Question

I am writing a kernel, so that i am starting with a hello world program in kernel.

I have written a hello world kernel in c++ and it compiles successfully.

But when i boot it, it does not show anything on screen.

What’s wrong with this code?

link.ld

OUTPUT_FORMAT("binary")
ENTRY(start)
SECTIONS{
. = 0x00100000;

.text :{
    *(.text)
}

.rodata ALIGN (0x1000) : {
    *(.rodata)
}

.data ALIGN (0x1000) : {
    *(.data)
}

.bss : {
    sbss = .;
    *(COMMON)
    *(.bss)
    ebss = .;
}
}

loader.asm

[BITS 32]

global start
extern _main

start:
    call _main
    cli 
    hlt

video.h

#ifndef VIDEO_H
#define VIDEO_H

class Video{
    public:
        Video();
        ~Video();
        void clear();
        void write(char *cp);
        void put(char c);
    private:
        unsigned short *videomem;
        unsigned int off;
        unsigned int pos;
};
#endif

video.cpp

#include "Video.h"

Video::Video(){
pos = 0;
off = 0;
videomem = (unsigned short*)0xb8000;
}

Video::~Video(){}

void Video::clear(){
unsigned int i;
for(i=0;i<(80*25);i++){
    videomem[i] = (unsigned short)' '|0x0700;
}
pos = 0;
off = 0;
}

void Video::write(char *cp){
char *str = cp, *ch;
for(ch=str;*ch;ch++){
    put(*ch);
}
}

void Video::put(char c){
if(pos>=80){
    pos = 0;
    off+=80;
}
if(off>=(80*25)){
    clear();
}

videomem[off+pos] = (unsigned short)c|0x0700;
pos++;
}

Kernel.cpp

#include "Video.h"
int _main(void){
Video vid;
vid.clear();
vid.write("Hello World!");
}

I am compiling it using this commands:

g++ -c video.cpp -ffreestanding -nostdlib -fno-builtin -fno-rtti -fno-exceptions
g++ -c Kernel.cpp -ffreestanding -nostdlib -fno-builtin -fno-rtti -fno-exceptions
nasm -f aout Loader.asm -o Loader.o
ld -T linker.ld -o Kernel.bin Loader.o Video.o Kernel.o

It does not give any errors.

If it is possible to debug, then please help me to how to debug.

I am booting it in virtual box.

Any help will be appreciated.

Answer 1

Have you tried qemu? When making a tiny os at uni, it proved to be the best program for this type of stuff.