Adjust GAMESTART logic, consolidate VIP sign-extensions

core/bus.c

@@ -159,6 +159,8 @@ static void busRead(VB *sim, uint32_t address, int type, int32_t *value) {
 
         case 2: /* Misc. I/O */
             *value = busReadMisc(sim, address, type);
+            if (type == VB_S8)
+                *value = (int8_t) *value;
             break;
 
         case 3: break; /* Unmapped */

core/vip.c

@@ -654,29 +654,29 @@ static int vipEmulateDisplay(VB *sim, uint32_t clocks) {
                 vipThrow(sim, 0x0010); /* FRAMESTART */
 
                 /* Game frame */
-                if (sim->vip.xp.xpen) {
                 if (sim->vip.xp.frame >= sim->vip.frmcyc) {
-                        sim->vip.dp.buffer ^= sim->vip.xp.toggle;
                     sim->vip.xp.frame = 0;
-                        sim->vip.xp.toggle  = 0;
 
-                        /* Initiate drawing procedure */
-                        if (sim->vip.xp.step == 0) {
-                            sim->vip.xp.enabled  = 1;
-                            sim->vip.xp.overtime = 0;
-                            sim->vip.xp.step     = 1;
                     vipThrow(sim, 0x0008); /* GAMESTART */
-                        }
+                    if (sim->vip.xp.step != 0) {
-
-                        /* Drawing procedure has run into overtime */
-                        else {
                         sim->vip.xp.overtime = 1;
                         vipThrow(sim, 0x8000); /* TIMEERR */
                     }
 
-                    } else sim->vip.xp.frame++;
+                    /* Initiate drawing procedure */
+                    else if (sim->vip.xp.xpen) {
+                        sim->vip.dp.buffer  ^= sim->vip.xp.toggle;
+                        sim->vip.xp.enabled  = 1;
+                        sim->vip.xp.overtime = 0;
+                        sim->vip.xp.step     = 1;
+                        sim->vip.xp.toggle   = 0;
                     }
 
+                }
+
+                /* Not a game frame */
+                else sim->vip.xp.frame++;
+
                 break;
 
             /* 0ms-3ms - Idle */
@@ -1316,8 +1316,13 @@ static void vipRead(VB *sim, uint32_t address, int type, int32_t *value) {
         *value = 0;
 
     /* I/O register */
-    else if (address < 0x60000)
+    else if (address < 0x60000) {
         *value = vipReadIO(sim, address, type);
+        switch (type) {
+            case VB_S8 : *value = (int8_t ) *value; break;
+            case VB_S16: *value = (int16_t) *value; break;
+        }
+    }
 
     /* Unmapped */
     else if (address < 0x78000)