OSDev.org

Hello guys! I get the multiboot info, but I can not get a pixel going on the screen no matter what, can someone tell me what I am doing wrong? I am still learning OSDev, it's messy cause it's just an initial release where I am just testing stuff out

my linker.ld
```
ENTRY(start)

SECTIONS {
. = 1K;

.boot :
{
*(.multiboot)
}

.text :
{
*(.text)
}
}
```

```
// grub.cfg

set timeout=10
set default=0
set gfxpayload=keep

menuentry "zxOS" {
multiboot /boot/kernel.bin
}
```

```asm
; multiboot.asm
section .multiboot
align 4
dd 0x1BADB002
dd (1<<0)|(1<<1)|(1<<2)
dd -(0x1BADB002 + ((1<<0)|(1<<1)|(1<<2)))

dd 0
dd 0
dd 0
dd 0
dd 0

dd 0
dd 1024
dd 768
dd 32
```

```asm
; boot.asm
global start
extern kernel_main

section .text
bits 32
start:
push ebx
push eax
call kernel_main
end:
hlt
jmp end
```

```cpp
#ifndef IO_H
#define IO_H

#include "stdint.h"

static inline void outb(uint16_t port, uint8_t val) {
__asm__ volatile ("outb %0, %1" : : "a"(val), "Nd"(port));
}

static inline uint8_t inb(uint16_t port) {
uint8_t ret;
__asm__ volatile ("inb %1, %0" : "=a"(ret) : "Nd"(port));
return ret;
}

#endif
```

```cpp
#ifndef MULTIBOOT_INFO_H
#define MULTIBOOT_INFO_H

#include "stdint.h"

typedef struct __attribute__((packed)) multiboot_info {
uint32_t flags;

uint32_t mem_lower;
uint32_t mem_upper;

uint32_t boot_device;

uint32_t cmdline;

uint32_t mods_count;
uint32_t mods_addr;

uint32_t syms[4];

uint32_t mmap_length;
uint32_t mmap_addr;

uint32_t drives_length;
uint32_t drives_addr;

uint32_t config_table;

uint32_t boot_loader_name;

uint32_t apm_table;

uint32_t vbe_control_info;
uint32_t vbe_mode_info;
uint16_t vme_mode;
uint16_t vbe_interface_seg;
uint16_t vbe_interface_off;
uint16_t vbe_interface_len;

uint64_t framebuffer_addr;
uint32_t framebuffer_pitch;
uint32_t framebuffer_width;
uint32_t framebuffer_height;
uint8_t framebuffer_bpp;
uint8_t framebuffer_type;
uint8_t color_info[5];
} multiboot_info_t;

#endif
```

```cpp
// kernel.c
#include "multiboot_info.h"
#include "stdint.h"
#include "io.h"

void kernel_main(uint32_t magic, multiboot_info_t* mbi) {

if (magic != 0x2BADB002) {}

if (mbi->flags & (1 << 12)) {

uint8_t rp = mbi->color_info[0];
uint8_t gp = mbi->color_info[2];
uint8_t bp = mbi->color_info[4];

uint32_t red_pixel = (255 << rp); // full red
uint32_t green_pixel = (255 << gp); // full green
uint32_t blue_pixel = (255 << bp); // full blue

uint32_t* framebuffer = (uint32_t*)(uint32_t)mbi->framebuffer_addr;

framebuffer[mbi->framebuffer_pitch / 4] = red_pixel;

while (1) {}
}

while (inb(0x64) & 0x02) {}
// Pulse CPU reset
outb(0x64, 0xFE);
}
```

```
all:
nasm -f elf32 src/asm/multiboot.asm -o output/multiboot.o
nasm -f elf32 src/asm/boot.asm -o output/boot.o

gcc -m32 -ffreestanding -fno-builtin -fno-pie -fno-stack-protector -Iinclude -g -c src/c/kernel.c -o output/kernel.o

ld -m elf_i386 --nmagic --output=iso/boot/kernel.bin --script=linker.ld \
output/multiboot.o output/boot.o output/kernel.o

grub-mkrescue -o os.iso iso
```

I run with qemu-system-x86_64.exe os.iso

What could the issue be? Any help is appreciated!

I haven’t checked your code, but are you sure you are not outputting the pixel? A single pixel is pretty small, and red is probably the least visible colour. You might want to try displaying a 10x10 block rather than a single pixel.

itsaxg wrote: ↑Tue Sep 30, 2025 11:56 amI get the multiboot info, but I can not get a pixel going on the screen no matter what, can someone tell me what I am doing wrong?

What does your debugger say about the multiboot info? Do the values look reasonable?

itsaxg wrote: ↑Tue Sep 30, 2025 11:56 am

Code: Select all

    . = 1K;

I'm not sure GRUB works properly if you tell it to load your kernel below 1M.

Thank you so much for the reply!
Switching to 1M seems to have fixed the issue!
Now the pixel shows up!