PC-98: Implement port 9A0h readback used to determine if the GDC is r…

…unning at 5MHz or not so that PC-9821 CD-ROM games can then program the correct GDC PITCH value for proper 256-color mode display

CHANGELOG

@@ -1,4 +1,11 @@
 Next:
+  - The reason some PC-9821 games have been giving bad PITCH values (double the
+    correct value) to the GDC in 256-color mode has been determined. They read
+    port 9A0h to determine if the GDC is running at 5MHz or not. That port 9A0h
+    index was not implemented and therefore gave the game the impression it was
+    running at 5MHz when it was not, therefore the doubled PITCH value. This fix
+    corrects the squeezed 256-color mode and allows it to appear properly.
+    (joncampbell123).
   - IMGMOUNT: If the file extension is HDI, assume a hard disk image even if the
     image file is small enough to qualify as a floppy disk image. There is an
     HDI image of PC-98 game "D.O. Doki Doki Disk 8" that is 2.2MB, small enough

src/hardware/vga_pc98_crtc.cpp

@@ -331,6 +331,9 @@ Bitu pc98_read_9a0(Bitu /*port*/,Bitu /*iolen*/) {
         case 0x07:      // EGC mode
             if (pc98_gdc_vramop & (1 << VOPBIT_EGC)) retval |= 1u;
             break;
+        case 0x09:      // GDC clock #0
+            if (gdc_5mhz_mode) retval |= 1u;
+            break;
         case 0x0A:      // Graphics mode + 256-color
             if (pc98_gdc_vramop & (1 << VOPBIT_VGA)) retval |= 1u;
             break;