Category : Alternate Operating Systems - Quarterdeck DesqView, CP/M, etc
Archive   : QTEC9306.ZIP
Filename : DVFLOPPY.TEC

ID:DK DESQview: Why Floppy Access is So Slow
Quarterdeck Technical Note #230 Filename: DVFLOPPY.TEC
by Vernon Balbert and Michael Bolton CompuServe: DVFLOP.TEC
Last revised: 6/25/92 Category: DV

Subject: This note explains why floppy drive access can cause other
applicatons in DESQview to slow down.


Q: Why is floppy disk access so slow when I'm in DESQview?

A: Actually, floppy disk access isn't slower under DESQview. Accessing a
floppy disk slows down everything else under DESQview, including DESQview
itself. There are two reasons for this. First, when you are in DESQview, a
certain amount of overhead is expected under any operation. However, this
doesn't explain why other operations aren't slowed nearly as much as floppy
access.

DOS is only loaded once: when you boot your machine. Programs normally make
calls to DOS when they want some service provided by DOS, such as reading from
and writing to disks. DOS is only intended to be run once at a time. DOS was
never intended to be a multi-tasking operating system. It was not written to
be re-entrant.

Re-entrancy is the ability of one piece of code to be able to be run many
times simultaneously while it is only loaded once. Thus, it is possible to
write a program in such a way that you can load it only once but it can be
accessed by many programs at the same time. DOS can't do this. It expects
only one thing to be happening at one time. When a floppy is accessed, the
program calls DOS and says to it, "I need to read the next 10240 bytes from
A:" or something to that effect. DOS complies and starts the transfer
process. A hard disk is sufficiently fast that you don't notice any slowdown
as DOS comes back with the information almost right away. On a floppy,
however, the access time is so slow (300 milliseconds, as opposed to 28
milliseconds or less for hard drives) that everything has to stop while the
data is being retrieved from the floppy. Since it takes a long time for any
information to get to or from the floppy, and since DOS is in use while the
floppy is being accessed, everything else slows down.

DOS has a way to indicate to other programs that a DOS call is in progress.
This is called the InDOS flag; when a program is using DOS, this flag is up,
or "set"; when no program is using DOS, the InDOS flag is down, or "clear".
Programs must inspect the status of the InDOS flag before they enter a DOS
call, and should proceed only when the InDOS flag is clear. DESQview honors
this convention, prohibiting more than one program from entering DOS
simultaneously, and thus avoids system crashes.

Floppy access is notoriously timing-sensitive. Because the original PC ran at
4.77 MHz, some computer manufacturers slow down their system clocks to 4.77
MHz during floppy I/O to assure floppy compatibility. Of course this slows
down all operations during floppy I/O. It is instructive to look at the First
Meg/Timings screen of Manifest during floppy access.


Q: How can I improve things?

A: Third-party utilities exist which allow you to format, diskcopy, and copy
to floppy disks without interfering with multi-tasking. These programs use the
BIOS or access the hardware directly for formatting, rather than using DOS
services. Since BIOS services can be interrupted more easily than DOS calls,
performance in other windows improves markedly. Such utilities can be found
on the Quarterdeck BBS (310-314-3227), on Compuserve in the QUARTERDECK section, or
on many large local BBSs.

Experiment with the DOS Buffer for EMS setting. The setting for DOS Buffer
for EMS (found in Setup DESQview / Advanced Options / Performance) should be
increased to 10 if you are on a machine that is connected to a network. If
you fall into neither of those categories and are using a 386 or higher
machine, try setting DOS Buffers for EMS to 0.


Summary
=======

While DESQview can multi-task, DOS can't. Because of this, when a floppy disk
is accessed, everything else in DESQview needs to wait until DOS comes back
with the information requested from the floppy.


************************************************************************
*This technical note may be copied and distributed freely as long as it*
*is distributed in its entirety and it is not distributed for profit. *
* Copyright (C) 1991 by Quarterdeck Office Systems *
************************ E N D O F F I L E *************************