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From: connery@bnrmtv.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Creating RAM Disks on the fly.
Message-ID: <1585@bnrmtv.UUCP>
Date: Tue, 7-Apr-87 22:29:07 EST
Article-I.D.: bnrmtv.1585
Posted: Tue Apr  7 22:29:07 1987
Date-Received: Sat, 11-Apr-87 05:00:29 EST
References: <3347@vrdxhq.UUCP>
Organization: Bell Northern Research, Mtn. View, CA
Lines: 63

> 
> I am looking for a way to generate a RAM disk from within a program,
> and then release it prior to exiting the same program.
> 
> Any information on the following would be appreciated :
> 
>    1) How can the last drive assigned be found ?  (If A: and B: are
> floppies, and C: and D: are the hard drive and an existing RAM disk,
> how do I find out that the next available drive is E: ??)
> 
>    2) How can I use this info (#1 above) and allocate RAM to create a
> RAM disk (E: in this example) for some arbitrary amount of memory ?
> 
>    3) How do I get rid of the RAM disk and return the RAM to the
> system when I am done using it ?
> 
>    4) How do I get the amount of RAM memory available in the system ?
> 
I doubt this is what you really want to do.  It would be better in most
instances to do the paging/swapping yourself in high memory.  However,
assuming this is really what you want to do:

1) You can find the value of LASTDRIVE, the last legal drive number by
   calling INT 21H function 0EH, set default disk drive.  If you set AH
   to the current drive so that it effectively does nothing, it returns
   AL=LASTDRIVE.  ie:

       mov ah,19h
       int 21h                   ; gets al=current drive
       mov dl,al
       mov ah,0Eh
       int 21h                   ; sets current drive from dl

   returns AL=LASTDRIVE.  It would be E: or 5 if pre-DOS 3.0 or if the
   user has no LASTDRIVE statement in CONFIG.SYS.  Skipping over A: and
   B: which are always used, you can determine whether a given drive
   is in use (real or redirected) by calling INT 21H function 36H with
   DL=the drive number (starting at 1 this time).  If the value returned
   in AX is 0xFFFF then the drive is not in use.  This process can take
   a little bit of time if you have a hard disk, and generally you want
   to start AFTER any hard disks, which can be obtained through the
   BIOS equipment check, but this should be enough to get you going.

2) Buy a good book on TSRs.  This is much too complex to explain here.
   Basically you are going to hook the DOS disk stuff and handle it in
   memory.  There is source for VDISK.ASM on the second DOS disk which
   should give you a start, but it's a device driver which won't do you
   much good.  If you can find source for one of the freeware TSR VDISKs
   that would probably get you pretty close.

3) See 2)

4) This is easy.  Use INT 21H function 62H to get the PSP address.  Its
   returned in BX.  This is the paragraph where this program loaded, ie.
   the start of available memory.  At offset 2 in the PSP is a word
   containing the top of memory segment address.  Subtract the two
   to get the amount of RAM (remember that these are in paragraphs).

...Glenn
-- 

Glenn Connery, Bell Northern Research, Mountain View, CA
{hplabs,amdahl,3comvax}!bnrmtv!connery