From 8b4bed0ba7f80387f22e7ec8a1a88f0648f62579 Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@member.fsf.org>
Date: Wed, 6 Jul 2022 07:31:59 +0300
Subject: [PATCH] added video bios vector detection and attempt to initialize
 it if it's missing

---
 efitest.asm |   61 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 58 insertions(+), 3 deletions(-)

diff --git a/efitest.asm b/efitest.asm
index 0feb61d..3ca78d0 100644
--- a/efitest.asm
+++ b/efitest.asm
@@ -138,6 +138,29 @@ start16:
 
 	call print_ivt
 
+detect_vbios:
+	; make sure we have video bios vector installed in 10h
+	mov ax, [10h * 4 + 2]
+	test ax, ax
+	jnz .notnull
+	call init_vbios
+	jnc vbios_present
+	jmp halt
+.notnull: ; beyond being non-zero, the video bios should really be at c0000h
+	cmp ax, 0c000h
+	jz vbios_present
+	; unexpected address, make sure it starts with 0aa55h
+	mov es, ax
+	xor bx, bx
+	mov ax, [es:bx]
+	cmp ax, 0aa55h
+	call init_vbios	; seems like garbage, try init
+	jc halt
+
+vbios_present:
+	mov si, msg_vbios_test
+	call ser_putstr
+
 	; run 16bit video bios test
 	mov ax, 13h
 	int 10h
@@ -175,8 +198,40 @@ start16:
 	jnz .yloop
 
 	; halt for ever
-.hang:	hlt
-	jmp .hang
+halt:	hlt
+	jmp halt
+
+init_vbios:
+	mov ax, 0c000h
+	mov es, ax
+	xor bx, bx
+	cmp word [es:bx], 0aa55h
+	jz .foundsig
+	mov si, err_vbios_notfound
+	call ser_putstr
+	mov ax, [es:bx]
+	call printhex16
+	mov al, 13
+	call ser_putchar
+	mov al, 10
+	call ser_putchar
+	stc
+	ret
+.foundsig:
+	mov si, msg_vbios_init
+	call ser_putstr
+	; don't bother with CRC, just call it
+	push es
+	push word 4
+	mov bp, sp
+	call far [bp]
+	add sp, 4
+	clc
+	ret
+
+msg_vbios_test db 'running video BIOS test',13,10,0
+msg_vbios_init db 'attempting to initialize video BIOS',13,10,0
+err_vbios_notfound db 'failed to initialize video BIOS, sig not found at c0000h: ',0
 
 print_ivt:
 	xor bx, bx
@@ -197,7 +252,7 @@ print_ivt:
 	call ser_putchar
 	add bx, 4
 
-	cmp bx, 64 << 2
+	cmp bx, 33 << 2
 	jnz .loop
 	ret
 
-- 
1.7.10.4