Newsgroups: comp.lang.pascal
Path: utzoo!utgpu!watserv1!watmath!dmurdoch
From: dmurdoch@watmath.waterloo.edu (Duncan Murdoch)
Subject: Re: Variable sized arrays?
Message-ID: <1991Apr26.180056.9188@watmath.waterloo.edu>
Organization: University of Waterloo
References: <1991Apr21.211234.17738@ux1.cso.uiuc.edu> <1991Apr22.000227.26061@bnlux1.bnl.gov> <1991Apr23.022206.29766@ux1.cso.uiuc.edu>
Date: Fri, 26 Apr 1991 18:00:56 GMT
Lines: 33


There were a couple of misconceptions in earlier postings about trying to
allocate contiguous blocks of size greater than 64K.

1.  Why a limit of 65521 on Getmem allocations?

This comes from the facts that, in versions 4.0 to 5.5, Getmem allocates
the block at the very first place that it can find it, and all routines
that use allocated blocks do 16 bit addressing only.  

If Getmem happens to allocate something at location 1234:000F, then the
65521st byte will be at address 1234:FFFF, and the 65522nd would wrap around
to 1234:0000.

In versions 4.0 to 5.5, the offset portion of an allocated address is
always in the range 0000 to 000F, hence the limit.

In version 6.0, the offset portion is always either 0000 or 0008, so 
presumably the limit is a bit bigger, but the manual doesn't mention it.
Here you'll have to watch out for the fact that you might skip some bytes
at the beginning to achieve the 8 byte granularity, but it should be
a bit easier to get contiguous allocations.

2.  Is a contiguous allocation good enough?

The answer in all versions is no.  Unless you do your own array addressing
in assembler, inline, or pointer arithmetic, there's no way to get at 
anything after the address wraps around.  You can't have an array bigger
than 64K if you want to be able to use it as an array.

Duncan Murdoch
dmurdoch@watstat.waterloo.edu (when it's up :-)