From 2079e614e993e5f68a48ff9d55e872157f4de2aa Mon Sep 17 00:00:00 2001
From: Joursoir <chat@joursoir.net>
Date: Thu, 19 Aug 2021 11:53:14 +0000
Subject: x86/boot: add boot sector

---
 arch/x86/boot/bootsect.s | 60 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 arch/x86/boot/bootsect.s

diff --git a/arch/x86/boot/bootsect.s b/arch/x86/boot/bootsect.s
new file mode 100644
index 0000000..ecbdf16
--- /dev/null
+++ b/arch/x86/boot/bootsect.s
@@ -0,0 +1,60 @@
+.code16							# Tell GAS to generate 16 bit code
+
+.global _start					# Make the symbol visible to ld
+
+.include "bios.inc"
+
+.set MAGIC, 0xAA55
+
+.section .text.bootentry		# Code that will start executing at
+								# special address (specified in linker)
+_start:
+	jmp $0x0, $_start2			# Normalize the start address
+								# CS = 0 and IP = _start2
+_start2:
+	mov %cs, %ax				# AX = CS = 0 (see above)
+	mov %ax, %ds				# Zero segment registers		
+	mov %ax, %es
+	mov %ax, %ss				# zero stack (?)
+	mov %ax, %sp				# zero stack pointer (?)
+								# Don't touch FS and GS register (why?)
+	cld							# Set direction flag for incrementing
+	mov %dl, boot_drive			# BIOS stores our boot drive in DL,
+								# so we remember it
+
+	BIOS_PRINT $boot_os_msg
+
+	mov $0x02, %ah				# Set BIOS read sector routine
+	mov boot_drive, %dl			# Read drive number from $boot_drive
+	mov $0x00, %ch				# Select cylinder 0
+	mov $0x00, %dh				# Select head 0 [has a base of 0]
+	mov $0x02, %cl				# Select sector 2 (next after the
+								# boot sector) [has a base of 1]
+	mov $0x01, %al				# Read 1 sectors
+	mov $0x9000, %bx			# Load sectors to ES:BS (0x9000)
+	int $0x13					# Start reading from drive
+	jc disk_error				# If carry flag set, bios failed to read
+
+	# FIXME: we must compare different register
+	cmp %al, %al				# If AL(sect. read) != <>(sect. expected)
+	jne disk_error				# then return disk error
+
+	jmp . # TODO: jump into the darkness
+
+disk_error:
+	BIOS_PRINT $disk_error_msg
+	jmp .
+
+# Global variables
+boot_os_msg:
+	.asciz "Booting mfsos...\r\n"
+
+disk_error_msg:
+	.asciz "Disk read error!"
+
+boot_drive:
+	.byte 0
+
+# Bootsector padding
+.space 512 - 2 - (. - _start), 0
+.word MAGIC
-- 
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