AUTHOR'S PREFACE



OS: Operating System. The resident system that runs the Atari.
     The OS resides in the 10K front cartridge slot under the hood in
     your Atari 800. It's not visible in the 400 without taking the cover
     apart (not recommended). The OS is the same for both the 400 and
     800. There are two versions of the OS currently in circulation: the
     older "A" ROMs and the newer "B" ROMs, released around
     January 1982. The new OS is almost identical to the old OS except
     that it corrects a few bugs and changes some addresses. Not all of
     your old software will run with the new OS. The differences
     between the two are better explained in Appendix Four.
     Although people often refer to the entire ROM area as the OS, this
     is not correct. The OS ROM is that portion of memory which holds
     the floating point package, the Atari character set, the device
     handlers, and both CIO and SIO. The actual operating system
     itself is the portion of the OS ROM which handles the I/O.

PMG, PM Graphics: Player/missile graphics. Players and
     missiles are special moveable, user-defined, colored screen
     objects. They are often used for games, animation, or special
     cursors. PM graphics are unique in that you can establish the
     manner (priority) in which they interact with the rest of the screen
     display and each other.

RAM: Random Access Memory. All memory below the OS area
     (0 to 49151) which is used for storage, programs, buffers,
     cartridges, DOS, IOCB, shadow registers, and registers for the
     special Atari chips. Random Access means you can get to and
     from these locations at random, not that they store information
     randomly!

ROM: Read Only Memory. That part of high memory (locations
     49152 to 65535) in which the special hardware chips and the OS
     reside. ROM is also used to describe cartridge memory such as the
     8K BASIC ROM, which cannot be user-altered (the cartridge
     ROM supersedes the RAM). You cannot alter most of the ROM,
     although some of the locations in the special Atari chips may be
     temporarily set to a new value.
     With both RAM and ROM, we refer to areas with lesser values as
     being in "low" memory and locations with larger values as being in
     "high" memory.

SIO: Serial Input/Output routines located in ROM. Controls
     serial operations including the 850 interface (R:) and cassette
     recorder (C:). Briefly, SIO controls the Atari peripherals as per
     the request placed in its Device Control Block (DCB) by the
     proper device driver. It is also accessed by FMS for data transfer.

VBI: VBLANK interrupt. A VBI is an interrupt that occurs