"MyCPU" is a homebrew CPU that was primary developed for fun. It is a simple 8-bit processor that is completely built with simple discrete logic gates of the 74xx CMOS series. Over the years the project has grown, and now MyCPU is the brain of a real Personal Computer. The project is totally open source, and everybody is invited to participate and contribute to the project.

Currently the computer consists of the
following main parts:

The processor performance is around 1 MIPS (where an integer has 8 bit). The CPU can be clocked up to 8 MHz. The speed is limited by the slow EPROM's used in the CPU design and by the slow external components. Because of the cable length and mismatched line impedance the processor bus cannot be clocked higher than 4 MHz, making it to the bottleneck.

The fully equipped system has a power consumption of less than 3.5 Watt. This is due to the fact that only CMOS chips are used and the power dissipation at 8 MHz clock frequency is near zero. Most power is consumed by the VGA Unit (1.3 Watt) that runs at 25 MHz.

The MyCPU is not only a hardware but also a software project. Today much software is available. The Operating System that resides in an EPROM is completely written in assembly. The OS is a mixture of DOS, Unix and Commodore 64, and provides most of the well-known commands to manage hard disks. MyCPU has its own file system that is able to handle up to 8 disks or partitions. Drivers for additional external hardware components like the Ethernet Controller can be loaded from disk. Furthermore MyCPU OS comes with a build-in basic interpreter that is compatible to the

basic interpreter of Commodore 64. Old pure basic programs that were written for C64 can be run on MyCPU without modification. MyCPU can be programmed in three more programming languages: For the fans of higher-level languages a "C"-Cross-Compiler is available. And hardcore freaks can write their programs directly in assembler or in the esoteric programming language "brainfuck" (MyCPU OS comes with a build-in JIT compiler for brainfuck). The following application software is currently available for MyCPU: Basic interpreter, brainfuck compiler, assembler, text editor, games and demo's, networking software like HTTP web server and a Telnet demon. And of course there are some more tools available that make life with MyCPU more convenient.

Where "MyCPU" stands for the processor itself, I use the term "MyCPU System" for the whole home-brew computer. The fully equipped MyCPU System currently consists of the following components (compare with figure[1], from left to right): MyCPU (5 boards), Interrupt Controller (1 board), Memory (2 boards), I/O (3 boards), IDE Interface (1 board), VGA Graphic (6 boards), Ethernet Controller (1 board) and the Power Supply (1 board). The system provides this interfaces to the outside world: PS/2 Keyboard, alpha-numeric LC-Display, 2 RS232 Interfaces, Parallel Printer Port (Centronics), 8 universal inputs and 8 universal outputs, VGA graphic interface (RGB, 15 pin high density Sub-D) and 10 MBit Ethernet (RJ45 twisted pair). Because I have not written the FTP server software yet, data communication with a PC is still done through a serial link via RS232 and special software. But note: You can now trash your PC since there is

everything available that you need to work stand-alone with the MyCPU System ;-). For example there is an assembler program available that runs on MyCPU and produces code for MyCPU. The evolutionary step is that the

assembler can assemble itself on MyCPU. So you can write software for MyCPU on MyCPU, and you are no more required to use a PC for special tasks.

The MyCPU processor (depicted in figure[2]) is a complex microcode machine (CISC), like any modern processor today. The internals of my CPU are shown in figure[3]. The core of the processor is the microcode engine that controls the data flow from and to the registers and arithmetic components. The processor has only 5 universal and 1 special purpose register: A, X, Y, P and SP are universal 8-bit registers, whereas the Program Counter (PC)

is a special purpose 16-bit register whose contents can be incremented by one through a dedicated signal. The P-register is only an internal register used by the microcode to store temporary data and is not visible to assembly programs. In my implementation the microcode covers many commands and addressing modes of the good old 6502 processor, but MyCPU is

not binary compatible to it. The microcode can be adopted to simulate other processors like Z80, 8051 or whatever; the only limitation is the number of internal CPU registers. For Z80 or 8051 it becomes necessary to store registers in external memory. The microcode was developed with the help of a "macro language". The Microcode-Compiler is written in "C" and does also some validity checks and optimisations on the generated microcode. For example the OP-Code "CLA" (clear accumulator) is constructed from the following micro-OP's:

 fetch_opcode_and_increment_pc,  move_const0_to_accu,  update_flags,  next_op.

The processor design is optimised for speed. With easy to obtain parts like usual EPROM's with 100ns propagation delay and the usual 74HCxxx series of logic gates MyCPU can be run with at least 4 MHz. It is possible to clock MyCPU even higher, but than some parts must be replaced, e.g. faster EPROM's are needed and some 74HC-chips must be replaced by faster 74AC-chips. I have successfully run a MyCPU at above 8 MHz, but that is only possible with a short bus length and a reduced number of peripheral boards. My goal was to have a fast and efficient processor that requires as less parts as possible by providing a superior high processing power. The MyCPU design is a good compromise. Of course it would be nice to have a 16-bit processor with 32-bit address range, but this would double the amount of required parts.

The MyCPU System is equipped with 1 MB RAM and 160 kB ROM. Since MyCPU has only an address range of 16 bit (that means 64 kB of linear memory can be accessed), it is necessary to do some bank switching to allow MyCPU to access all the memory. The ROM

is partitioned into chunks of 32 kB. The EPROM that contains the Operating System Kernel is always visible and is mapped to the address range 0000h - 7FFFh. The other four 32 kB ROM partitions can be mapped to 8000h - FFFFh. These ROM partitions

contain the filesystem driver, the shell and the Basic Interpreter. The RAM can also be mapped into the area between 8000h and FFFFh. Thus, a program that is loaded from disk can have a maximum size of 32 kB. That is why it is not possible to run Linux on MyCPU (there are some other limitations also, e.g. Linux requires at least a 16 bit processor). There are some more special features on the memory board: The RAM can be split into 16kB chunks that can be paged into the area 4000h to 7FFFh. Furthermore the two lowest 256 byte pages, which are the zero-page and the stack-page, can also be bank-switched. This allows the use of more stack memory and enables MyCPU to run multitasking operating systems. I have successfully ported the RTOS "Pico]OS" to MyCPU. OK, that is only half of the truth. Remember MyCPU has a Harvard like architecture, except that there are no two separate buses to access program and data memory. It's more like the 8051; MyCPU supports two different read signals, one for code- and one for data-memory. If you are interested in how memory bank switching really works please read the Memory-Selfbuild-Guide that can be found on my website. Physically the Memory Unit consists of two boards: The baseboard provides 512 kB RAM and 160 kB ROM. The optional extension board adds 512 kB more RAM and a second paging unit.

Figure 3: Internal architecture of MyCPU

Figure[4] shows how a minimalist computer system with MyCPU looks like. At the left side you see the MyCPU cube. The two other boards are the memory baseboard with 512 kB RAM and the RS232 interface board that mainly consists of two commercial 16C550 UART's and some glue-logic. User communication is done with a terminal program like Hyperterm on MS Windows or minicom on Linux. A second serial link is used for file transfer between MyCPU and a PC. The PC must run special server software for this purpose. When the System is powered on it searches for boot files on the PC. The user can choose one of four

software packages that is then loaded into a RAM-disk on MyCPU. With this minimalist computer system the user can write and save texts, write Basic programs, write and assemble Assembler programs, play games and run network services like the http server and Telnet over a SLIP link.

Figure 4: Smallest possible MyCPU computer system

All had begun in February 2001. I was learning for my diploma examination in digital electronics and got bored. I wanted to do something productive with the knowledge I just obtained. So I decided to construct an electronic calculator machine by using only fundamental digital gates of the 74-TTL series from which I still had some hands full in a drawer. At this point it was still nothing special to construct a calculator, many people would have done this before. It was just an education project for me.

Figure [6]: MyCPU in an early phase. You see the prototype of the VGA Graphic Unit and besides the Multi-I/O-Unit and the Interrupt Controller.

Which way to go? Because I already had some experience with the very simple but powerful 6502 CPU, the idea was not to implement the calculator algorithms in hardware but to construct some programmable machine that should be similar to the 6502. If I could port the Commodore 64 Basic Interpreter to my machine I would get the floatingpoint routines for my calculator for free.



This article was printed in
Embedded Projects Journal 03.

8 bit are enough! To keep the design as simple as possible 8 bit should be enough for my machine. Maybe you still remember the Commodore 64 and the powerful games and demos that ran on it. And this with only 64 kB RAM and less than 1 MHz clock frequency. That should be possible for my machine, too. My philosophy was and is until today to use only the simple gate parts. No higher integrated components like GAL's, PAL's and FPGA's are allowed in my design (this would be too easy and was already done by many other people). The only higher integrated parts that are permitted are EPROM's because I needed them to store the calculator program anyway.
How to start? I sketched the schematics on a piece of paper. The draft of the CPU was finished within two weeks (while I was still learning for my examination). My idea was to have a micro code based machine. The advantage is that only a few components are required. The core logic would reside in a re-programmable EPROM. And because EPROM's are so universal I also constructed the ALU (Arithmetic Logic Unit) with some big EPROM's that are simply used as lookup-tables. Thus an 8x8-bit multiplication can be done within only one clock cycle.
Simulation! But before I would start constructing the processor in hardware (what would be an exhausting task) I wanted to test my design. My approach was to write a CPU emulation in the programming language "C". Version 1 of the emulator simulated every single gate of the processor, including the timing constraints. The result was that the design was functional!

What's with software? The first emulator simulated the processor (gate by gate), 32 kB ROM and 32 kB RAM. User interaction was done by a simulated serial terminal. The test software simply read some input from the terminal and outputted it immediately again. The first "stupid" version of my today's kShell was born! (This was around April 2001; I already finished my diploma with best degree). And now: Hardware! Because all went so fine I started constructing "my CPU" in hardware. The first set-up consisted of the CPU, a memory board and a couple of LED's for I/O. As next I constructed the interrupt controller and the RS232 I/O board. The keyboard interface and the VGA Unit followed, and at the beginning of 2002 I had a fully featured "Personal Computer" on my desk. A friend of mine was so enthused of my project that he recommended me to share the project with others. So I started my website yet in September 2001.
In the following years I improved the design and wrote the self-build guides. I spent much time with developing the application software. Even today I am not finished with the CPU design, and I will possibly never be. There are still too much things to improve. My slogan is: "Others have a model railway in their cellar, I have my CPU instead."

Figure [7]: MyCPU running an old Basic game by Commodore.