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From: andyross@ddsw1.MCS.COM (Andrew Rossmann)
Newsgroups: comp.sys.ibm.pc
Subject: More questions, and book data
Message-ID: <26659545-2cd2.8comp.ibmpc-1@ddsw1.MCS.COM>
Date: 31 May 90 23:00:05 GMT
References: <2381@zipeecs.umich.edu> <30347@cup.portal.com>
Lines: 20

>  Resp: 7 of 7 by cliffhanger at cup.portal.com
>Author: [Cliff C Heyer]
>
>I've been reading several MS-DOS developer books
>(Duncan, Norton, etc.)  but have not been able to get
>a really good "feel" as to why pre 3.3 DOS has the 
>32MB limit, and have the following questions:

  The DOS INT25H and INT26H disk read/write routines work by specifying
which sector to read. It used a 16-bit word to do this. Since the maximum
is 65536 (FFFFh), and each sector is 512 bytes, this is 65536 * 512 =
33554432 (/1048576 [1M] = 32M!)
  Under the 3.31, 3.30+, and 4.x versions, there is an alternate format
used that uses 32-bit numbers for the sectors. This gives: 4294967295 * 512
= 2199023255552 (/ 1048576 =2097152M, /1073741824 [1G] = 2098G,
/109951162776 [1T] = 2T!!!) M=Mega, G=Giga, T=Tera.
  Realistically, I don't believe this is really possible.

  Andrew Rossmann
  andyross@ddsw1.MCS.COM