From 412925a790caf53738d002d51cab1ff4f7c0ae26 Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Sun, 18 Oct 2020 14:25:06 -0400
Subject: [PATCH] switch from multiboot to multiboot2

---
 Rsconscript       |  2 +-
 src/boot.S        | 30 +++++++++++++++---------------
 src/kernel_main.c | 42 ++++++++++++++++++++++++++++++++++++------
 3 files changed, 52 insertions(+), 22 deletions(-)

diff --git a/Rsconscript b/Rsconscript
index af7e747..b0c3b61 100644
--- a/Rsconscript
+++ b/Rsconscript
@@ -24,7 +24,7 @@ build do
           File.open("#{tmpdir}/iso/boot/grub/grub.cfg", "wb") do |fh|
             fh.write(<<EOF)
 menuentry "HOS" {
-  multiboot /hos.elf
+  multiboot2 /hos.elf
 }
 EOF
           end
diff --git a/src/boot.S b/src/boot.S
index 06bfd9e..3c04a19 100644
--- a/src/boot.S
+++ b/src/boot.S
@@ -1,10 +1,8 @@
-/* Declare constants for the multiboot header. */
-.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
-.set MEMINFO,  1<<1             /* provide memory map */
-.set VID, 1<<2
-.set FLAGS,    ALIGN | MEMINFO | VID  /* this is the Multiboot 'flag' field */
-.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
-.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+/* 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
@@ -15,18 +13,20 @@ forced to be within the first 8 KiB of the kernel file.
 */
 .section .multiboot
 .align 4
-.long MAGIC
-.long FLAGS
+.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
-.long 0
-.long 0
-.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
diff --git a/src/kernel_main.c b/src/kernel_main.c
index 4637631..d176a27 100644
--- a/src/kernel_main.c
+++ b/src/kernel_main.c
@@ -1,12 +1,42 @@
 #include <stdint.h>
 
-void kernel_main(uint32_t mbinfo)
+uint32_t * fb;
+uint32_t pitch;
+uint32_t width;
+uint32_t height;
+
+static uint32_t * process_mbinfo_tag(uint32_t * mbinfo)
 {
-    volatile uint32_t * mbi = (volatile uint32_t *)mbinfo;
-    volatile uint32_t * fb = (volatile uint32_t *)mbi[88 / 4];
-    uint32_t pitch = mbi[96 / 4];
-    uint32_t width = mbi[100 / 4];
-    uint32_t height = mbi[104 / 4];
+    uint32_t type = mbinfo[0];
+    uint32_t size = mbinfo[1];
+    switch (type)
+    {
+    case 0u:
+        return 0u;
+
+    case 8u:
+        fb = (uint32_t *)mbinfo[2];
+        pitch = mbinfo[4];
+        width = mbinfo[5];
+        height = mbinfo[6];
+        break;
+    }
+    return (uint32_t *)(((uint32_t)mbinfo + size + 7u) & 0xFFFFFFF8u);
+}
+
+static void process_mbinfo(uint32_t * mbinfo)
+{
+    /* Skip multiboot2 boot information header. */
+    mbinfo += 2u;
+    do
+    {
+        mbinfo = process_mbinfo_tag(mbinfo);
+    } while (mbinfo != (uint32_t *)0u);
+}
+
+void kernel_main(uint32_t * mbinfo)
+{
+    process_mbinfo(mbinfo);
     for (uint32_t row = 0u; row < height; row++)
     {
         for (uint32_t col = 0u; col < width; col++)