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I want to draw a colored line in asm. I have to use AT&T syntax, on an x86 intel linux unit. I got pretty far already, but I'd like to know how to enter VGA-mode, or Mode-X, and how to put a pixel on the screen. Is there a function for that in a standard C library (like printf)?

Thanks a lot for your help. :)

 
.bss # Declaring uninitialized variables
 
.data # Declaring initialized variables
 
.text # Actual assembly code and constants
 
intro: .asciz "Draw a line in VGA\n\n" clr: .asciz "Give a color \n" optns: .asciz "red (1), blue (2), white (3)\n" res .asciz "%d" ent: .asciz "\n" .global main # Tell kernel where to start (visible from outside) 
 
main: pushl %ebp # Push base pointer movl %esp, %ebp # Initialize base pointer pushl $intro # Push the string address call printf # Call the printf routine from C library addl $8, %esp
 
pushl $clr # push color question on the stack call printf # Print it subl $4, %esp # Reserve stack space for variable leal -4(%ebp), %eax # Load address of stack var in eax pushl %eax # Push second argument of scanf pushl $rets # Push first argument of scanf call scanf # Call scanf movl 4(%ebp), %ecx # mov the result in ecx cmpl $1, %ecx je red cmpl $2, %ecx je blue jne white 
 
red: #... still working on this
 
 movl 0013h, %eax # enter 320x200x256 mode int 10h # IS THIS CORRECT? movl $0, %ebx # set X to 0 movl $0, %ecx # set Y to 0 call draw # Call line routine movl 0003h, %eax # IS THIS CORRECT? int 10h # return to text mode movl $0, (%esp) # Make esp 0, indicating succesful termination call exit # Exit the program 
 
draw:
 call putpixel # pushl %ebp # Push the base pointer # movl %esp, %ebp # Initialize base pointer inc %ebx # increment X inc %ecx # increment Y cmpl $200, %ecx # check if Y => 200 jge end # if Y=> 200, jump to end jmp draw # loop 
 
putpxl: #has to put a pixel at (%ebx, %ecx) and return to draw # should use the color from the main routine
 
end: movl %ebp, %esp # Remove local variables popl %ebp # Remove base pointer ret # return to main routine

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2 Answers 2

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Best thing you could do would be to use a higher-level library, like SDL or Allegro. This way, your program will work on top of X11 and on non-VGA framebuffers (e.g: displays on embedded ARM-based devices).

Svgalib allowed to program for VGA and so-called SVGA graphics cards. Its development stopped years ago.

There are two ways to access VGA hardware:

  • Using registers
  • Using the VGA BIOS: this is the one you've tried. VGA BIOSes are written to run in real mode. To invoke them from protected mode, you would need to switch to vm86 mode. which is what LRMI (Linux Real Mode Interface, the opposite of DPMI) did.
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Thank you. I'll try the vm86 mode. I think that what's the goal of my assignment... Master was very vague though :(
@CurveBall: are you sure you need to go VGA? maybe your teacher wanted you to use the framebuffer?
He specifically pointed me to VGA/Mode-X. This is the reference he gave me (and particularly VGA/Mode-X): archive.gamedev.net/reference/list.asp?categoryid=130 But that's on DOS and since osgx said that linux is a protected OS, I guess that's not possible. Maybe he doesn't know it doesn't work on linux... He asked me what OS I'd use, I said Linux, and he responded something like "This reference will probably suite for Linux as well"... Maybe I should contact him to tell him about this problem? BTW: I appreciate you guys' help a lot :)
Just in case you weren't sure, the parts where you commented # IS THIS CORRECT? were, in fact, correct. Unfortunately, in Protected Mode you aren't allowed to communicate with hardware like this (your code would have been just fine on MS-DOS)
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You are not allowed to do direct video (graphic mode) output in linux (protected OS). it was possible only in a dos (real-mode OS).

You can use framebuffer device or SDL, or libX11. But there is no reason why to do work with high-level libraries from the asm.

Int 10 is not allowed in linux directly (in user mode; the kernel must do low-level accesses).

e.g. this is 5 year old exact the same discussion http://www.gamedev.net/topic/368196-linux-assembler-int-10h/

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Thanks a lot! :) So basically, what you're saying is that this is not possible in a simple way on linux without higher level libraries?
Yes, outputting of video or changing of video mode is not possible without help of kernel. Easiest way to get this help is to use library. But You can use library only for outputing prerendered pixmap and generate the pixmap with assembler.

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