose R14L + R14W + R14D + R14 +
General Purpose R15L + R15W + R15D + R15 +

This table requires some explanation. Registers suffixed with * may only be utilised in 32 bit assembly. Similarly, registers suffixed with + may be utilised in 64 bit assembly only. Each row is a register, and each column represents a different number of bits. For example, the EAX register can be accessed as an 8 bit register with AL, 16 bit as AX, 32 bit as EAX, and 64 bit as RAX.

The exception here is the AL, BL, CL, DL, AH, BH, CH and DH registers. These actually refer to the same register. Let's take AL and AH for example. The AL register refers to the first 8 bits of the AX register, and the AH register refers to the second 8 bits. This is called the Low byte and the High byte.

In addition to the registers above, there are two others which can also be accessed as 8, 16, 32 and 64 bit registers:

Description 16 bit 32 bit 64 bit
Instruction Pointer IP EIP RIP

These registers should not be written to under normal usage though, as the CPU uses these to maintain its internal state. The instruction pointer points to the instruction that the CPU is currently executing, whereas the CPU Flags register holds all of the current flags, such as the result of a cmp (comparison).

That concludes this post on CPU registers. I don't think I can quite believe I'm posting this actually.... a year ago I would never have suspected I'd be learning about how to program the CPU itself in assembly.

Art by Mythdael