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Article viewer

Simple ASM program



Written by:dimport
Published by:sefo
Published on:2003-06-21 07:19:46
Topic:Assembly
Search OSI about Assembly.More articles by dimport.
 viewed 13687 times send this article printer friendly

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This simple ASM program is designed to show you some of the syntax, and get you accustomed to how the code is written.

This example prints a character to the screen

An assembler is required to turn this into an executable application. Look here for details of downloading assemblers

.MODEL SMALL
 
.CODE

MOV AH,2 ;COMMAND TO PRINT CHAR
MOV DL,'J' ; CHARACTER TO PRINT
INT 21h ;INTERRUPT TO PRINT CHAR
INT20h ; RETURN SAFELY TO DOS
 
END


This article was originally written by JagWire

Did you like this article? There are hundreds more.

Comments:
typedeaF
2006-07-20 17:47:15
Horrible?
The comments tell you exactly whats going on... if you already understand whats going on :/ Even then, they are still vague.

This program makes use the the DOS service dispatcher which is called by making a software interrupt to vector 0x21. The particular service is called "Display Character" which is service 0x02. This is why AH is loaded with 2. The Print Character service also expects the character to be loaded in DL. Interrupt vector 0x20 is one of several ways to return control back to the program.

A lot could be said alone on software interrupts and the interrupt vector table before even getting into the particular services the DOS service dispatcher.

Learning assembly isnt about memorizing code snippits and instructions, its about memory addressing and memory management.

typedeaF
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Blogs: (People who have posted blogs on this subject..)
jackier
jackier on Mon 13th Oct 10am
111
sefo
Sneak - encryption on Fri 17th Nov 12pm
I'm developing the win32 version of sneak: http://snarkles.net/scripts/sneak/sneak. php The ASM source code is available on cyberarmy svn (for members only - free) Check the forum for details: http://www.cyberarmy.net/forum/sneak/mes sages/295244.
sefo
Geek Toolbar on Mon 13th Nov 8am
This a very simple and small toolbar I wrote in my spare time. I use the same set of tools very often and I find it very annoying to look for them in the start menu, on the desktop or in explorer. http://www.osix.net/modules/folder/index .php?tid=134
sefo
BinScan and Alternate Data Stream on Thu 27th Jul 9am
BinScan I created this tool to quickly identify modifications in the PE, use of a TLS callback and Alternate Data Streams. -> Some modifications done in the PE structure of an executable can prevent debuggers or other tools to open the executable.
sefo
Wmf Creator on Wed 26th Jul 7am
Now that the blog is online, I'll be able to share two or three tools I wrote. The first one I'd like to share is WMF Creator. I already put a link in the comments of my article: Wmf Exploit but I thought it would look nicer here. This tool will tak

Test Yourself: (why not try testing your skill on this subject? Clicking the link will start the test.)
Reverse Engineering by Geek_Freek

A test to check your assembly and reversing skills.
Assembly Language - non compiler specific by TroPe

You can test your assembly knowledge by taking this test. It starts out relatively easy, but gets progressively hards very quickly! If you know assembly, or just want to see what you DONT know about assembly, this test is for you. A more advanced assembly


     
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