gcc realmode

[driechers]

hey i am working on making an os and i am wondering if anybody knows the option to get the gcc compiler to work in 16 bit or real mode.


any help wold be helpful

[gerard4143]

I look at the very same thing and the closest I could get was using the asm directive .code16...Hope this helps...Gerard4134

i.e. __asm__ (".code16\n\t"); in C ocde

or

.code16

in assembler

[gerard4143]

Here's an example of making a boot disk that will display G4143 in the top left hand corner and then loop forever....G4143

Just copy the code, compile and then place a floppy in the floppy drive and execute and presto a bootable floppy that will boot up and display 'G4143'....Note the hex or assembler is 16 bit...

#include<stdio.h>
#include<stdlib.h>
#include<fcntl.h>

/*
.code16

.section .data

.scetion .text
	.global _start
_start:
			movw	$0xb800, %ax
			movw	%ax, %es

			movb	$0x41, %es:0
			movb	$0x1f, %es:1

			movb	$0x34, %es:2
			movb	$0x1f, %es:3

			movb	$0x31, %es:4
			movb	$0x1f, %es:5

			movb	$0x34, %es:6
			movb	$0x1f, %es:7

			movb	$0x33, %es:8
			movb	$0x1f, %es:9
loop1:
			jmp	loop1
*/

//displays G4143 at boot up and the loop forever

char boot_buf[512] =	{                         
0xb8,0x00,0xb8,0x8e,0xc0,0x26,0xc6,0x06,0x00,0x00,0x47,0x26,0xc6,0x06,0x01,0x00,0x1f,0x26,0xc6, 
0x06,0x02,0x00,0x34,0x26,0xc6,0x06,0x03,0x00,0x1f,0x26,0xc6,0x06,0x04,0x00,0x31,0x26,0xc6,0x06, 
0x05,0x00,0x1f,0x26,0xc6,0x06,0x06,0x00,0x34,0x26,0xc6,0x06,0x07,0x00,0x1f,0x26,0xc6,0x06,0x08, 
0x00,0x33,0x26,0xc6,0x06,0x09,0x00,0x1f,0xeb,0xfe		
			};

int main(int argc, char**argv)
{
	int floppy_desc;

boot_buf[510]=0x55;//to make the floppy/image bootable
boot_buf[511]=0xaa;//to make the floppy/image bootable

floppy_desc=open("/dev/fd0",O_RDWR);
lseek(floppy_desc,0,SEEK_CUR);
write(floppy_desc,boot_buf,512);
close(floppy_desc);
}

[gerard4143]

Also if you haven't already install 'Qemu launcher' this piece of software will allow you to boot your OS image on your own machine...I use it and find it simple and easy to use...Gerard4143

[driechers]

thanks
ten chars

[driechers]

wow this is great the code answered 6 other questions of mine

[gerard4143]

Check out this link:

Back to Basics: Creating a Bootloader from Scratch - R&D by Michiel van Oosterhout

Gerard4143

[Rubberman]

You can use nasm, which is an 8086 assembler for Linux. You should be able to install it from your package manager.

[gerard4143]

Do you know how to compile an assembler file so that it strips all the header/symbol information. I want just the text section remaining without any extra information...Gerard4143

Note I would prefer GNU as assembler instructions but if you know how to do it in NASM I'll take that...Gerard4143

[Rubberman]

Do you know how to compile an assembler file so that it strips all the header/symbol information. I want just the text section remaining without any extra information...Gerard4143

Note I would prefer GNU as assembler instructions but if you know how to do it in NASM I'll take that...Gerard4143

Sorry. I haven't personally used an assembler on a Linux system, other than assembler directives in a kernel module with GCC.