In this blog post I’m going to talk about some computers that came out in the late 70’s and early 80’s. These are some of the earliest home computers and utilized mostly 8-bit CPUs. Schematics are available and I’ve done some deep-diving into the circuitry to see how these machines were created. The first microcomputer I put my hands on was the Commodore Pet. I have used commodore 64s, TRS-80s, an Apple IIc and I own a Macintosh. I also used many variations of the PC XT.
The Apple I
I have never actually seen an Apple I computer. According to my research about 200 units were sold. This computer came mostly in kit form, but Steve Jobs sold 50 units to the Byte store fully assembled. You can download the manual, which includes a schematic in the back, by clicking here. When you look at the schematic you can see the 6502 processor and the ROM and RAM sections. There are some jumpers on the board that allow you to use a 6800 CPU instead of the 6502. They’re using an MK4096 memory for RAM. That’s a dynamic RAM chip that is organized as 4096 x 1 bit. So it takes 8 chips to represent 8k bytes of memory. I count 16 chips in that row of memory (click for larger image):
So that’s 8k of RAM. The PROMs are used for booting up the machine, the schematic shows two chips. Note 11 indicates that they are 256 x 4 bits wide, so there’s 256 bytes of bootstrap program. This computer didn’t ship with any device to store your programs on, so you would need to interface with something like a cassette drive or paper tape reader or something. The competition for this computer when it was built was the IMSAI 8080 or the Altair 8800. Both of which used front panel switches to input a program. There is a video output circuit and a keyboard can be attached to the Apple I motherboard which gave it an advantage over the competition.
The Commodore Pet
Somewhere around 1978 or so my family visited an electronic engineer that we used to be neighbors of. He had just bought a Commodore Pet (Personal Electronic Transactor). I had never touched a computer before this and I was amazed at all the things it could do. We played a lot of computer games that night. The machine used an audio cassette tape to store programs onto. This could take up to 30 minutes to load a program (at about 75 bits per second). You can see the cassette tape machine next to the square keyboard:
Photograph by Rama, Wikimedia Commons, Cc-by-sa-2.0-fr, CC BY-SA 2.0 fr
The schematics for this machine can be found here. This computer also uses the 6502 CPU. The original PET used the MOS 6550 RAM, which is 1k x 4. The first computers came with either 4k or 8k of memory and articles on-line indicate that the chips can be replaced with the 2114. All of these RAM chips are static ram, no refresh logic was necessary but the chips were more expensive. You can find more history on the PET at the wiki site by clicking here.
The TRS-80 was built by Tandy/Radio-Shack and it used the Z80 CPU. You can find a schematics inside the technical manual by clicking here. The schematics show that this computer used the 2102 static RAM. I’m surprised that they didn’t take advantage of the dynamic RAM refresh logic built into the Z80 CPU. The initial memory of a TRS-80 was also 4k in size:
The computer can be upgraded by replacing the chips with bigger chips and re-configuring the selector wires. You can see the 4k or 16k configuration jumpers in places on the schematic (see X71 blocks below):
The TRS-80 also stored programs using a tape drive. In 1977 this was the cheapest medium to store data on. The hard drive was far too expensive for personal computer use and the floppy was still a bit out of the price range of most people. The 5 1/4″ floppy drive came onto the scene in 1978 (click here).
Space invaders was one of the first video games when arcades become all the rage. I was in high-school when arcades opened near me and I spent many quarters on these machines. The space invaders game used the 8080 CPU and the schematic can be downloaded by clicking here. This computer used the Intel 2107 memory which is a 4k x 1 dynamic RAM. The board has 16 chips, so there is 8k bytes of memory. When looking at the schematics for space invaders, you’ll notice that the hardware is designed specifically for the game itself (since that’s all it really does). So there’s a synthesizer circuit for the explosions and missile sounds, etc:
There’s a special video circuit too. Apparently the monitor is mounted sideways in the video game console so the aspect ratio is vertical. So the program that runs actually plots the pixels sideways (so the aliens appear right-side up). You can download the assembly language for this game by clicking here. The EPROMs for this machine contain the running program and there are straps that allow between 4k to 64k of EPROM storage:
Apparently, the game can be upgraded in the field if Midway decided to update the program. A local technician could pull the old chips off the board and insert new chips. This, of course, never happened because arcades went out of style in what seemed like a nano-second. I remember my favorite arcade disappearing only 3 or 4 years after they opened.
Here’s the coin input circuit:
I traced the route and it looks like it’s multiplexed (74153 is a multiplexer) and fed into another circuit on the motherboard. I’m betting that this drives the interrupt line and there is probably a small program that increments the coin count. Was that displayed on the screen? Why yes it was (see “Credit”):
The arcade game uses a black and white display and the program renders one dot per bit using 7k of RAM (224 x 256 pixels). The color that you see in the picture is a plastic cellophane overlay that was adhered to the screen. The Wiki on Space Invaders has an interesting story on the challenges of building this game (click here). Here’s an interesting excerpt from the Wiki:
Because microcomputers in Japan were not powerful enough at the time to perform the complex tasks involved in designing and programming Space Invaders, Nishikado had to design his own custom hardware and development tools for the game. He created the arcade board using new microprocessors from the United States. The game uses an Intel 8080 central processing unit, features raster graphics on a CRT monitor and monaural sound hosted by a combination of analog circuitry and a Texas Instruments SN76477 sound chip. Despite the specially developed hardware, Nishikado was unable to program the game as he wanted—the Control Program board was not powerful enough to display the graphics in color or move the enemies faster—and he considered the development of the hardware the most difficult part of the whole process. While programming the game, Nishikado discovered that the processor was able to render the alien graphics faster the fewer were on screen. Rather than design the game to compensate for the speed increase, he decided to keep it as a challenging game play mechanism.
That’s all for now. I have a collection of other computers that I’ll talk about in a future post. If you enjoyed this blog post, please hit the like button!