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ROM (READ ONLY MEMORY)
ROM causes the PET to perform most of its operations. In each PET, 14K of
ROM
contains a series of
programs written by Commodore which scan the keyboard, print the display, control inputfoutput, count
the real time clock, and execute commands that the user types in. Read Only memories are not only
the lowest cost memory for storing this data, but also give the user the most protection and the fastest
operation of his machine. This is because the operating system memory Is indestructible from the
keyboard, or from the user's program. Not only is the machine available to run basic from the moment it is
powered on, but also the user program cannot damage the basic operating system.
1/0 MEMORY
The second type of memory is that which is devoted to Input/output operations. This memory contains 110
devices called PIA* and VIA** which allow the PET to individually control the bits that manipulate the
computer. Except when special 110 operations are desired, the user should not allow his program to
interfere in any way with these areas. The operating system automatically handles these locations in
order to perform legitimate Input/Output operations.
USER READWRITE MEMORY· R.A.M.(RANDOM ACCESS MEMORY)
The third type of memory is the User Program Memory Space. (We will call this area RAM
throughout this book.) In a standard 8K PET, it is located from location $0000 to hexidecimal
$1FFF. A detailed map of all the memory is included in figure 2.3, showing where the ROM,
RAM, 110, and Screen Memory are located from a programming standpoint. As you can see
by the map, the first 1024 bytes of memory are reserved for the operating system to use
for its various tasks, including the buffering of data from the cassettes and other 110 devices.
The message "7167 BYTES FREE" is a result of an analysis of BASIC which starts at
locxation 1024 and cycles through the memory to determine which locations are available, thereby,
performing a check on whether or not the ReadlWrite Memory is working correctly.
If the number was less than 7167, you may have a hardware problem. 11 the number is
greater than 7167, you probably have added your own memory. BASIC can automatically check
up to 32K of RAM as long as the added memory is continuous to the memory that comes
furnished with the PET. This memory is really the working memory in the machine; it is where programs
are loaded and BASIC holds all of the program variables.
Later on, we will discuss some techniques to expand this memory by using tape files and
program overlays.
SCREEN MEMORY
The screen memory is physically composed of the same kind of chips that are used to
make up the PET's standard memory. It is constantly being used by the CRT control electronics,
which takes the individual bytes of memory and uses them to address a special character generator
ROM, thus displaying characters on the screen.
As mentioned during the power-up discussion, this process is totally automatic, and the programmer has
no direct control over it.
*PIA - Peripheral Interface Adaptor
* *VIA - Versatile Interface Adaptor
For information about these and related chips, see 6502 Hardware Manual.
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