🚀 go-pugleaf

In article <26038@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes:
>Books explain int 21h calls as being more 
>functional and portable than int 13h calls.

Int 13h is a BIOS call.  It was written for 
use by the operating system.  Int 21h is an
operating system call (PC/MS-DOS).  Int 13h
was not intended for direct use via an
application program.

>A point of confusion is what do compilers
>output. The C compiler gives you a choice of
>what to use, but I assume other languages
>may use either 13h or 21h when you compile
>a program, and you might not know which one
>is used(?) Also 21h gives more than one
>way of doing disk I/O (FCBs, Fcn. 44 &
>_dos_ interface calls). Do you use switches
>to control which is used in some languages?

First of all, file I/O is not part of the C
language per se; no file I/O is included in
the C language at all.  So the compiler does
not emit code to call file I/O services.
C programs usually perform file I/O by calling
functions in a "standard" library, in other
words, by calling read(), write(), fread(),
printf(), etc.  These function SHOULD NOT
call int 13h, but should make all requests
via standard DOS calls (int 21h).  This is
true of MSC & Turdo C libraries.

>Further, books say 13h is for floppies only.

Not so.  There are some functions provided
by int 13h that are only for floppies,
some that are only for fixed disk, some
new ones that are only for ESDI disks, and
many that are for any disk.

>Also, do 13h and 21h access installed device
>drivers the same way, or does 13h speak
>the WD1003 interface language only and
>bypass any installed device drivers.

Original intent was that the disk device
driver(s) perform the actual dirty work by
making calls to int 13h, but there is
certainly nothing to prevent a driver from
taking over int 13h and providing its own
services.

>But I'm wondering if there other important
>benefits of one method vs. the other.

Always use the int 21h calls in your application
programs.  If you MUST do absolute reads &
writes, use int 25h & int 26h (also DOS calls),
but only if you must.  The only time it makes
sense to use int 13h is: (A) if you are
writing a disk device driver or disk caching
program (B) if you need a function not
provided by DOS (like format a track), or
(C) if you are writing a TSR, there are
certain reasons for using and/or watching
the int 13h vector.

>Perhaps I have not found the right book that
>really explains all this stuff.

Good conclusion.  Unfortunately, neither have
I, but I'm sure that it's because I haven't
been looking.  Standard advice is to start
with one of Peter Norton's books.  If you do,
make sure it is a recent one; disk drive
software in MS-DOS and various BIOSes have
changed a lot since the PC was introduced.
Here are some invaluable references:

The MS-DOS Encyclopedia:
  stupid, arragant name, outrageous $75
  price for paperback book, but every
  MS-DOS systems programmer should have
  one.

The MS-DOS (or PC-DOS) technical reference
manual:
  You can live without it if you have the
  MS-DOS Encyclopedia.  If you are an
  MS-DOS application programmer, you can
  get by with this instead.

The IBM PC and/or AT tech. ref. manual:
  Contains original PC/AT BIOS listings,
  with some hardware info as well.

PD1:<MSDOS.SYSUTL>INTER190.ARC
  Useful listing of interrupt service
  calls, many are otherwise undocumented
  for the general public.  Calls are
  documented for DOS and BIOS, as well
  as many other applications and system
  software gadgets that run on the same
  architecture.

===========================================
pipkins@imagen.com
All opinions expressed are mine only.
Remember what my "Papaw" used to say,
"I've already told you more than I know!"