From 85e5b76aa31f56de2435d69b404d8f93086a174c Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Mon, 19 Oct 2020 21:42:14 -0400
Subject: [PATCH] Move multiboot2 header from assembly to C.

---
 src/boot.S             | 30 ------------------------
 src/link.ld            | 53 +++++++++++++++---------------------------
 src/multiboot2.h       | 37 +++++++++++++++++++++++++++++
 src/multiboot_header.c | 12 ++++++++++
 4 files changed, 68 insertions(+), 64 deletions(-)
 create mode 100644 src/multiboot2.h
 create mode 100644 src/multiboot_header.c

diff --git a/src/boot.S b/src/boot.S
index 3c04a19..90b5149 100644
--- a/src/boot.S
+++ b/src/boot.S
@@ -1,33 +1,3 @@
-/* Declare constants for the multiboot2 header. */
-.set MAGIC, 0xE85250D6       /* 'magic number' lets bootloader find the header */
-.set NTAGS, 1 /* including terminating tag */
-.set LENGTH, 4 * 4 + NTAGS
-.set CHECKSUM, -(MAGIC + LENGTH) /* checksum of above, to prove we are multiboot */
- 
-/* 
-Declare a multiboot header that marks the program as a kernel. These are magic
-values that are documented in the multiboot standard. The bootloader will
-search for this signature in the first 8 KiB of the kernel file, aligned at a
-32-bit boundary. The signature is in its own section so the header can be
-forced to be within the first 8 KiB of the kernel file.
-*/
-.section .multiboot
-.align 4
-.long MAGIC                 /* magic */
-.long 0                     /* architecture */
-.long LENGTH                /* header_length */
-.long CHECKSUM
-.short 5
-.short 0
-.long 20
-.long 0
-.long 0
-.long 0
-.long 0 /* padding */
-.short 0
-.short 0
-.long 8
- 
 /*
 The multiboot standard does not define the value of the stack pointer register
 (esp) and it is up to the kernel to provide a stack. This allocates room for a
diff --git a/src/link.ld b/src/link.ld
index 38ddd94..66b9c93 100644
--- a/src/link.ld
+++ b/src/link.ld
@@ -1,43 +1,28 @@
-/* The bootloader will look at this image and start execution at the symbol
-   designated as the entry point. */
 ENTRY(_start)
  
-/* Tell where the various sections of the object files will be put in the final
-   kernel image. */
 SECTIONS
 {
-	/* Begin putting sections at 1 MiB, a conventional place for kernels to be
-	   loaded at by the bootloader. */
-	. = 1M;
+    . = 1M;
  
-	/* First put the multiboot header, as it is required to be put very early
-	   early in the image or the bootloader won't recognize the file format.
-	   Next we'll put the .text section. */
-	.text BLOCK(4K) : ALIGN(4K)
-	{
-		*(.multiboot)
-		*(.text)
-	}
+    .text BLOCK(4K) : ALIGN(4K)
+    {
+        *(.multiboot)
+        *(.text)
+    }
  
-	/* Read-only data. */
-	.rodata BLOCK(4K) : ALIGN(4K)
-	{
-		*(.rodata)
-	}
+    .rodata BLOCK(4K) : ALIGN(4K)
+    {
+        *(.rodata)
+    }
  
-	/* Read-write data (initialized) */
-	.data BLOCK(4K) : ALIGN(4K)
-	{
-		*(.data)
-	}
+    .data BLOCK(4K) : ALIGN(4K)
+    {
+        *(.data)
+    }
  
-	/* Read-write data (uninitialized) and stack */
-	.bss BLOCK(4K) : ALIGN(4K)
-	{
-		*(COMMON)
-		*(.bss)
-	}
- 
-	/* The compiler may produce other sections, by default it will put them in
-	   a segment with the same name. Simply add stuff here as needed. */
+    .bss BLOCK(4K) : ALIGN(4K)
+    {
+        *(COMMON)
+        *(.bss)
+    }
 }
diff --git a/src/multiboot2.h b/src/multiboot2.h
new file mode 100644
index 0000000..e8cd8d9
--- /dev/null
+++ b/src/multiboot2.h
@@ -0,0 +1,37 @@
+#ifndef MULTIBOOT2_H
+#define MULTIBOOT2_H
+
+#include <stdint.h>
+
+#define MULTIBOOT2_MAGIC 0xE85250D6u
+#define MULTIBOOT2_ARCHITECTURE_I386 0u
+
+typedef struct {
+    uint32_t magic;
+    uint32_t architecture;
+    uint32_t header_length;
+    uint32_t checksum;
+} multiboot2_header_t;
+#define multiboot2_header(magic, architecture, header_length) \
+    {(magic), (architecture), (header_length), (uint32_t)(0x100000000u - (magic) - (architecture) - (header_length))}
+#define multiboot2_header_default() \
+    multiboot2_header(MULTIBOOT2_MAGIC, MULTIBOOT2_ARCHITECTURE_I386, sizeof(multiboot2_header_t))
+
+typedef struct {
+    uint16_t type;
+    uint16_t flags;
+    uint32_t size;
+} multiboot2_tag_t;
+#define multiboot2_end_tag() {0u, 0u, 8u}
+
+typedef struct {
+    multiboot2_tag_t header;
+    uint32_t width;
+    uint32_t height;
+    uint32_t depth;
+    uint32_t _padding;
+} multiboot2_framebuffer_tag_t;
+#define multiboot2_framebuffer_tag(width, height, depth) \
+    {{5u, 0u, 20u}, width, height, depth}
+
+#endif
diff --git a/src/multiboot_header.c b/src/multiboot_header.c
new file mode 100644
index 0000000..93a0d44
--- /dev/null
+++ b/src/multiboot_header.c
@@ -0,0 +1,12 @@
+#include "multiboot2.h"
+
+/* Multiboot2 Header. */
+struct {
+    multiboot2_header_t header;
+    multiboot2_framebuffer_tag_t framebuffer_tag;
+    multiboot2_tag_t end_tag;
+} multiboot_header __attribute__((section(".multiboot"))) = {
+    multiboot2_header_default(),
+    multiboot2_framebuffer_tag(0u, 0u, 0u),
+    multiboot2_end_tag(),
+};