From a485d59fdaf7b768f95539991d72421fb0ec85de Mon Sep 17 00:00:00 2001
From: Carlos Maiolino <cem@toxiclabs.cc>
Date: Thu, 17 Jul 2025 19:20:26 +0200
Subject: Restructure project folder

Signed-off-by: Carlos Maiolino <cem@toxiclabs.cc>
---
 src/boot/bootloader.asm | 94 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 src/boot/bootloader.asm

(limited to 'src/boot/bootloader.asm')

diff --git a/src/boot/bootloader.asm b/src/boot/bootloader.asm
new file mode 100644
index 0000000..f42e702
--- /dev/null
+++ b/src/boot/bootloader.asm
@@ -0,0 +1,94 @@
+ORG	0x7c00
+BITS	16
+
+CODE_SEG equ	gdt_code - gdt_start	; 0x8
+DATA_SEG equ	gdt_data - gdt_start	; 0x10
+_start:
+	jmp short start
+	nop
+
+; Setup dummy BIOS Parameter block so a dumb BIOS does not corrupt us
+; if it starts messing up with the parameter block
+	times 33 db 0
+
+start:
+	; Set CS to 0 with label step2 as its beginning offset
+	jmp 0:step2
+
+step2:
+	; Setup segments
+
+	cli		; Disable interrupts
+	mov ax, 0
+	mov ds, ax
+	mov es, ax
+	mov ss, ax
+	mov sp, 0x7c00	; Stack segment starts right before ds
+			; and grows down to 0x0. This works because
+			; Intel implements a full descending stack
+	sti		; Enable interrupts
+
+.load_protected:
+	cli
+	lgdt[gdt_table]
+	mov eax, cr0
+	or eax, 0x1
+	mov cr0, eax
+	jmp CODE_SEG:start_32
+
+; GDT table description
+gdt_start:
+gdt_null:	; First segment Descriptor is always 0
+	dd 0x0
+	dd 0x0
+
+;Second segment descriptor... Code segment - 0x8
+gdt_code:
+	dw 0xffff	; Segment limit 0-15
+	dw 0		; Base address 0-15
+	db 0		; Base 16-23
+	db 0x9a		; Access byte field 0b10011010
+
+	; We don't have a 4 bits type, so use a byte type
+	; to set both Segment limit high bits and flags
+
+	db 11001111b	; Segment limit 16-19
+			; Flags 0-3
+	db 0		; Base address high bits 24-31
+
+;Second segment descriptor... Data segment - 0x10
+gdt_data:
+	dw 0xffff	; Segment limit 0-15
+	dw 0		; Base address 0-15
+	db 0		; Base 16-23
+	db 0x92		; Access byte field 0b10010010
+	db 11001111b	; Segment limit 16-19
+			; Flags 0-3
+	db 0		; Base address high bits 24-31
+gdt_end:
+
+gdt_table:
+	dw gdt_end - gdt_start - 1
+	dd gdt_start
+
+
+[BITS 32]
+
+; No access to BIOS from now on....
+start_32:
+	; Set all segments to the same as the DATA_SEG
+	mov ax, DATA_SEG
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+
+	; Set the stack pointer and base pointer further in mem
+	mov ebp, 0x00200000
+	mov esp, ebp
+	jmp $
+
+; Fill in to the end and add bootloader signature
+	times 510 - ($ - $$) db 0
+	dw 0xAA55
-- 
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