memdisk: Constrain input drive numbers both by equipment byte and INT 13hsyslinux-3.32-pre4

Apparently on some BIOSes, INT 13h return a bogus number of floppy drives
when the real value is zero (probably because the code doesn't check the
validity bit in the equipment byte.)  Do it ourselves if we need to.

1 files changed, 20 insertions, 2 deletions

diff --git a/memdisk/setup.c b/memdisk/setup.c
index 556d1996..f29325a1 100644
--- a/memdisk/setup.c
+++ b/memdisk/setup.c
@@ -551,6 +551,7 @@ void setup(syscall_t cs_syscall, void *cs_bounce)
   int total_size, cmdlinelen;
   com32sys_t regs;
   uint32_t ramdisk_image, ramdisk_size;
+  int bios_drives;
 
   /* Set up global variables */
   syscall = cs_syscall;
@@ -723,11 +724,28 @@ void setup(syscall_t cs_syscall, void *cs_bounce)
 
   if ( regs.eflags.l & 1 ) {
     printf("INT 13 08: Failure, assuming this is the only drive\n");
-    pptr->drivecnt = 1;
+    pptr->drivecnt = 0;
   } else {
     printf("INT 13 08: Success, count = %u, BPT = %04x:%04x\n",
 	   regs.edx.b[0], regs.es, regs.edi.w[0]);
-    pptr->drivecnt = regs.edx.b[0]+1;
+    pptr->drivecnt = regs.edx.b[0];
+  }
+
+  /* Compare what INT 13h returned with the appropriate equipment byte */
+  if ( geometry->driveno & 0x80 ) {
+    bios_drives = rdz_8(BIOS_HD_COUNT);
+  } else {
+    uint8_t equip = rdz_8(BIOS_EQUIP);
+
+    if (equip & 1)
+      bios_drives = (equip >> 6)+1;
+    else
+      bios_drives = 0;
+  }
+
+  if (pptr->drivecnt > bios_drives) {
+    printf("BIOS equipment byte says count = %d, go with that\n", bios_drives);
+    pptr->drivecnt = bios_drives;
   }
 
   /* Discontiguous drive space.  There is no really good solution for this. */