[Platform] [PC-98] VSync interrupt handler

Starting with the simple refresh rate-oblivious code from TH01, until
we've figured out what the rest of the master.lib code is doing and
have valid reasons to include it. Also extending the second counter to
32-bit because we *might* be measuring some processes that could take
longer than 19:35 minutes…

Part of P0232, funded by [Anonymous].

platform/x86real/pc98/vsync.cpp

@@ -0,0 +1,55 @@
+#include <stdlib.h>
+#include "platform.h"
+#include "x86real.h"
+#include "platform/x86real/pc98/vsync.hpp"
+
+volatile uint16_t vsync_count_16;
+volatile uint32_t vsync_count_32;
+static void interrupt (*vsync_callback_old)(...);
+bool vsync_active;
+
+static void interrupt vsync_intfunc(...)
+{
+	vsync_count_16++;
+	vsync_count_32++;
+	_outportb_(0x00, 0x20); // End of Interrupt
+	_outportb_(0x64, _AL); // VSync interrupt trigger
+}
+
+void __cdecl vsync_init(void)
+{
+	if(vsync_active) {
+		return;
+	}
+	disable();
+	vsync_callback_old = getvect(0x0A);
+	setvect(0x0A, vsync_intfunc);
+
+	// Disable all interrupts from 0x08 to 0x0F except for 0x0A
+	_outportb_(0x02, (_inportb_(0x02) & 0xFB));
+
+	_outportb_(0x64, _AL); // VSync interrupt trigger
+	enable();
+	vsync_active = true;
+	atexit(vsync_exit);
+}
+
+void __cdecl vsync_exit(void)
+{
+	if(!vsync_active) {
+		return;
+	}
+	disable();
+
+	// Reenable all interrupts from 0x08 to 0x0F except for 0x0A
+	_outportb_(0x02, (_inportb_(0x02) | 0x04));
+
+	setvect(0x0A, vsync_callback_old);
+	enable();
+	vsync_active = false;
+}
+
+bool vsync_is_active(void)
+{
+	return vsync_active;
+}

platform/x86real/pc98/vsync.hpp

@@ -0,0 +1,17 @@
+// VSync interrupts
+// ----------------
+
+// Incremented by 1 on every VSync interrupt. Can be manually reset to 0 to
+// simplify frame delay loops.
+extern volatile uint16_t vsync_count_16;
+extern volatile uint32_t vsync_count_32;
+
+// Sets up a VSync interrupt that increments [vsync_count_16] and
+// [vsync_count_32] every time it's called. Also registers vsync_exit() to be
+// called at process termination.
+void __cdecl vsync_init(void);
+
+// Removes the VSync interrupt handler set up by vsync_init().
+void __cdecl vsync_exit(void);
+
+bool vsync_is_active(void);