My4TH hardware features:

• 8-bit discrete logic CPU
• 8 to 14 MHz clock frequency
• 32 KB ROM, 32 KB RAM
• up to 256 KB EEPROM
• 1-bit NOR gate "ALU"
• 6300 8-bit additions / sec
• 4 memory chips
• 16 CMOS logic chips
• RS-232 with 4800 baud
• I2C-bus for extensions
• 6 digital inputs
• 5 digital outputs
• PCB size 10 x 10 cm

My4TH software features:

• 16-bit Forth system
• Supports the Forth 2012 standard
• Core and core extension words
• Double number word set
• Block word set
• Integrated text editor

This is the world's smallest^* Forth computer with the fewest components and a discrete CPU:

The computer is controlled by a VT100 compatible serial terminal. After switching on, My4TH displays a welcome message and the Forth prompt. Now you can immediately start entering your own Forth program:

This is the block diagram of the My4TH computer. As you can see, there is no CPU or microcontroller, just memory chips and simple CMOS logic chips:

Now look at the picture below. You will see all the blocks from the diagram above:

But one question remains: How does it all work?

It all starts with the microcode address counter, which is something like the "heart" of the computer. Each machine instruction is composed of up to 128 microinstructions, whereby a microinstruction consists of two parts: an 8-bit control word and an optional 8-bit data word. The machine instruction (the "OP code") to be executed is applied to address lines A8 to A14 of the EPROM, and the microcode address counter is connected to A0 to A7. The microcode address counter counts through all microinstructions of the current OP code.

In every "even" clock cycle (when A0 is zero), the first part of the microinstruction, namely the control word, is stored in the control word latch. 5 of the 8 signals of this control word are used to control the data flow through the processor by selecting the source and destination device. The final data transfer occurs in the following "odd" clock cycle when A0 is one. Since in this second clock cycle the EPROM itself can also serve as a data source, it is easy to store constant values defined by the microcode in the various registers and the RAM. However, since any address can be input into the EPROM in this cycle, it is possible to perform random read accesses to the EPROM so that the application program (the Forth interpreter) can be executed from the EPROM.

As you can see, the main purpose of all the logic is to transfer data from the various sources to the various destinations under the control of the microcode. The CPU registers such as PC, SP, Accu, etc. are all stored in RAM. The programme counter is incremented via a look-up table in the EPROM.

Data manipulation is done via a single NOR gate on the D0 data line. You may already know that really any logical function can be derived from the NOR function, even complex functions like addition and subtraction. But to process all the bits of a byte, My4TH must loop over all the data bits and process bit by bit. The required data shift function is also implemented as a look-up table in the EPROM.

Conditional jumps are only possible within the 128 microcode positions of an OP code. When the microcode needs to perform a conditional jump, it first loads the target address (which ranges from 0 to 127) into the lower 8 bits of the 16-bit address latch. A control signal coming from the control word latch then enables the conditional loading of the new address into the microinstruction counter depending on the value of D0 within the odd clock cycle.

If you are more interested in the microcode, I recommend to study the document "The MyNOR ADD instruction" where I describe how I implemented the ADD instruction in the microcode for the MyNOR computer. Since MyNOR and My4TH are very similar, the microcode of the ADD function works exactly the same on My4TH. But to increase performance, I cheated a little on the My4TH by implementing the one-bit adder via a small look-up table in the EPROM.

Now we get down to business: The schematic. Click on the image to download it as a better readable PDF file.

Bill of material:

Want to build your own? All you need is available for download.

Don't want to hunt for all the components yourself?   Buy a My4TH kit at Budgetronics!