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From: Mark Sullivan <m...@marksullivan.net>
To: "txrxlabs@googlegroups.com" <txrxlabs@googlegroups.com>
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Date: Sat, 10 Nov 2012 15:33:57 -0600
Message-ID: <20121110153357.373537@MARXHP>
Subject: Question for an Atmel Guru
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As I understand it, the Atmel 8 bit ATMEGA microcontrollers have a dedicated=
 I/O address space from 0x20 to 0x5F.
Some of these addresses are physically wired to peripherals like timer=
 registers, I/O ports, etc.
My question is, if an I/O space address is not associated with a peripheral,=
 then can I use it as RAM or is it completely unimplemented?

For example, the ATMega8A I/O addresses 0x19, 0x1A, and 0x1B (aka data=
 addresses 0x39, 0x3A, and 0x3B) are marked "reserved" in the data sheet. =
 Can I use these locations as RAM?

My reason for asking is that I want to use the SBI and CBI instructions on=
 some working variables and these instructions can only address the first 32=
 bytes of I/O space.

And, yes, I already know the data sheet says not to do this.  

- Mark Sullivan -