Merge pull request #4677 from nanshiki/pc98gdcstatus

Fixed to set the drawing bit of the GDC status.
joncampbell123 · Dec 18, 2023 · 5c9579d · 5c9579d
2 parents 92628d6 + 49f9145
commit 5c9579d
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Showing 3 changed files with 20 additions and 11 deletions.
diff --git a/include/pc98_gdc.h b/include/pc98_gdc.h
@@ -174,6 +174,10 @@ struct PC98_GDC_state {
     bool                    doublescan;                 /* 200-line as 400-line */
 
     bool                    dbg_ev_partition;
+
+    double                  drawing_end;
+    int                     dot_count;
+    uint8_t                 drawing_status;
 };
 
 typedef union pc98_tile             egc_quad[4];

diff --git a/src/hardware/vga_pc98_gdc.cpp b/src/hardware/vga_pc98_gdc.cpp
@@ -150,6 +150,7 @@ PC98_GDC_state::PC98_GDC_state() {
     horizontal_back_porch_width = 0;
     vertical_front_porch_width = 0;
     vertical_back_porch_width = 0;
+    drawing_status = 0;
     reset_fifo();
     reset_rfifo();
     draw_reset();
@@ -656,16 +657,12 @@ uint8_t PC98_GDC_state::read_status(void) {
     if (rfifo_has_content())
         ret |= 0x01; // data ready
     else if (fifo_read == fifo_write) {
-#if 0 // THIS IS CAUSING SEVERE PERFORMANCE ISSUES, DISABLED!
-	// According to
-	// [http://hackipedia.org/browse.cgi/Computer/Platform/PC%2c%20NEC%20PC%2d98/Collections/Undocumented%209801%2c%209821%20Volume%202%20%28webtech.co.jp%29/io%5fdisp%2etxt]
-	// bit 3 (0x08) is supposed to indicate when the GDC is drawing. Perhaps the contributer who's
-	// pull request added this found a PC-98 game that failed to run without it. Re-enable when a
-	// higher performance implementation is possible. Also, recent commits in 2022 added actual
-	// GDC drawing functionality, so perhaps this should mirror that too?
-        initRand();
-        if (rand()%20<1) ret |= 0x08;
-#endif
+        if(drawing_status) {
+            if(PIC_FullIndex() > drawing_end) {
+                drawing_status = 0;
+            }
+            ret |= drawing_status;
+        }
     }
 
     return ret;

diff --git a/src/hardware/vga_pc98_gdc_draw.cpp b/src/hardware/vga_pc98_gdc_draw.cpp
@@ -7,6 +7,7 @@
 #include "logging.h"
 #include "pc98_gdc.h"
 #include "pc98_gdc_const.h"
+#include "pic.h"
 #include <math.h>
 
 /* do not issue CPU-side I/O here -- this code emulates functions that the GDC itself carries out, not on the CPU */
@@ -191,7 +192,8 @@ void PC98_GDC_state::draw_dot(uint16_t x, uint16_t y) {
     //       The GDC is documented to issue 16-bit read/modify/write when drawing, so
     //       that's what this code should do. Additionally, drawing with 8-bit memio
     //       and EGC causes minor artifacts.
-    if ((pc98_gdc_vramop & 0xE) == 0xA || (pc98_gdc_vramop & 0xE) == 0xE) {
+    // NOTE: It seems that the same behavior as EGC is required for GRCG+RMW.
+    if ((pc98_gdc_vramop & 0xE) == 0xA || (pc98_gdc_vramop & 0xC) == 0xC) {
         if(dot) {
             // REPLACE. COMPLEMENT or SET
             if(draw.mode == 0x00 || draw.mode == 0x01 || draw.mode == 0x03) {
@@ -219,6 +221,7 @@ void PC98_GDC_state::draw_dot(uint16_t x, uint16_t y) {
             }
         }
     }
+    dot_count++;
 }
 
 void PC98_GDC_state::pset(void) {
@@ -450,6 +453,7 @@ void PC98_GDC_state::box(void) {
 }
 
 void PC98_GDC_state::exec(uint8_t command) {
+    dot_count = 0;
     switch(draw.ope & 0xf8) {
         case 0x00:
             pset();
@@ -473,4 +477,8 @@ void PC98_GDC_state::exec(uint8_t command) {
             break;
     }
     draw_reset();
+    // GDC status drawing bit
+    drawing_status = 0x08;
+    // uPD7220's 1-dot drawing time is 800ns
+    drawing_end = PIC_FullIndex() + (0.0008 * dot_count);
 }