22
STAR*TECH JOURNAL/OCTOBER 1982
SEGNGREMLIN's
ROM Line (Part 1)
Welcome to the ROM Line. In this first installment, and upcoming
ones, we will devote this column to the theory of microprocessor-based
video games. We'll explore, in each installment, a new aspect of the
workings of video games, and explain in detail how the parts fit together
to form a whole game. The knowledge you gain can be applied to any
video piece, since they are all essentially similar. So let's begin with a
brief introduction to that area of electronics which make video games
possible - computers. (We'll assume that most of you are familiar
with the whys and hows of TTL circuitry for the moment.)
All computers are composed of six basic elements, each working in
unison to perform a desired task. The circuitry in video games is
composed of a least these six elements in order to function as a
computer. But, because video games are more than just a computer,
they need a few additional circuits to make them work properly. These
will be covered in upcoming installments also. Any computer, then,
must have these six basic elements:
1.
2.
3.
4.
5.
6.
Microprocessor
Memory
Input Port
Output Port
Clock Circuit
Power Supply
We'll start, in Part 1 of the ROM Line, with the microprocessor.
Then, in the segments to follow, we'll tackle the theories of the other
basic elements.
Microprocessor - The Great Synthesis
Imagine designing a huge circuit consisting of all the digital functions
available today in TTL integrated circuits. With this circuit, you could
move data in a shift register, count incoming signals, AND or NOR
two or more signals together - you name it; this circuit could do it.
Essentially, what we would have designed, using these TTL functions ,
is a microprocessor - a synthesis of TTL gates, counters, registers,
etc., shrunk to a minute size and placed in one IC package. To operate
the microprocessor requires a set of instructions which are designed to
activate any of the digital functions designed into the processor. Just as
there are different kinds of microprocessors, so are there different
instruction sets for each. In GREMLIN/ SEGA games, we use the
Z80 microprocessor, and we will concentrate on this device in this
column. An explanation of the Z80 will also serve to explain other
types of processors - they are similar in basic operation.
Now, some basics about the Z80. lthas 16 address and 8 data lines,
14 control lines and 2 connections for power ( + 5 volts and ground).
The 16 address lines operate in only one direction - from the
processor to the external circuit elements. The 8 data lines, however,
are bi-directional, over which the microprocessor sends and receives
data or instructions.
Here is how the Z80 operates. First, as with all microprocessors,
the device is reset when power is first applied. When you first turn on a
game, the picture on the screen will be scrambled. Then, a moment
later, the reset circuit resets the processor and forces it to start at the
beginning of its program, and the screen displays the correct information.
What happens next is that the processor, under program control,
addresses the program memory and receives from memory the data or
instruction at that address. This information is acted upon by the
processor to do a number of tasks; for example, the instruction may tell
the processor to add 2 numbers, or to send data to the sound board
output port. Whatever the instruction, the processor must receive it
first, then act on it. This procedure is controlled by the microprocessor's
control lines.
In troubleshooting microprocessor circuits, there are certain lines
which must be checked. The data bus should be inspected for any
pulled-hi or-lo signal; check the address bus in the same way. You will
need an oscilloscope to make these checks accurately. Check for the
master timing signal on the Z80 at pin 6. This is the system clock which
ensures that all timing and control signals occur where they are
supposed to. Don't forget to check for +5 volts on the processor also.
We have covered, in a basic way, the first of the six elements in any
computer -- the microprocessor. In the next issue, we'll take a look at
memory circuits. Until then, keep tech' ing!
ATARI
"DIG DUG" CREDIT MOD/NEW REV PCBs
Solution:
Do the following modification (for new revision
boards only). There are two ways you can
identify new and old revision PCBs.
1. Older PCBs have large 40-pin chips located
near their centers. Newer PCBs have these
chips along one edge, away from the edge
connector.
Modification:
I. Cut the trace between the custom chip in
location 11 P and the capacitor labeled C3 l.
Refer to the detail in Figure 1.
2. Jumper pin 6 ofIC 6H (74LS00) to the feed
through hole shown in Figure l.
3. Jumper pin 11 of IC 7D (74LS32) to pin
4+5 of IC 6H.
Wire from
/6H/Pln 6
Figure 1
Problem:
Four credits for one coin.
11 p
10 P
~
~
Rlll
Rl75
/y
~
Feed·lhru hole
C31
~ Cul
Trace