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2022-08-26
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PEEK/POKE/SYS -- Part I
==== ==== === ==== =
by: Joel Ellis Rea
This series is designed for people
who are familiar with BASIC program-
ming, and would like to learn more a-
bout the memory locations that can be
used with the PEEK function, and the
POKE and SYS statements.
Beginners, listen up...
A PEEK works like this:
'LET W = PEEK(1024)' sets the variable
W equal to the contents of memory
location 1024.
A POKE works like this:
'POKE 1025,W' sets memory location
1025 equal to the value of the
variable W, which must be within the
range 0 to 255.
A SYS works like this:
'SYS 49152' attempts to execute
machine language code starting at
memory location 49152.
------
Well, that's all the easy stuff. What
follows is usually very technical, so
pay close attention.
------
The 64's memory consists of 65,536
bytes (1K = 1,024; 64K = 64 * 1,024 =
65,536), some of which is ROM (Read-
Only Memory) which cannot be changed,
some is I/O (Input/Output) Chip Regis-
ters which control hardware functions
of the machine, and most is RAM (Ran-
dom-Access Memory), which can be writ-
ten to, and which "forgets" its con-
tents when the power is turned off.
The PEEK function can read RAM, ROM
and most I/O. The POKE statement can
write to RAM and many I/O locations.
The SYS statement transfers control to
a machine-language subroutine located
in ROM or RAM.
This month, we will cover five of
the most important locations in the
whole 64, locations 0 - 4.
--------------------------------------
Location: 0 Hexadecimal: $0000
Official Label: D6510 Type: I/O
Useful BASIC commands: None
This byte MUST be kept at its nor-
mal value (such that PRINT PEEK(0) AND
63 prints 47) in order for the 64 to
be able to function correctly. It is
the Data Direction Register for the
6510 processor's On Board Bi-direc-
tional I/O port (Location 1). Its
bits determine which of the 6 I/O port
lines are input and which are output.
The normal value means that bits 0-3
and 5 are output (1) and bit 4 is in-
put (0).
--------<continued in part 2>---------
<x to exit>