Topic       : The ATARI Compendium
Author      : Scott Sanders / JAY Software
Version     : 1.25 (20/6/2003)
Subject     : Documentation
Nodes       : 1117
Index Size  : 32614
HCP-Version : 6
Compiled on : Atari
@charset    : UTF-8
@lang       : en
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                              Memory Map Usage


The information in this appendix provides a useful reference to the
memory locations of the Atari computer series. While most documented
locations have stayed backwardly compatible, some have changed in meaning.
Software programmers directly accessing these locations should carefully
consider the possibility that a location may move or not even exist in
a newer version of the OS. For this reason many OS functions exist to
manipulate system variables, vectors, interrupts, and devices. These
should always be used, if possible, as an alternative to directly
accessing hardware registers, vectors, interrupts, and variables.

WARNING!

In addition to those considerations mentioned above, directly accessing
hardware registers can cause damage to hardware if not done correctly. In
particular, improper use of the Falcon030 video registers could damage an
attached monitor. Likewise, use of the floppy and hard drive registers can
cause data loss and drive damage. For these reasons, it is strongly
recommended that you avoid using hardware registers when possible, and
when otherwise unavoidable, they should be used with extreme care.

Conventions

For each Atari computer that a specific hardware location is valid for,
the appropriate box will be shaded. Following is a key to several
abbreviations and concepts used in this guide:


BYTE          Occupies one byte (8 bits).


WORD          Occupies one WORD (16 bits).


LONG          Occupies one longword (32 bits).


OW            Occupies the odd WORD of a LONG.


EW            Occupies the even WORD of a LONG.


OB            Occupies the odd BYTE of a WORD.


EB            Occupies the even BYTE of the WORD.


ROM           Location is Read-Only Memory


RAM           Location is Read-Write Memory


I/O           Location is hardware-mapped


VME           Location addresses VME address space


N/A           Not applicable


RO            Read-only location


WO            Write-only location


RW            Read-write location


RSVD          Reserved


Unassigned    Either not assigned or undocumented (hardware developers
              should always consult Atari before mapping a third-party
              device to a hardware location).