Page 0
$00-$7F
Operating system zero-page. The entire first half of page zero is reserved for the operating system.
$80-$FF
Free zero-page. The top half of page zero is free if BASIC is disabled. BASIC uses all but $CB-$D1. The floating point math routines use $D4-$FF. If the floating point arithmetic package is not used this memory is free.
Page 1
$100-1FF
This is the 6502 stack. The stack pointer initialized to $1FF and moves downward as the stack is filled.
Pages 2-5
$200-$47F
This area is used for operating system database variables. Parts which are not used in some particular programs, such as the cassette buffer or printer buffer, may then be used for other purposes. See the O.S. equate listing for these locations.
$480-$57D ($480-$6FF if no floating point)
This is called the user work space. It is free to be used by programs. If the floating point arithmetic package is not used the user work space extends to $6FF. This area is used by BASIC.
$57E-$5FF
This area is used by the floating point arithmetic package. It is free if the package is not used.
Page 6
$600-6FF
Atari has solemnly sworn never to put anything in this page of memory.
Page 7-the screen region
$700
This is called the boot region. Most machine language programs which don't use DOS load at this address. DOS extends from $700-$1CFB.
MEMLO
The address pointed to by the O.S. database variable MEMLO [$02E7,2 (743)] is the first byte of free memory. This pointer is usually changed by any program's initialization routine. For example, upon power-up, MEMLO points to $700. When DOS loads in, DOS changes MEMLO to point to $2A80. If an AUTORUN.SYS program then loads in just above DOS, such as DISKIO, it will usually change MEMLO to point above itself. One important reason for this is to protect the program from BASIC. BASIC uses memory starting at MEMLO.
MEMTOP
MEMTOP [$2E5,2 (741)] is set by the O.S. whenever a graphics mode is entered. The graphics region is at the very top of ram and extends downward. The address MEMTOP points to depends on how much memory the screen region uses.
APPMHI
APPMHI [$0E,2 (14)] should be set by any program to point to the highest address required by the program. If the O.S. cannot set up a screen without going below APPMHI it will return a not-enough-memory-for-screen-mode error.
The cartridge slots
$8000 (32768)
This is the beginning of the 8K bytes used by the right cartridge slot of the 800. This is also where 16K cartridges begin. If there is no cartridge here it is ram.
$A000 (40960)
This is the beginning of the left cartridge of the 800 or the only cartridge slot on all other models. This is where the BASIC ROM resides in the XL/XE models. This area is RAM is there is no cartridge or BASIC is disabled on XL/XE models.
above the cartridges
$C000-$CFFF (49152-53247)
This area is empty on the 800. Sometimes special ROM chips, such as Omnimon are wired in here. On the XL/XE models $C000-C7FF is free ram if there are no cartridges. On XL/XE models, the O.S. ROM starts at $C800
$D000-$D7FF (53248-57373)
This area is taken up by the hardware chips. The chips actually take only a fraction of this space. If these addresses are further decoded there is space for many, many more hardware chips. For example, The PIA chip uses 256 bytes of memory but needs only 4 bytes. There is room for 64 PIA chips in this reserved memory.
$E000-E3FF (57344-58367)
This is the location of the ATASCII character set.
$E400-FFF7 (58368-65527)
This is the operating system ROM
$FFF8-$FFFF (65528-65535)
These last 8 bytes contain the addresses of the interrupt vectors. Upon power up the 6502 gets a reset pulse and looks up the reset routine here.