From:	"A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Date:	Thu, 11 May 1995 12:09:27 BST
Subject:	Re: Checking for disk in drive

  morphine AT server DOT cs DOT jhu DOT edu (Michael Phelps) wrote (Subject: Re: Checking for
disk in drive):-

> DJ recommended to "Try to read the first sector with BIOS (retry if
> error==6). If that fails, the disk isn't in the drive.". I tried this
> method, using what I could get from the Info about the _go32 calls and Ralf
> Brown's interrupt list. I have managed (at least, in a crude way) to figure
> out if a disk is in the drive or not:-

  Phelps tried this:-
  AH = 0x02; /* BIOS disk read */      AL = 0x01; /* read one sector */
  BX = (int)disk_buffer; /* pointer to buffer???? */
  CH = 0x00; /* track zero?? */        CL = 0x00; /* sector zero?? */
  DH = 0x00; /* head zero?? */         DL = 0x00; /* drive A?? */
  ES = _go32_my_ds(); /* fill in DS */
  call interrupt 0x13

  He found by trial and error that:-
> ...the number of sectors read ... is always zero. ... 0x80 (disk time out)
> is the result if there is no disk in the drive, and 0x06 is the result from
> the first trial after inserting a valid disk, and 0x02 for all subsequent
> reads (that is, assuming you keep the disk in). So I guess this routine
> can't tell the difference between a good disk in the drive or a bad disk.

  `PC Interrupts' by Ralf Brown & Jim Kyle says that, on exit:- CARRY = 0 for
OK, 1 for error; AH = 2 for "address mark not found", 6 for "disk changed
(floppy)", 0x80 for "timeout (not ready)".
  All 3 of my interrupt books say that the buffer address should be input in
ES:BX; but Phelps seems to have input it in DS:BX. Perhaps that is why he got
AH=2 error exits and why always he always had zero sectors read.

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