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Re: [avr-gcc-list] How to (efficeiently !!!) test a bit within amulti-by

From: Martijn van Balen
Subject: Re: [avr-gcc-list] How to (efficeiently !!!) test a bit within amulti-byte integer ?
Date: Fri, 4 Nov 2005 16:34:29 +0100


I just remembered something that might be of use. This is a quote
from your first message:

>I just ran into a weird problem that I hardly expected.
>I wrote a trivial routine that takes a 32bit unsigned int, which holds a
>memory address, and shifts the lower 19 bits, out to a port pin (this
>feeds cascaded shift registers, that in turn drive the 19 address lines
>of a memory chip).

For a long time I always put address bit 0 on address pin A0 of the memory,
bit 1 on A1, etc. But this is not always needed! Especially for a RAM
device where you the AVR is the only one having access, the address lines
can be swapped as you like. The same holds for the data lines. For ROM memory
I have use this as well to enable better PCB routing and just swapped the contents
before flashing.

In your case if your reverse the order of A0-A18 to A18-A0, you could start with
outputting bit 0. I'm not sure if this results in smaller/faster code in your case,
but I found it a nice extra design parameter I could play with.


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