247,248



247,248        F7,F8          ZTEMP4
     Temporary register.

249,250        F9,FA          ZTEMP3
     Temporary register.

251            FB             RADFLG
     Also called DEGFLG. When set to zero, all of the trigonometric
     functions are performed in radians; when set to six, they are done
     in degrees. BASIC's NEW command and RESET both restore
     RADFLG to radians.

252,253        FC,FD          FLPTR
     Points to the user's FP number.

254,255        FE,FF          FPTR2
     Pointer to the user's second FP number to be used in an
     operation.

     End of the page zero RAM.

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PAGE ONE: THE STACK
Locations 256 to 511 ($100 to $1FF) are the stack area for the OS, DOS
and BASIC. This area is page one. Machine language JSR, PHA and
interrupts all cause data to be written to page one, and RTS, PLA and
RTI instructions all read data from page one. On powerup or RESET,
the stack pointer is initialized to point to location 511 ($1FF). The stack
then pushes downward with each entry to 256 ($100). In case of
overflow, the stack will wrap around from 256 back to 511 again.

---------------------------------------------------------------------------
PAGES TWO TO FOUR
Locations 512 to 1151 ($200 to $47F) are used by the OS for working
variables, tables and data buffers. In this area, locations 512 to 553
($200 to $229) are used for interrupt vectors, and locations 554 to 623
($22A to $26F) are for miscellaneous use. Much of pages two through
five cannot be used except by the OS unless specifically noted. A
number of bytes are marked as "spare", i.e., not in use currently. The
status of these bytes may change with an Atari upgrade, so their use is
not recommended.

There are two types of interrupts: Non-Maskable Interrupts (NMI)
processed by the ANTIC chip and Interrupt Requests (IRQ) processed
by the POKEY and the PIA chips. NMI's are for the VBLANK interrupts
(VBI's; 546 to 549, $222 to $225), display list interrupts (DLI) and
RESET key interrupts. They initiate the stage one and stage two
VBLANK procedures; usually vectored through an OS service routine,
they can be vectored to point to a user routine. IRQ's are for the timer