From 958e4457914c5e5646dc145ee18f1bc616a9520f Mon Sep 17 00:00:00 2001
From: Jonathan Campbell <jonathan@castus.tv>
Date: Fri, 1 Sep 2023 22:38:54 -0700
Subject: [PATCH] VGA DS START command to force start address

---
 CHANGELOG                 |  2 ++
 include/vga.h             |  6 ++++++
 src/debug/debug.cpp       | 25 ++++++++++++++++++++++++-
 src/hardware/vga_draw.cpp | 15 +++++++++++++++
 4 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG b/CHANGELOG
index 57e3c538392..62bbfba018a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 Next version:
   - Add "VRD" debugger command to force redraw of the VGA screen.
+  - Add VGA debug set commands to force a video start address and another
+    to clear all debug settings.
 
 2023.09.01
   - Disable by default message confirmation after snapshot and AVI video
diff --git a/include/vga.h b/include/vga.h
index 4de8296be2c..f8982913b62 100644
--- a/include/vga.h
+++ b/include/vga.h
@@ -649,6 +649,11 @@ typedef struct VGA_Complexity_t {
 	}
 } VGA_Complexity;
 
+typedef struct VGA_Override_t {
+	bool			enable = false;
+	uint32_t		start = ~uint32_t(0u);
+} VGA_Override;
+
 typedef struct VGA_Type_t {
     VGAModes mode = {};                              /* The mode the vga system is in */
     VGAModes lastmode = {};
@@ -672,6 +677,7 @@ typedef struct VGA_Type_t {
     VGA_Memory mem;
     VGA_LFB lfb = {};
     VGA_Complexity complexity = {};
+    VGA_Override overopts = {};
 } VGA_Type;
 
 
diff --git a/src/debug/debug.cpp b/src/debug/debug.cpp
index b005514f415..34edb043c80 100644
--- a/src/debug/debug.cpp
+++ b/src/debug/debug.cpp
@@ -304,6 +304,9 @@ static FPU_rec oldfpu;
 
 void VGA_DebugRedraw(void);
 
+void VGA_DebugOverrideStart(uint32_t ofs);
+void VGA_ResetDebugOverrides(void);
+
 bool IsDebuggerActive(void) {
     return debugging;
 }
@@ -2811,7 +2814,27 @@ bool ParseCommand(char* str) {
             return false;
         }
 
-	if (command == "FONTDUMP") { // Dump font RAM to file
+	if (command == "DS") {
+		std::string cmd2;
+		while (*found == ' ') found++;
+		stream >> cmd2;
+		while (*found != 0 && *found != ' ') found++;
+		while (*found == ' ') found++;
+
+		if (cmd2 == "START") {
+			VGA_DebugOverrideStart(strtoul(found,NULL,16/*hexadecimal*/));
+			VGA_DebugRedraw();
+		}
+		else if (cmd2 == "X") {
+			VGA_ResetDebugOverrides();
+			VGA_DebugRedraw();
+		}
+		else {
+			DEBUG_ShowMsg("Unknown VGA debug command");
+			return false;
+		}
+	}
+	else if (command == "FONTDUMP") { // Dump font RAM to file
 		/* Rule: If the file extension is .BIN, write the entire contents of bitplane 2 where the font resides (64KB).
 		 *       If the file extension is .BMP, write only the visible font characters in a neat 8x8 (256) matrix.
 		 *       If you run this when EGA/VGA graphics are active, you will get "jibberish" based on the graphics on
diff --git a/src/hardware/vga_draw.cpp b/src/hardware/vga_draw.cpp
index 42ccc996449..2eddc3b3a42 100644
--- a/src/hardware/vga_draw.cpp
+++ b/src/hardware/vga_draw.cpp
@@ -3691,6 +3691,10 @@ static void VGA_DisplayStartLatch(Bitu /*val*/) {
     vga.config.real_start=vga.config.display_start & vga.mem.memmask;
     vga.draw.bytes_skip = vga.config.bytes_skip;
 
+    if (vga.overopts.enable) {
+	    if (vga.overopts.start != ~uint32_t(0u)) vga.config.real_start = vga.overopts.start & vga.mem.memmask;
+    }
+
     /* TODO: When does 640x480 2-color mode latch foreground/background colors from the DAC? */
     if (machine == MCH_MCGA && (vga.other.mcga_mode_control & 2)) {//640x480 2-color mode MCGA
         VGA_DAC_UpdateColorPalette();
@@ -6202,6 +6206,7 @@ bool IsDebuggerActive(void);
 void VGA_DebugRedraw(void) {
 	if (IsDebuggerActive()) {
 		RENDER_EndUpdate(true);
+		vga.draw.lines_done = vga.draw.lines_total;
 		PIC_RemoveEvents(VGA_Other_VertInterrupt);
 		PIC_RemoveEvents(VGA_VerticalTimer);
 		PIC_RemoveEvents(VGA_PanningLatch);
@@ -6212,6 +6217,16 @@ void VGA_DebugRedraw(void) {
 	}
 }
 
+void VGA_DebugOverrideStart(uint32_t ofs) {
+	vga.overopts.start = ofs;
+	vga.overopts.enable = true;
+}
+
+void VGA_ResetDebugOverrides(void) {
+	vga.overopts.start = ~uint32_t(0ul);
+	vga.overopts.enable = false;
+}
+
 void VGA_CheckScanLength(void) {
 	switch (vga.mode) {
 		case M_EGA: