From f50ae14f9e2b6fc736693c866bcbf20a594b5f62 Mon Sep 17 00:00:00 2001
From: Wayne Warthen <wwarthen@gmail.com>
Date: Sun, 5 Jan 2025 19:55:37 -0800
Subject: [PATCH] Video Driver Fixes

Thanks and credit to Phillip Summers for finding these and suggesting solutions.
- VGA driver initialization fixed.
- CVDU probe fixed to not rely on ready bit of CVDU
---
 Source/HBIOS/cvdu.asm | 88 ++++++++++++++++++++++++++++++++-----------
 Source/HBIOS/vga.asm  |  2 +-
 2 files changed, 66 insertions(+), 24 deletions(-)

diff --git a/Source/HBIOS/cvdu.asm b/Source/HBIOS/cvdu.asm
index 9b4d6f64..ae6254b8 100644
--- a/Source/HBIOS/cvdu.asm
+++ b/Source/HBIOS/cvdu.asm
@@ -106,7 +106,7 @@ CVDU_INIT1:
 	CALL	PRTDEC
 	PRTS("KB$")
 	CALL	CVDU_LOADFONT		; LOAD FONT DATA FROM ROM TO CVDU STRORAGE
-	CALL	CVDU_VDARES
+	CALL	CVDU_VDAINI
 	CALL	KBD_INIT		; INITIALIZE KEYBOARD DRIVER
 
 	; ADD OURSELVES TO VDA DISPATCH TABLE
@@ -342,28 +342,70 @@ CVDU_RDX:
 ; ON RETURN, ZF SET INDICATES HARDWARE FOUND
 ;
 CVDU_PROBE:
-	; WRITE TEST PATTERN $A5 $5A TO START OF VRAM
-	LD	HL,0			; POINT TO FIRST BYTE OF VRAM
-	LD	C,18			; ADDRESS REGISTER PAIR
-	CALL	CVDU_WRX		; UPDATE VRAM ADDRESS POINTER
-	LD	A,$A5			; INITIAL TEST VALUE
-	LD	B,A			; SAVE IN B
-	LD	C,31			; DATA REGISTER
-	CALL	CVDU_WR			; WRITE VALUE TO LOC 0, ADR PTR INCREMENTS
-	CPL				; INVERT TEST VALUE
-	CALL	CVDU_WR			; WRITE INVERTED VALUE TO LOC 1
-	; READ TEST PATTERN BACK TO CONFIRM HARDWARE EXISTS
-	LD	HL,0			; POINT TO FIRST BYTE OF VRAM
-	LD	C,18			; ADDRESS REGISTER PAIR
-	CALL	CVDU_WRX		; UPDATE VRAM ADDRESS POINTER
-	LD	C,31			; DATA REGISTER
-	CALL	CVDU_RD			; GET BYTE AT LOC 0, ADR PTR INCREMENTS
-	CP	B			; CHECK IT
-	RET	NZ			; ABORT IF BAD COMPARE
-	CALL	CVDU_RD			; GET BYTE AT LOC 1
-	CPL				; INVERT IT
-	CP	B			; CHECK FOR INVERTED TEST VALUE
-	RET				; RETURN WITH ZF SET BASED ON CP
+	LD	B,CVDU_PROBE_DATA_SIZE / 2	; LOOP COUNTER
+	LD	HL,CVDU_PROBE_DATA		; START OF DATA
+CVDU_PROBE1:
+	LD	C,(HL)				; REGISTER
+	INC	HL
+	LD	A,(HL)				; BYTE
+	INC	HL
+	OUT	(C),A				; WRITE IT
+	LD	DE,25
+	CALL	VDELAY
+	DJNZ	CVDU_PROBE1
+;
+	IN	A,(CVDU_DATA)
+	LD	C,A
+	LD	DE,25
+	CALL	VDELAY
+	IN	A,(CVDU_DATA)
+	CPL
+;
+	CP	C				; COMPARE OK?
+	RET
+
+CVDU_PROBE_DATA:
+	; SET VRAM ADDRESS TO $0000
+	.DB	CVDU_REG, 18		; ADR LOW
+	.DB	CVDU_DATA, 0
+	.DB	CVDU_REG, 19		; ADR HI
+	.DB	CVDU_DATA, 0
+	; WRITE PATTERN
+	.DB	CVDU_REG, 31		; VRAM DATA
+	.DB	CVDU_DATA, $A5
+	.DB	CVDU_DATA, $5A
+	; RESET VRAM ADDRESS TO $0000
+	.DB	CVDU_REG, 18		; ADR LOW
+	.DB	CVDU_DATA, 0
+	.DB	CVDU_REG, 19		; ADR HI
+	.DB	CVDU_DATA, 0
+	; SETUP TO READ PATTERN
+	.DB	CVDU_REG, 31		; VRAM DATA
+;
+CVDU_PROBE_DATA_SIZE	.EQU	$ - CVDU_PROBE_DATA
+;
+;;;	; WRITE TEST PATTERN $A5 $5A TO START OF VRAM
+;;;	LD	HL,0			; POINT TO FIRST BYTE OF VRAM
+;;;	LD	C,18			; ADDRESS REGISTER PAIR
+;;;	CALL	CVDU_WRX		; UPDATE VRAM ADDRESS POINTER
+;;;	LD	A,$A5			; INITIAL TEST VALUE
+;;;	LD	B,A			; SAVE IN B
+;;;	LD	C,31			; DATA REGISTER
+;;;	CALL	CVDU_WR			; WRITE VALUE TO LOC 0, ADR PTR INCREMENTS
+;;;	CPL				; INVERT TEST VALUE
+;;;	CALL	CVDU_WR			; WRITE INVERTED VALUE TO LOC 1
+;;;	; READ TEST PATTERN BACK TO CONFIRM HARDWARE EXISTS
+;;;	LD	HL,0			; POINT TO FIRST BYTE OF VRAM
+;;;	LD	C,18			; ADDRESS REGISTER PAIR
+;;;	CALL	CVDU_WRX		; UPDATE VRAM ADDRESS POINTER
+;;;	LD	C,31			; DATA REGISTER
+;;;	CALL	CVDU_RD			; GET BYTE AT LOC 0, ADR PTR INCREMENTS
+;;;	CP	B			; CHECK IT
+;;;	RET	NZ			; ABORT IF BAD COMPARE
+;;;	CALL	CVDU_RD			; GET BYTE AT LOC 1
+;;;	CPL				; INVERT IT
+;;;	CP	B			; CHECK FOR INVERTED TEST VALUE
+;;;	RET				; RETURN WITH ZF SET BASED ON CP
 ;
 ;----------------------------------------------------------------------
 ; MOS 8563 DISPLAY CONTROLLER CHIP INITIALIZATION
diff --git a/Source/HBIOS/vga.asm b/Source/HBIOS/vga.asm
index 171b32c2..97470ff5 100644
--- a/Source/HBIOS/vga.asm
+++ b/Source/HBIOS/vga.asm
@@ -138,7 +138,7 @@ VGA_INIT1:
 	; HARDWARE INITIALIZATION
 	CALL 	VGA_CRTINIT		; SETUP THE VGA CHIP REGISTERS
 	CALL	VGA_LOADFONT		; LOAD FONT DATA FROM ROM TO VGA STORAGE
-	CALL	VGA_VDARES
+	CALL	VGA_VDAINI
 	CALL	KBD_INIT		; INITIALIZE KEYBOARD DRIVER
 
 	; ADD OURSELVES TO VDA DISPATCH TABLE