In 1975, Walzniac designed a breakthrough arcade game with 44 simple chips, but did not use a microprocessor, just because he could not use these chips at the time. One year later, MOS6502 and Zilogz80 were listed, and his Apple 1 started a microcomputer revolution. About which processor is better debate still fierce. But more interesting is whether these devices are really necessary: if they never appear, what happens?
Can only 40 old-fashioned TTL chips enable MSIHHz miniature computers with video and sound?
The beginning of this project is to explore 30-40 simple logic chips. It has become a universal 8-bit microcomputer without any microprocessor drives it. Initially, it was designed on a large board in early 2017, but soon, it turned all it into a small printed circuit board. It has 64 colors with VGA-compatible 60 Hz video, which can display full-screen images, scrolling images, and play sound. Soon it will run games like PAC MAN and Space Invaders. But for fun, it will still be compatible with the design of the breadboard design.
This is now owned:
8-bit system constructed by TTL chip (74LS) in the 1970s
34 TTL IC or 930 logic doors for CPU
No microcontroller, no complex chip (such as 74181 alu)
16-bit virtual CPU runs the transplant version of the classic game, wozmon, Basic, ...
8 virtual CPUs run 6502 programs without blinking
Only simple ICs, such as And / OR, 4-bit adder, multiplexers, registers, etc.
6.3 MHz. Can be pushed to 15 MHz via 74F logic and faster RAM
32kb 70ns ram
Harvard architecture with EPROM, used for kernel and cold storage
74HCT version of power consumption is 2.5W or less than 0.5W
RISC with flow wire: 1 instruction for each clock (sometimes 2 ...)
Using diode decoding instructions
NICE instruction set: add / sub / and / or / xor, condition jump, many useful addressing mode
60Hz 64 color VGA and 4 channels operate through software
Originally designed and manufactured on weld breadboard within 6 weeks
Build has become a contradiction. In order to reduce the number of components, each hardware feature is basically software-defined: video, audio and I / O are handled by software. Pixel level video. Sampling level audio, 4 channels. Even the application itself runs in a non-existing 16-bit processor (that is, the interpreter).
However, there is no microprocessor chip that runs any virtual subsystem software: they all are processed by TTL circuitry, act as simple CPU and fast switching between these tasks. It is not only effective, but it is smaller, faster, including the first IBM PC than the microcomputer at the time. No microprocessor may be better than any time.
Some concepts to think before starting:
How much? 4, 8, 16, 32 ...
Software generated video or hard wiring?
Harvard or Vonnoiman architecture?
Single cycle or multi-cycle? assembly line?
Is Alu chip?
Is the Skit ALU still wide?
One experience is that a minimum four-bit system can be completed in 10 chips, and an 8-bit chip does not require more than 20 chips, and the 16-bit chip will double this figure. Not all unit chip counts have doubled, but through the expansion bus, you also need more addressing mode so that all units make sense. For example, for 256 words of memory and absolute addressing, four bitter words can work well, but for large memory, you need to dynamically construct the address. In addition, additional features are required, such as high speed, stack pointers, interrupts, custom ALUs, or videos, can also add more chips.
The easiest concept
A concept that may be the easiest, which uses a minimalist 4-bit TTL CPU from TTL and SRAM TrinketPro to TrinketPro for early breadboard VGA. Then, a working system of approximately 25 chips will be obtained, and if a custom ALU is created, approximately 30. It is really good for PAC Man and Space Invaders, but that will also be extreme: No chance has any fast scrolling, fast color change, smooth moving object, large object, and so on. Upgrading the CPU part to 8 bits will not bring too much impact, because there is still a communication bottleneck between these two parts. If you do this, you will definitely get more than 40 chips. Try this 25 chip concept is very interesting (do it with a 74'181 alu chip) because Space Invaders from such a small system is very attractive.
Bull
Look at it from another angle. If you look at the early video generation circuit, it already contains many CPU elements: there are counters, ROM, RAM, and bus. Therefore, first, the extended video portion is to be considered to combine the video with the CPU logic. The software will bite synchronize and pixel signals in the correct time. Dead time can be used for application code.
This choice does not know what will be, it may make the timing of the application software very complicated, but it is possible to make more interesting video possible because all the flexibility is in the software. It also requires the CPU part very fast.
Alu
The ALU is an arithmetic and logical unit, or an internal calculator in the processor. For ALU, you can choose to use 74181 or scroll it yourself. But they become difficult to obtain, so if they need them, copying this project will become a bit difficult. And they are still quite complex and large, 600 million chips. Therefore, using a simpler block to perform a custom ALU more interesting. Of course, you can cut the function, such as only add and NAND. But this extreme way compromise software is not very like.
Instruction table
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Apprecianted prior art and thanks:
Don't pretend that this is brand new. Many things can be learned from similar projects. There are many projects, but the most influential projects are:
Ben Eater's breadboard computer, used for very clear teaching video, and showing the dashboard items do not have to be like the nest of the mouse
The MT15 of Dieter Mueller, because he can break things into the most basic, then build it from there. His webpage reads like poetry
Radical Brad's QUARK 85, realized that should not be satisfied with 8x8 color LED output
Alexis Bezverkhyy's BFM3 understands the availability of parts of such a project. Most home brewed CPUs are one-off project, which needs to be avoided in design.
Steve Chamberlin's 4-bit Nibbler, used to show its simplicity
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Source: HackADAY.IO
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