14
STAR*TECH JOURNAL/NOVEMBER 1982
SEGNGREMLIN'S
ROM LINE (PART 2)
Memory Circuits
In the last installment, we discussed the heart of computer
systems - the microprocessor. We said the microprocessor
is a synthesis of many TTL functions, and that the device is
unique in that it can be programmed. Programming the
processor requires that we have some place to keep the
instructions and data the microprocessor needs to operate.
This is where the memory circuits enter our discussion.
These are the devices that hold the program codes for the
microprocessor.
Memory circuits, as they appear in video games, are of
two main types : RAM (Random Access Memory) and ROM
(Read Only Memory). RAM is memory that allows data to
be put into (written) or taken out (read) of the device. ROM
only allows data to be taken out (read) by the processor.
Let's look at ROM more closely.
ROM circuits consist of EPROMs (Erasable
Programmable ROM) and PROMs (Programmable ROM).
EPROMS are those ICs with a small window in the top
center of its package. When the chip is programmed with
certain data, this data can be erased by exposing the chip to
ultraviolet light. Then, the same device can be used again
and again. PRO Ms, on the other hand, are programmed just
once - the data they contain cannot be changed. ROM
memory, then, is perfect for holding the program instructions
for a certain game.
RAM memory is different in that the processor can send
information to it and have it stored there. At a later time, the
processor can retrieve that same data from the RAM. So,
RAM is used in video games to act as temporary storage for
the characters generated on the monitor. This video RAM,
as it is called, can be changed rapidly to reflect changes in
the game action on the screen. Unlike ROM data, the
information in a RAM will become lost when power is cut
off to these circuits. That is why some games have a battery
back-up system that maintains power to the RAM long after
the game power is turned off.
All memory circuits, ROM or RAM, must be addressed
by the processor to locate a unique portion of that memory.
The microprocessor typically advances its address values to
locate successive instructions in memory. Or, the processor
may address the portion of video RAM that corresponds to
the center of the video picture to change a character there.
The movement of data in memory, then, is not at all
haphazard; it is systematic and ordered, with a specific
location and direction.
This brief introduction to the computer memory circuits
touches on the two type of memory ICs you'll see in almost
any video game. In the next installment, we'll look at the
clock circuitry in a computer and see how it keeps a video
game computer well timed. Catch us in December!
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