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From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Memory accessing performance
Date:	Fri, 21 Dec 2001 09:48:07 +0200
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Alex Oleynikov wrote:
> 
> I've got a question related to memory accessing performance when using DPMI.
> Currently I am developing an application which talks to PLCs via Modbus
> protocol and I need to create a map of Modbus registers in my program.
> Every register is two bytes of data and there are quite a few of them (up to
> 50000). First thing that comes to the mind is to use dynamic arrays of short
> integers, but wouldn't it be much slower compared to 32-bit integers? (since
> the compiler generates a 32-bit code which is more efficient accessing
> 32-bit memory locations, right?).

I don't understand your concerns: it sounds like you have an array of 100KB
of data, and you want to access it as fast as possible?  If so, I don't see
any problem: just allocate 100KB of memory at the beginning, and then access
the array as usual.  malloc already takes care of a proper alignment for the
allocated memory, so the memory accesses will be as fast as they get.

There's no penalty for memory access in DPMI mode; assuming that your
machine has enough memory to not start paging when you allocate 100KB, it's
just you, the CPU, and the bare metal, with nothing in between.

> Those registers must be updated about
> every 250ms, so I'd rather sacrifice size to gain performance.

How does this update process work?  That is, how do you talk to the device
itself?  Depending on how you do it, the actual bottleneck might be
communicating with the device, not updating 100KB of your program's memory.

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