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> Linux > Simple mathmatics written in Assembly code using nasm |
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DavidHalliday 19 Jan 2007, 17:25
I'm aware that I'm using nasm http://nasm.sourceforge.net/ assembler (differnt from most people here) but this seemed like the best forum I can find. I'm running Ubuntu Linux
I have had a go at a tutorial I got in a linux format magazine http://www.linuxformat.co.uk/ and I have found a similar tutorial on this site: http://docs.cs.up.ac.za/programming/asm/derick_tut/ I'm haveing difficulty with adapting this to do something with numbers. In short all I want to do is make a simple assembly example (for teaching) that does the following: 1: takes two numbers input (optional as I'm guessing this is complex?) 2: adds them together 3: Displays the result I've had a go at it but without luck yet. Can anyone tell me if i'm making some stupid/simple mistake or give me a pointer to a website with a simple solution. So far all I have found (in 4 hours of googling) have been: helo world tutorials (got that working good  )
tutorials that look confusingly different from nasm examples that are too complex for my level. And i found the nasm manual too technicxal for my relative beginner status Any help would be much apreciated. Here is my code: Code: section .data
section .text
global _start
_start:
mov eax, 5 ; set eax to 5
mov ebx, 10 ; set ebx to 10
mov ecx, 0 ; set ecx to 0
mathfunc:
dec eax ; decrease eax by 1
add ecx, ebx ; add ebx to ecx (store in ecx)
cmp eax, 1 ; Compare eax with 1
je output ; If so, jump to output
jmp mathfunc ; And jump back to start of function
output:
mov eax, 4 ; tell kernel to output
mov ebx, 1 ; use standard output
; ecx already contains output (i think)
mov edx, 5 ; output length?
int 80h
finish:
mov eax, 1 ; Kernel 'exit prog' routine
mov ebx, 0
int 80h ; Call kernel I should point out that I'm a teacher who has tried many things in the world of computers but this is my first dip into assembly. I'm aiming to use this as an example for some students who have to learn the basics of what assembly is for a course (and i figured learn by doing was the best option). This has proved far harder than I ever imagined... or I'm just having a day of stupidity. Once I get this sorted I'll write up notes and get distributing them for others like me |
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19 Jan 2007, 17:25 |
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rugxulo 19 Jan 2007, 20:33
http://www.drpaulcarter.com/pcasm/
This covers NASM and 32-bit Pmode programming (Linux GCC, DJGPP, OpenWatcom, Borland, MSVC). |
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19 Jan 2007, 20:33 |
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DavidHalliday 22 Jan 2007, 16:51
vid wrote: for int80h/5, ECX should contain pointer to buffer, which holds data to write. in your case, ECX holds just some number (50?), not pointer to data. I was hopeing that there was an easy way to convert integer to text. the closest (easy/simple to demonstrate) thing I could find was manipulating hex values. But thanks for the help |
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22 Jan 2007, 16:51 |
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DavidHalliday 22 Jan 2007, 17:24
rugxulo wrote: http://www.drpaulcarter.com/pcasm/ Thanks for that link It was just what I needed. I was able to use the library functions (and c code) to wrap arround my assembly which is perfect for the demmonstration I want to do. I'd love to learn to do assembly properly but I think that will have to be put asside. Most importantly I can teach my students more than/better than I was taught myself (which Is something I always aim to do) Before Friday I'd never tried to make assembly code work. My code still isn't perfect but I'm happy 
Just in case someone has the same situation as me my reccommendation is to get both the pdf and the example code file from this site: http://www.drpaulcarter.com/pcasm/ If it helps anyone then here is my code etc: Code: %include "asm_io.inc" ;library with input/output routines section .data section .text global asm_main asm_main: mov eax, 5 ; set eax to 5 mov ebx, 10 ; set ebx to 10 mov ecx, 0 ; set ecx to 0 mathfunc: dec eax ; decrease eax by 1 add ecx, ebx ; add ebx to ecx (store in ecx) cmp eax, 0 ; Compare eax with 1 je output ; If so, jump to output jmp mathfunc ; And jump back to start of function output: mov eax, ecx ; eax is used to store the value output by print_int call print_int ; output integer call print_nl ; output new line finish: mov eax, 1 ; Kernel 'exit prog' routine mov ebx, 0 int 80h ; Call kernel Using the example code file I was able to get my code (which I had to modify slightly) working using the following instructions (you may have to check the order): Code: gcc -c driver.c nasm -f elf MYCODE.asm gcc -o OUTPUT.FILE driver.o MYCODE.o asm_io.o If you change the MYCODE.asm file then you have to do the last two steps to make the changes to the output. Hope this helps others... and not just to cheet on their homework (Which seems to be what most people asking many of the questions I have been googling are after). |
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22 Jan 2007, 17:24 |
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vid 22 Jan 2007, 19:10
you might be interested in my project FASMLIB. It is portable library for 32bit assembler. Something like functions in asm_io.inc, but more advanced, with little better API etc.
Also, functions are called exactly same way on linux and windows, and code using my library is portable over these OSes. If you are interested, mail me at vid (at) inamail (dot) sk, i can explain details and usage to you. or even better, contact me with ICQ. |
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22 Jan 2007, 19:10 |
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