VFRCRATE PITSYNC command

CHANGELOG

@@ -2,6 +2,12 @@ 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.
+  - Add "PITSYNC" option to VFRCRATE command. "VFRCRATE PITSYNC ON"
+    directs the system timer interrupt to synchronize tick rate with
+    vertical refresh rate of VGA emulation if the game or demo set
+    the timer tick rate to a value close enough to vertical refresh.
+    This is intended for games or demos that use the system timer for
+    a vsync interrupt.
 
 2023.09.01
   - Disable by default message confirmation after snapshot and AVI video

src/hardware/timer.cpp

@@ -29,6 +29,9 @@
 #include "setup.h"
 #include "control.h"
 
+extern bool VGA_PITsync;
+extern double vga_fps;
+
 // This is only set in PC-98 mode and only if emulating PC-9801.
 // There is at least one game (PC-98 port of Thexder) that depends on PC-9801 PIT 1
 // behavior where the counter cycles at all times whether or not the PC speaker is
@@ -308,13 +311,26 @@ static bool latched_timerstatus_locked;
 
 unsigned long PIT_TICK_RATE = PIT_TICK_RATE_IBM;
 
+pic_tickindex_t VGA_PITSync_delay(void);
+
 static void PIT0_Event(Bitu /*val*/) {
 	PIC_ActivateIRQ(0);
 	/* NTS: "Days of Thunder" leaves PIT 0 in mode 1 for some reason, which triggers once and then stops. "start" does not advance in that mode.
 	 *      For any non-periodic mode, this code would falsely detect an ever increasing error and act badly. */
 	if (pit[0].mode == 2 || pit[0].mode == 3) {
 		pit[0].track_time(PIC_FullIndex());
 
+		/* If enabled option and VGA refresh rate is close to PIT 0 timer tick rate,
+		 * make them line up so that demos that use PIT0 for vsync can run without
+		 * shearing artifacts */
+		if (VGA_PITsync) {
+			pic_tickindex_t vga_delay = 1000.0 / vga_fps;
+			if (fabs(vga_delay - pit[0].delay) < (vga_delay * 0.05)) {
+				PIC_AddEvent(PIT0_Event,VGA_PITSync_delay());
+				return;
+			}
+		}
+
 		/* event timing error checking */
 		pic_tickindex_t err = PIC_GetCurrentEventTime() - pit[0].start;
 

src/hardware/vga.cpp

@@ -170,6 +170,8 @@ void VGA_CaptureStartNextFrame(void);
 void VGA_CaptureMarkError(void);
 bool VGA_CaptureValidateCurrentFrame(void);
 
+bool                                VGA_PITsync = false;
+
 unsigned int                        vbe_window_granularity = 0;
 unsigned int                        vbe_window_size = 0;
 
@@ -528,6 +530,13 @@ void VGA_SetCGA4Table(uint8_t val0,uint8_t val1,uint8_t val2,uint8_t val3) {
     }
 }
 
+void SetPITSync(char *x) {
+	if (!strncasecmp(x,"off",3))
+		VGA_PITsync = false;
+	else if (!strncasecmp(x,"on",3))
+		VGA_PITsync = true;
+}
+
 void SetRate(char *x) {
     if (!strncasecmp(x,"off",3))
         vga_force_refresh_rate = -1;
@@ -574,11 +583,14 @@ class VFRCRATE : public Program {
 			WriteOut("  SET rate  Lock to integer frame rate, e.g. 15\n");
 			WriteOut("  SET rate  Lock to decimal frame rate, e.g. 29.97\n");
 			WriteOut("  SET rate  Lock to fractional frame rate, e.g. 60000/1001\n\n");
+			WriteOut("  PITSYNC <ON|OFF> Make PIT timer tick at refresh rate if close enough\n\n");
 			WriteOut("Type VFRCRATE without a parameter to show the current status.\n");
 			return;
 		}
         if (cmd->FindString("SET",temp_line,false))
             SetRate((char *)temp_line.c_str());
+        if (cmd->FindString("PITSYNC",temp_line,false))
+            SetPITSync((char *)temp_line.c_str());
 #if defined(USE_TTF)
         if (TTF_using()) PIC_AddEvent(&resetSize, 1);
 #endif

src/hardware/vga_draw.cpp

@@ -5428,6 +5428,15 @@ void FreeRawImage(void) {
 	rawshot.free();
 }
 
+pic_tickindex_t VGA_PITSync_delay(void) {
+	pic_tickindex_t current_time = PIC_GetCurrentEventTime();
+	pic_tickindex_t dt = current_time - vga.draw.delay.framestart;
+	pic_tickindex_t et = vga.draw.delay.vrstart - dt;
+	if (et < (vga.draw.delay.vtotal/2.0)) et += vga.draw.delay.vtotal;
+//	LOG_MSG("dt %.3f et %.3f\n",double(dt),double(et));
+	return et;
+}
+
 static void VGA_VerticalTimer(Bitu /*val*/) {
 	double current_time = PIC_GetCurrentEventTime();