Pass Multiboot structure pointer as argument

src/arch/x86_64/boot.asm

@@ -19,6 +19,7 @@ section .text
 bits 32
 start:
     mov esp, stack_top
+    mov edi, ebx
 
     call check_multiboot
     call check_cpuid

src/arch/x86_64/long_mode_init.asm

@@ -20,7 +20,7 @@ bits 64
 long_mode_start:
     call setup_SSE
 
-    ; call rust main
+    ; call rust main (with multiboot pointer in rdi)
     call rust_main
 .os_returned:
     ; rust main returned, print `OS returned!`

src/lib.rs

@@ -24,7 +24,7 @@ use core::fmt::Write;
 mod vga_buffer;
 
 #[no_mangle]
-pub extern fn rust_main() {
+pub extern fn rust_main(multiboot_address: usize) {
     // ATTENTION: we have a very small stack and no guard page
     use vga_buffer::{Writer, Color};