add multiboot section

_config.yml

@@ -11,6 +11,10 @@ tagline:             "rust, kernels, and more"
 paginate:            10
 baseurl:             ""
 
+markdown: redcarpet
+redcarpet:
+  extensions: ["no_intra_emphasis", "fenced_code_blocks", "autolink", "tables", "with_toc_data", "superscript", "footnotes"]
+
 # Assets
 #
 # We specify the directory for Jekyll so we can use @imports.
@@ -33,3 +37,4 @@ navigation_pages:
   Archive: '/archive'
 
 contact: '/contact'
+

_posts/2015-07-22-rust-os-boot.md

@@ -1,27 +1,63 @@
 ---
 layout: post
-title: "[DRAFT] Rust OS Part 1: Booting"
-related_posts:
+title: '[DRAFT] Rust OS Part 1: Booting'
+related_posts: null
 ---
+## Multiboot
+Fortunately there is a bootloader standard: the [Multiboot Specification][multiboot].  So our kernel just needs to indicate that it supports Multiboot and every Multiboot-compliant bootloader can boot it. We will use the [GRUB 2] bootloader together with the Multiboot 2 specification ([PDF][Multiboot 2]).
 
-Fortunately there is a bootloader standard: the [Multiboot
-Specification][multiboot]. So our kernel just needs to indicate that it supports
-Multiboot and every Multiboot-compliant bootloader can boot it. We will use the [GRUB 2] bootloader together with the [Multiboot 2] specification. So let's begin!
+To indicate our Multiboot 2 support to the bootloader, our kernel must contain a _Multiboot Header_, which has the following format:
 
-To indicate our Multiboot 2 support to the bootloader, our kernel must contain a *Multiboot Header*, which has the following format:
+Field         | Type            | Value
+------------- | --------------- | ----------------------------------------
+magic number  | u32             | 0xE85250D6
+architecture  | u32             | 0 for i386, 4 for MIPS
+header length | u32             | total header size, including tags
+checksum      | u32             | -(magic + architecture + header length)
+tags          | variable        |
+end tag       | (u16, u16, u32) | (0, 0, 8)
 
-Field | Size in byte
-------|-----
-magic number | 4
+Converted to x86 assembly it looks like this (Intel syntax):
 
-Offset | Type | Field Name
--------|------|-----------
-0      | u32  | magic
-4      | u32  | architecture
-8      | u32  | header_length
-12     | u32  | checksum
-16-XX  |      | tags
+```nasm
+header_start:
+    dd 0xe85250d6                ; magic number (multiboot 2)
+    dd 0                         ; architecture 0 (protected mode i386)
+    dd header_end - header_start ; header length
+    ; checksum
+    dd 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))
+
+    ; insert optional multiboot tags here
+
+    ; required end tag
+    dw 0    ; type
+    dw 0    ; flags
+    dd 8    ; size
+header_end:
+```
+
+If you don't know x86 assembly, here is some quick guide:
+
+- `header_start` and `header_end` are _labels_ that mark a memory location. We use them to calculate the header length easily
+- `dd` stands for `define double` (32bit) and `dw` stands for `define word` (16bit)
+- the additional `0x100000000` in the checksum calculation is a small hack[^fn-checksum_hack] to avoid a compiler warning
+
+We can already compile it using `nasm`. As it produces a flat binary by default, the resulting file contains just our 24 bytes (in little endian if you work on a x86 machine):
+
+```
+> nasm boot.asm
+> hexdump -x boot
+0000000    50d6    e852    0000    0000    0018    0000    af12    17ad
+0000010    0000    0000    0008    0000
+0000018
+```
+
+
+## Booting
+
+
+[^fn-checksum_hack]: The formula from the table, `-(magic + architecture + header length)`, creates a negative value that doesn't fit into 32bit. By subtracting from `0x100000000` instead, we keep the value positive without changing its truncated value. Without the additional sign bit(s) the result fits into 32bit and the compiler is happy.
 
 [multiboot]: https://en.wikipedia.org/wiki/Multiboot_Specification
-[GRUB 2]: http://wiki.osdev.org/GRUB_2
-[Multiboot 2]: http://nongnu.askapache.com/grub/phcoder/multiboot.pdf
+[grub 2]: http://wiki.osdev.org/GRUB_2
+[multiboot 2]: http://nongnu.askapache.com/grub/phcoder/multiboot.pdf

_sass/_layout.scss

@@ -3,7 +3,7 @@
 // Styles for managing the structural hierarchy of the site.
 
 .container {
-  max-width: 38rem;
+  max-width: 45rem;
   padding-left:  1.5rem;
   padding-right: 1.5rem;
   margin-left:  auto;