add initial support for DRIVER.SYS

currently only skeleton logic and implementation of install check & get drive data table list [returns ddt* which allows mapping DOS drive # to BIOS drive # for drives handled by default DOS block driver]

kernel/inthndlr.c

@@ -1922,6 +1922,7 @@ extern intvec FAR ASM BIOSInt19;
 /* WARNING: modifications in `r' are used outside of int2F_12_handler()
  * On input r.AX==0x12xx, 0x4A01 or 0x4A02
  * also handle Windows' DOS notification hooks, r.AH==0x16 and r.AH==0x13
+ * along with DRIVER.SYS/DRIVPARAM r.AH=0x08 calls
  */
 VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *pr)
 {
@@ -2203,6 +2204,30 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *pr)
 #endif
     return;
   }
+  else if (r.AH == 0x08) /* DRIVER.SYS / DRIVPARAM */
+  {
+    switch (r.AL)
+    {
+    case 0x00:                 /* installation check */
+      r.AL = 0xff;             /* installed, 0=not installed */
+      break;
+
+    case 0x01:                 /* add new block device */
+	  /* TODO, see push_ddt() */
+      break;
+
+    case 0x02:                 /* execute device driver request */
+	  /* TODO */
+      break;
+
+    case 0x03:                 /* get drive data table */
+      r.DS = FP_SEG(&nul_dev);
+	  r.DI = FP_OFF(getddt(0));
+      break;
+    }
+
+    return;
+  }
   /* else (r.AH == 0x12) */
 
   switch (r.AL)

kernel/main.c

@@ -340,6 +340,9 @@ STATIC void init_kernel(void)
   LoL->lastdrive = 26;
 
   /*  init_device((struct dhdr FAR *)&blk_dev, NULL, 0, &ram_top); */
+  /*  WARNING: dsk_init() must be called prior to update_dcb() to ensure
+      _Dyn (start of Dynamic memory block) is the start of drive data table (see getddt() in dsk.c)
+   */
   blk_dev.dh_name[0] = dsk_init();
 
   PreConfig();