block: Check for read/write requests over 64K

The standard BIOS disk read/write request interface should never get a
request for more than 64K of data.  Explicitly check for overly large
requests and reject them.  This way, the low-level drivers do not need
to check for or attempt to handle very large requests.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/block.c b/src/block.c
index 43af305..3f7ecb1 100644
--- a/src/block.c
+++ b/src/block.c
@@ -485,6 +485,10 @@
 {
     ASSERT16();
     int ret, origcount = op->count;
+    if (origcount * GET_GLOBALFLAT(op->drive_gf->blksize) > 64*1024) {
+        op->count = 0;
+        return DISK_RET_EBOUNDARY;
+    }
     u8 type = GET_GLOBALFLAT(op->drive_gf->type);
     switch (type) {
     case DTYPE_FLOPPY:
diff --git a/src/disk.c b/src/disk.c
index fe2e2c3..0e0af24 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -173,6 +173,7 @@
     struct disk_op_s dop;
     dop.drive_gf = drive_gf;
     dop.command = CMD_RESET;
+    dop.count = 0;
     int status = send_disk_op(&dop);
     disk_ret(regs, status);
 }
@@ -322,6 +323,7 @@
     struct disk_op_s dop;
     dop.drive_gf = drive_gf;
     dop.command = CMD_ISREADY;
+    dop.count = 0;
     int status = send_disk_op(&dop);
     disk_ret(regs, status);
 }