implement hardware bug in timer

fixes PCM samples in nintendo's games and some homebrew

include/v810_mem.h

@@ -51,6 +51,8 @@ typedef struct {
     BYTE CCSR;      //Com Cnt Stat Reg,     0x02000004
     BYTE CCR;       //Com Controll Reg,     0x02000000
     HWORD hwRead;   //Hardware input read timer
+    uint8_t timerSkipState;
+    int8_t timerSkipTimer;
 } V810_HREGDAT;
 
 //Export some data structs...

source/common/v810_cpu.c

@@ -388,6 +388,7 @@ int serviceInt(unsigned int cycles, WORD PC) {
                 tHReg.tCount += tHReg.tTHW + 1; //reset counter
                 tHReg.TCR |= 0x02; //Zero Status
             }
+            tHReg.timerSkipState = 0;
             tHReg.TLB = (tHReg.tCount&0xFF);
             tHReg.THB = ((tHReg.tCount>>8)&0xFF);
         }

source/common/v810_mem.c

@@ -514,6 +514,22 @@ void hcreg_wbyte(WORD addr, BYTE data) {
         ) zstat = 0; // Clear the ZStat Flag...
 
         tHReg.TCR = (((data|0xE4)&0xFD)|zstat);
+
+        if (tHReg.tCount == tHReg.tTHW + 1) {
+            // Due to a hardware bug, when the timer is turned off then back on,
+            // it may reload early. The frequency with which it does this depends on
+            // the reload value. This seems to only happen on the 20 microsecond timer.
+            // The precise mechanism for this is unclear, as there seems to be a blip
+            // every 400-ish round-trips. However, this is close enough.
+            if ((data & 0x08) == 0 && tHReg.timerSkipState == 0) {
+                tHReg.timerSkipState = 1;
+            } else if ((data & 0x18) == 0x18 && tHReg.timerSkipState == 1) {
+                const u8 SKIPS[] = {0, 4, 2, 3, 1};
+                if (tHReg.timerSkipTimer > 0 || tHReg.tTHW % 5 == 0) tHReg.tCount--;
+                if (tHReg.timerSkipTimer <= 0) tHReg.timerSkipTimer = SKIPS[tHReg.tTHW % 5];
+                tHReg.timerSkipTimer--;
+            }
+        }
         break;
     case 0x02000024:    //WCR
         //~ dtprintf(3,ferr,"\nWrite  BYTE HCREG WCR [%08x]:%02x ",addr,data);