Pass multiboot pointer as argument to rust_main

posts/DRAFT-remapping-the-kernel.md

@@ -8,6 +8,25 @@ TODO
 ## The Multiboot Information Structure
 When a Multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information structure in the `ebx` register. We can use it to get information about available memory and loaded kernel sections.
 
+First, we need to pass this pointer to our kernel as an argument to `rust_main`. To find out how arguments are passed to functions, we can look at the [calling convention of Linux]:
+
+[calling convention of Linux]: https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI
+
+> The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9
+
+So to pass the pointer to our kernel, we need to move it to `rdi` before calling the kernel. Since we're not using the `rdi`/`edi` register in our bootstrap code right now, we can simply set the `edi` register right after booting (in `boot.asm`):
+
+```nasm
+start:
+    mov esp, stack_top
+    mov edi, ebx       ; Move Multiboot info pointer to edi
+```
+Now we can add the argument to our `rust_main`:
+
+```rust
+pub extern fn rust_main(multiboot_information_address: usize) { ... }
+```
+
 ## Start and End of Kernel
 We can now use the ELF section tag to calculate the start and end address of our loaded kernel:
 

src/arch/x86_64/boot.asm

@@ -19,6 +19,9 @@ section .text
 bits 32
 start:
     mov esp, stack_top
+    ; Move Multiboot info pointer to edi to pass it to the kernel. We must not
+    ; modify the `edi` register until the kernel it called.
+    mov edi, ebx
 
     call test_multiboot
     call test_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

@@ -23,7 +23,7 @@ extern crate spin;
 mod vga_buffer;
 
 #[no_mangle]
-pub extern fn rust_main() {
+pub extern fn rust_main(multiboot_information_address: usize) {
     // ATTENTION: we have a very small stack and no guard page
     vga_buffer::clear_screen();
     println!("Hello World{}", "!");