From 5b8595c6c63fcc12fbf8e1ad32986b7e59bb3d75 Mon Sep 17 00:00:00 2001
From: Joursoir <chat@joursoir.net>
Date: Thu, 26 Aug 2021 17:39:31 +0000
Subject: x86/boot/bootsect: add a filled GDT structure

---
 arch/x86/boot/bootsect.s | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/arch/x86/boot/bootsect.s b/arch/x86/boot/bootsect.s
index 6f90746..eedf3bd 100644
--- a/arch/x86/boot/bootsect.s
+++ b/arch/x86/boot/bootsect.s
@@ -4,6 +4,10 @@
 
 .include "bios.inc"
 
+# Define some constants for the GDT segment descriptor offsets
+.set CODESEG, gdt_code - gdt_start
+.set DATASEG, gdt_data - gdt_start
+
 .set MAGIC, 0xAA55
 
 .section .text.bootentry		# Code that will start executing at
@@ -48,7 +52,41 @@ disk_error:
 	BIOS_PRINT $disk_error_msg
 	jmp .
 
+# Global Descriptor Table (contains 8-byte entries)
+gdt_start:
+gdt_null:						# The mandatory null descriptor
+	.quad 0x0
+
+gdt_code:						# The code segment descriptor
+	# Base = 0x0, limit = 0xfffff
+	# 1st flags: (present)1 (privilege)00 (descriptor type)1 -> b1001
+	# Type flags: (code)1 (conforming)0 (readable)1 (accessed)0 -> b1010
+	# 2nd flags: (granularity)1 (size)1 (64-bit seg)0 (AVL)0 -> b1100
+	.word 0xffff				# Limit (bits 0-15)
+	.word 0x0					# Base (bits 0-15)
+	.byte 0x0					# Base (bits 16-23)
+	.byte 0b10011010			# 1st flags, type flags
+	.byte 0b11001111			# 2nd flags, limit (bits 16-19)
+	.byte 0x0					# Base (bits 24-31)
+
+gdt_data: 						# the data segment descriptor
+	# Same as code segment except for the type flags:
+	# Type flags: (code)0 (direction)0 (writable)1, (accessed)0 -> b0010
+	# P.S: direction bit: 0 the segment grows up
+	.word 0xffff				# Limit (bits 0-15)
+	.word 0x0					# Base (bits 0-15)
+	.byte 0x0					# Base (bits 16-23)
+	.byte 0b10010010			# 1st flags, type flags
+	.byte 0b11001111			# 2nd flags, limit (bits 16-19)
+	.byte 0x0					# Base (bits 24-31)
+gdt_end:
+
 # Global variables
+gdt_descriptor:
+	# Size of GDT, always less one of the true size
+	.word gdt_end - gdt_start - 1
+	.long gdt_start 			# Start address of our GDT
+
 boot_real_mode_msg:
 	.asciz "Started mfsos in 16-bit real mode\r\n"
 
-- 
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