AUTHOR'S PREFACE



(LSB) and 176 (MSB). 176 times 256 equals 45056, plus 234 equals
45290


LEAST-MOST STORAGE
The Atari uses the convention of storing addresses in the LSB/MSB
manner in memory (i.e., the smaller part is in the first memory
location). For example, locations 88 and 89 store the lowest address of
the screen memory. Let's say the numbers found there are 22 and 56,
respectively. To get the decimal address, you take the MSB (stored in
89) and multiply it by 256, then you add it to the LSB at 88. In our case
that's 56 * 256 equals 14336, plus 22 equals 14358. This is the address
of the upper left corner of the screen. A simple way to do this in BASIC
is:
BYTE = PEEK (88) + PEEK (89) * 256

The reverse (to break up a decimal location into MSB and LSB) is done
by:
MSB = INT (BYTE/256):LSB = BYTE - MSB * 256

This process is easier for assembly language programmers who use
hexadecimal numbers, since the right two digits are always the LSB
and the two left of them are the MSB. For example:
$D016 (hexadecimal for 53270) equals 16 (LSB) and D0 (MSB)

$16 equals 22 in decimal, and $D0 equals 208 decimal. Multiply the
MSB by 256 and add 22 and you get 53270. Throughout the map
portion of this book I have provided both decimal and hexadecimal
numbers together for ease of reference. In 8K BASIC, you can use
decimal numbers only with POKE, and PEEK will return only decimal
values to you.
Hexadecimal is a base 16 used instead of the normal base ten system
because it is more suited to the eight-bit structure of the computer. So,
when we say 2175 in decimal, what we really mean is:
10000  1000  100  10  1
    0     2    1   7  5

In hex, the same number is $87F. That breaks down to:
4096  256  16  1
   0    8   7  F

Rather than multiply each next step up by ten, we multiply by 16.
Okay, but where do we get "F" from? Well, if base ten has the numbers
zero to nine, base 16 will have to have some letters added to the end to
make up for the extra numbers:
Decimal  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 
Hex      0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F