|106|$006A|RAMTOP|RAM Size|both

RAM size, defined by powerup as passed from [TRAMSZ] (location 6), given in the total number of available pages (one page equals 256 bytes, so PEEK(106) * 256 will tell you where the Atari thinks the last usable address --byte-- of RAM is). [MEMTOP] (741,742; $2E5. $2E6) may not extend below this value. In a 48K Atari, [RAMTOP] is initialized to 160 ($A0), which points to location 40960 ($A000).

The user's highest address will be one byte less than this value.
This is initially the same value as in location 740. PEEK(740) / 4 or PEEK(106) / 4 gives the number of 1K blocks. 

You can fool the computer into thinking you have less memory than you actually have, thus reserving a relatively safe area for data (for your new character set or player/missile characters, for example) or machine language subroutines by: 

{{{POKE(106), PEEK(106): REM # of pages you want to reserve.}}}

see also: [TRAMSZ], [MEMTOP]