Atari XL User Manual

Memory Management
75
the memory. You can tryout PEEK and POKE by
using them to control the display.
Type in these instructions:
GRAPHICS O:POKE 82,20
The cursor will jump to the middle of the screen,
and from now on you will only be able to use the
right-hand side of the display. This is because the
XL uses location 82 to control the width of the
screen - this location normally contains the value
2, resulting in a2 column left-hand margin. The
POKE command placed a 20 in this location,
causing the left-hand margin to move to column 20.
If you now type:
PRINT PEEK(82)
the XL will print the answer 20, telling you that
the number in location 82 is 20. (It should be, as
you've just put it there!) Before you continue, type:
POKE 82,2
which will reset the margins to their normal
positions.
What makes the POKE and PEEK commands so
important is that not all the memory addresses of
the XL are used for memory. Some of the addresses.
are used to point to the control registers of the
special chips which control the sound and graphics
facilities. This means that the chips can be
controlled simply by writing a number into these
control registers using the POKE command. These
special chi ps will be described in the following
chapters.

Atari XL Specifications

General

Brand	Atari
Model	XL
Category	Desktop
Language	English