commit 9ff35b028aa13ebe40ab84158d22faa85a5bf20d
Author: Leonid Krivoshein <klark@altlinux.org>
Date:   Fri Mar 23 00:58:26 2018 +0300

    Add description for MBR/GPT and boot flag detection

diff --git a/rpm/extlinux-config b/rpm/extlinux-config
index db90250..3562736 100755
--- a/rpm/extlinux-config
+++ b/rpm/extlinux-config
@@ -39,12 +39,18 @@ print_version() {
 	exit
 }
 
+# Read 32-bit little-endian from GPT header and output its as decimal number.
+# GPT header begins in the second sector of the disk. $1 is offset.
+#
 read_gpt() {
 	local offset=$(($sector_size + $1))
 	dd if="$dev" bs=1 skip=$offset count=4 2>/dev/null |
 		hexdump '-e"%u"'
 }
 
+# Read single byte from partition table element and output its as decimal number.
+# GPT partition table begins in the third sector of the disk. $1 is offset.
+#
 read_entry() {
 	local offset=$(($sector_size * 2 + $1))
 	local x="$(dd if="$dev" bs=1 skip=$offset count=1 2>/dev/null)"
@@ -106,13 +112,24 @@ verbose "Device name: $dev"
 verbose "Partition number: $partition"
 verbose "Sector size: $sector_size"
 
+#
+# First, 2-bytes MBR-signature at address 510 (end of the first sector) is read.
+# It should be read as 0x55, 0xAA.
+# https://en.wikipedia.org/wiki/Master_boot_record#Sector_layout
+#
 magic="$(dd if="$dev" bs=2 skip=255 count=1 2>/dev/null | base64)"
 [ "$magic" = "Vao=" ] ||
 	fatal "Unknown partition table"
 
-magic="$(dd if="$dev" bs=8 skip=64 count=1 2>/dev/null)"
+#
+# Then 8-bytes GPT-signature "EFI PART" is checked at the beginning of the second
+# sector of the disk.
+# https://en.wikipedia.org/wiki/GUID_Partition_Table#Partition_table_header_(LBA_1)
+#
+magic="$(dd if="$dev" bs=1 skip=$sector_size count=8 2>/dev/null)"
 if [ "$magic" = "EFI PART" ]; then
 	partition_table='gpt'
+	# Number of partition entries, 4 bytes little-endian.
 	magic=$(read_gpt 80)
 else
 	partition_table='msdos'
@@ -125,16 +142,26 @@ verbose "Partition table: $partition_table"
 	fatal "invalid partition number: $partition"
 
 if [ "$partition_table" = "msdos" ]; then
+	# MBR partition table starts at offset 446.
+	# Each element of the table occupying 16 bytes.
 	magic=$((16 * ($partition - 1) + 446))
 	magic="$(dd if="$dev" bs=1 skip=$magic count=1 2>/dev/null | base64)"
+	# First byte of the element is 0x80 for the boot partition.
+	# https://en.wikipedia.org/wiki/Master_boot_record#PTE
 	[ "$magic" = "gA==" ] ||
 		fatal "partition not bootable"
 	mbr_bin='mbr.bin'
 else
+	# Size of a single partition entry, 4 bytes little-endian.
 	magic=$(read_gpt 84)
+	# Attribute flags (8 bytes little-endian) starts at offset 48 of the entry.
+	# https://en.wikipedia.org/wiki/GUID_Partition_Table#Partition_entries_(LBA_2-33)
 	magic=$(($magic * ($partition - 1) + 48))
+	# We interested only low byte of the attribute flags and only bit "2" (mask 0x04).
 	magic=$(read_entry $magic)
 	magic=$(($magic & 4))
+	# If the bit "2" of the attribute flags is set,
+	# partition is marked as bootable in the Legacy BIOS.
 	[ "$magic" = "4" ] ||
 		fatal "partition not bootable"
 	mbr_bin='gptmbr.bin'