a cow based x86_64 operating system, using limine and stivale2
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#include <stivale2.h>
#include <stddef.h>
#include <stdint.h>
// We need to tell the stivale bootloader where we want our stack to be.
// We are going to allocate our stack as an uninitialised array in .bss.
static uint8_t stack[4096];
// stivale2 uses a linked list of tags for both communicating TO the
// bootloader, or receiving info FROM it. More information about these tags
// is found in the stivale2 specification.
// stivale2 offers a runtime terminal service which can be ditched at any
// time, but it provides an easy way to print out to graphical terminal,
// especially during early boot.
// Read the notes about the requirements for using this feature below this
// code block.
static struct stivale2_header_tag_terminal terminal_hdr_tag = {
// All tags need to begin with an identifier and a pointer to the next tag.
.tag = {
// Identification constant defined in stivale2.h and the specification.
// If next is 0, it marks the end of the linked list of header tags.
.next = 0
// The terminal header tag possesses a flags field, leave it as 0 for now
// as it is unused.
.flags = 0
// We are now going to define a framebuffer header tag, which is mandatory when
// using the stivale2 terminal.
// This tag tells the bootloader that we want a graphical framebuffer instead
// of a CGA-compatible text mode. Omitting this tag will make the bootloader
// default to text mode, if available.
static struct stivale2_header_tag_framebuffer framebuffer_hdr_tag = {
// Same as above.
.tag = {
// Instead of 0, we now point to the previous header tag. The order in
// which header tags are linked does not matter.
.next = (uint64_t)&terminal_hdr_tag
// We set all the framebuffer specifics to 0 as we want the bootloader
// to pick the best it can.
.framebuffer_width = 0,
.framebuffer_height = 0,
.framebuffer_bpp = 0
// The stivale2 specification says we need to define a "header structure".
// This structure needs to reside in the .stivale2hdr ELF section in order
// for the bootloader to find it. We use this __attribute__ directive to
// tell the compiler to put the following structure in said section.
__attribute__((section(".stivale2hdr"), used))
static struct stivale2_header stivale_hdr = {
// The entry_point member is used to specify an alternative entry
// point that the bootloader should jump to instead of the executable's
// ELF entry point. We do not care about that so we leave it zeroed.
.entry_point = 0,
// Let's tell the bootloader where our stack is.
// We need to add the sizeof(stack) since in x86(_64) the stack grows
// downwards.
.stack = (uintptr_t)stack + sizeof(stack),
// Bit 1, if set, causes the bootloader to return to us pointers in the
// higher half, which we likely want.
.flags = (1 << 1),
// This header structure is the root of the linked list of header tags and
// points to the first one in the linked list.
.tags = (uintptr_t)&framebuffer_hdr_tag
// We will now write a helper function which will allow us to scan for tags
// that we want FROM the bootloader (structure tags).
void *stivale2_get_tag(struct stivale2_struct *stivale2_struct, uint64_t id) {
struct stivale2_tag *current_tag = (void *)stivale2_struct->tags;
for (;;) {
// If the tag pointer is NULL (end of linked list), we did not find
// the tag. Return NULL to signal this.
if (current_tag == NULL) {
return NULL;
// Check whether the identifier matches. If it does, return a pointer
// to the matching tag.
if (current_tag->identifier == id) {
return current_tag;
// Get a pointer to the next tag in the linked list and repeat.
current_tag = (void *)current_tag->next;
// The following will be our kernel's entry point.
void _start(struct stivale2_struct *stivale2_struct) {
// Let's get the terminal structure tag from the bootloader.
struct stivale2_struct_tag_terminal *term_str_tag;
term_str_tag = stivale2_get_tag(stivale2_struct, STIVALE2_STRUCT_TAG_TERMINAL_ID);
// Check if the tag was actually found.
if (term_str_tag == NULL) {
// It wasn't found, just hang...
for (;;) {
asm ("hlt");
// Let's get the address of the terminal write function.
void *term_write_ptr = (void *)term_str_tag->term_write;
// Now, let's assign this pointer to a function pointer which
// matches the prototype described in the stivale2 specification for
// the stivale2_term_write function.
void (*term_write)(const char *string, size_t length) = term_write_ptr;
// We should now be able to call the above function pointer to print out
// a simple "Hello World" to screen.
term_write("Bincows beta", 12);
// We're done, just hang...
for (;;) {
asm ("hlt");