From: Lizzy Fleckenstein <eliasfleckenstein@web.de>
Date: Wed, 21 Sep 2022 18:45:29 +0000 (+0200)
Subject: Enable unreal mode
X-Git-Url: https://git.lizzy.rs/?a=commitdiff_plain;h=51d0e1d5254fc13146820ce4b58c02bcc3111934;p=loadnothing.git

Enable unreal mode
---

diff --git a/stage1/boot.asm b/stage1/boot.asm
index 6144c2a..29a1849 100644
--- a/stage1/boot.asm
+++ b/stage1/boot.asm
@@ -90,9 +90,39 @@ boot:
 	cmp al, STAGE2SECTORS ; Have we read as many sectors as we requested?
 	jne stage2_error
 
+	; enable to unreal mode
+	; https://wiki.osdev.org/Unreal_Mode
+	cli                   ; no interrupts
+	push ds               ; save real mode
+
+	lgdt [gdtinfo]        ; load gdt register
+
+	mov eax, cr0          ; switch to pmode by
+	or al, 1              ; set pmode bit
+	mov cr0, eax
+
+	jmp $+2               ; tell 386/486 to not crash
+
+	mov bx, 0x08          ; select descriptor 1
+	mov ds, bx            ; 8h = 1000b
+
+	and al, 0xFE          ; back to realmode
+	mov cr0, eax          ; by toggling bit again
+
+	pop ds                ; get back old segment
+	sti
+
 	jmp STAGE2START       ; Hand over control to stage 2
 
 hello db 'Welcome to loadnothing stage 1!', 13, 10, 0 ; \r\n\0
 error db 'Error reading stage 2 from disk', 13, 10, 0 ; \r\n\0
 
+gdtinfo:
+	dw gdt_end - gdt - 1   ; last byte in table
+	dd gdt                 ; start of table
+
+gdt dd 0, 0        ; entry 0 is always unused
+flatdesc db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0
+gdt_end:
+
 times (446 - ($ - $$)) db 0x00