AUTHOR'S PREFACE

What exactly is a memory map? It is a guide to the memory locations in
your computer. A memory location is one of 65536 storage places
called bytes in which a number is stored. Each of these bytes holds a
number for programs, data, color, sound, system operation, or is
empty (i.e., has a zero in it), waiting for you to fill it with your own
program.
Each byte is composed of eight bits, each of which can be either a one
(on) or a zero (off). The alterable area of memory you use for your
programs is called the Random Access Memory (RAM), while the area
used by the Atari to run things is called the Read Only Memory
(ROM). Although some of the memory locations in the special Atari
chips were designed to be written to like the RAM, the rest of the ROM,
including the Operating System ROM, cannot be altered by you since
it contains routines such as the floating point mathematics package
and the input/output routines.
I hope that the reader is familiar enough with his or her Atari to
understand some of these rudimentary uses of a memory map. It is not
the scope of this manual to fully explain how to use PEEK and POKE
statements; refer to your BASIC manual. Briefly, however, PEEK
allows you to look at the value stored in any one memory location. If
you want that value to be printed to the screen, you must preface the
PEEK statement with a PRINT statement such as:

PRINT PEEK (708)

If you haven't changed your color registers, this will return the number
40 to your screen. All bytes in the Atari can hold a number between
zero and 255. POKE allows you to place a value into a byte, such as:

POKE 755,4

By doing this you will have turned your text upside down! You can
return it to normal by:

POKE 755,2

Similarly, POKE 710,80 will turn your screen dark purple! As with
PEEK, POKE can only involve numbers between zero and 255. You will
not be able to POKE into most of the ROM locations since the numbers
in many of them are "hard-wired," "burned" into the chip, and cannot
be changed in this manner.

So how does the Atari (or other eight-bit microcomputers, for that
matter) store a number larger than 255? By breaking it down into two
parts; the Most Significant Byte (MSB), which is the number divided
by 256 and rounded down to the nearest whole number, and the Least
Significant Byte (LSB), which is the original number minus the MSB.
The Atari knows to multiply the MSB by 256 and add the LSB to get the
number. For example, the number 45290 is stored as two parts: 234