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Octocontrabass wrote:The Multiboot specifications include example code. Which part do you not understand?

Caffeine wrote:

Octocontrabass wrote:The Multiboot specifications include example code. Which part do you not understand?

The part I am mainly confused on (at least right now) is what the argument addr would be and how I would find out addr in the function cmain() in the Kernel example code found here: https://www.gnu.org/software/grub/manua ... ot-kernels.

I am sorry for all the confusion.

Octocontrabass wrote:When GRUB jumps to your kernel's entry point, the address is in the EBX register. You need to write assembly code that will take the address in EBX and put it somewhere so you can use it.

In the i386 psABI, parameters are passed to functions by pushing them on the stack. The example boot.S code pushes EBX onto the stack to pass the address as a parameter to the cmain function.

Caffeine wrote:All I can find online is passing c functions to assembly, not the other way around.

Octocontrabass wrote:

Caffeine wrote:All I can find online is passing c functions to assembly, not the other way around.

The sample code in the Multiboot specification demonstrates how to call a C function from assembly.

There are also plenty of other examples out there. Perhaps you need to change the keywords you use when you search.

And it's a good idea to check the i386 psABI for the details.

Octocontrabass wrote:

Caffeine wrote:All I can find online is passing c functions to assembly, not the other way around.

The sample code in the Multiboot specification demonstrates how to call a C function from assembly.

There are also plenty of other examples out there. Perhaps you need to change the keywords you use when you search.

And it's a good idea to check the i386 psABI for the details.

push eax

Octocontrabass wrote:The pointer to the Multiboot information structure is in EBX, not EAX.

void initializeVGA(unsigned long addr) {
    unsigned char* location = (unsigned char*)addr + (320 * 100 + 100);
    *location = 4;

}

Octocontrabass wrote:The address is the address of the Multiboot information structure, not the address of the framebuffer. You need to parse the multiboot information structure to find the framebuffer address.

For example:

Code: Select all

void example( uintptr_t addr )
{
    multiboot_info_t * mbi = (multiboot_info_t *)addr;
    uint8_t * framebuffer = (uint8_t *)mbi->framebuffer_addr;
    switch( mbi->framebuffer_bpp )
    {
        case 8:
            framebuffer[mbi->framebuffer_pitch * 100 + 1 * 100] = 4;
            break;
        case 15:
        case 16:
            framebuffer[mbi->framebuffer_pitch * 100 + 2 * 100] = 0xff;
            break;
        case 24:
            framebuffer[mbi->framebuffer_pitch * 100 + 3 * 100] = 0xff;
            break;
        case 32:
            framebuffer[mbi->framebuffer_pitch * 100 + 4 * 100] = 0xff;
            break;
        default:
            // error
            break;
    }
}

unsigned int *framebuffer;
multiboot_info_t *mbi;

void initializeVGA(unsigned long addr) {
    mbi = (multiboot_info_t *)addr;  // Get the multiboot information from the address
    framebuffer = (unsigned int *)mbi->framebuffer_addr;  // Get the framebuffer address

}

void plotPixle(int x, int y, unsigned char color) {
    switch (mbi->framebuffer_bpp) {
    case 8:
        framebuffer[mbi->framebuffer_pitch * y + 1 * x] = 4;
        break;
    case 15:
        break;
    case 16:
        framebuffer[mbi->framebuffer_pitch * y + 2 * x] = 0xff;
        break;
    case 24:
        framebuffer[mbi->framebuffer_pitch * y + 3 * x] = 0xff;
        break;
    case 32:
        framebuffer[mbi->framebuffer_pitch * y + 4 * x] = 0xff;
        break;
    default:
        // error
        break;
    }

}

Octocontrabass wrote:Yes, those hex values determine the color. However, this example code only sets one byte, and pixels are usually more than one byte. (Most common seems to be 32bpp, which is four bytes per pixel.)

Octocontrabass wrote:Have you tried adding 1 to the array index to set the second byte?