From 2348172240f59017b4e1f4e2ee87868e03f2b42e Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@member.fsf.org>
Date: Mon, 4 Jul 2022 21:33:58 +0300
Subject: [PATCH] switched to 16bit mode, but int 10h hangs

---
 efitest.asm |   55 ++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 46 insertions(+), 9 deletions(-)

diff --git a/efitest.asm b/efitest.asm
index bae6aa2..ae804dd 100644
--- a/efitest.asm
+++ b/efitest.asm
@@ -116,7 +116,7 @@ mmapbuf: times 4096 db 0
 str_hello db 'hello!',13,10,0
 
 	align 4
-gdtlim	dw 23
+gdtlim	dw 31
 gdtbase	dq gdt
 
 	align 8
@@ -128,7 +128,9 @@ gdt:	; 0: null segment
 	; 2: data - base:0, lim:4g, G:4k, 32bit, avl, pres|app, dpl:0, type:data/rw
 	dd 0x0000ffff
 	dd 0x00cf9200
-
+	; 3: code16
+	dd 0x0000ffff
+	dd 0x00009a00
 
 	bits 32
 start32:
@@ -151,13 +153,8 @@ start32:
 	and eax, 0fffffeffh
 	wrmsr
 
-	mov edi, 0a0000h
-	mov ecx, 16000
-	mov eax, 00ff0000h
-	rep stosd
-
-	mov esi, str_hello
-	call ser_putstr
+	; load 16bit code segment and jump to 16bit code
+	jmp 0x18:start16
 
 .hang:	hlt
 	jmp .hang
@@ -186,4 +183,44 @@ ser_putstr:
 
 
 	align 4
+	; real mode IDTR pseudo-descriptor pointing to the IVT at addr 0
+	dw 0
+rmidt:	dw 3ffh
+	dd 0
+
+	bits 16
+start16:
+	; disable protection
+	mov eax, cr0
+	and eax, 0fffffffeh
+	mov cr0, eax
+	; load cs <- 0
+	jmp 0:.loadcs0
+.loadcs0:
+	; zero data segments
+	xor ax, ax
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	; move stack to the top of 640k
+	mov ax, 9000h
+	mov ss, ax
+	xor sp, sp
+
+	; run 16bit video bios test
+	mov ax, 13h
+	;int 10h
+
+	mov ax, 0a000h
+	mov es, ax
+	xor di, di
+	mov cx, 32000
+	mov ax, 6767h
+	rep stosw
+
+.hang:	hlt
+	jmp .hang
+
+	align 4
 end:
-- 
1.7.10.4