4 Bit Processor Schematic

The above picture shows the 4 bit adder.

4 bit processor schematic.

Alu flags are stored in the 4 bit flags register a 173. To preform subtraction it uses 2s compliment math. It was the first commercially produced microprocessor and the first in a long line of intel cpus. More instructions could have been implemented but 4 bit instructions means 16 is the limit.

Mp 4 have low encoded horizontal microcode. These are two halves of a single 244 octal driver. The add one is done by using the extra bit on the first full adder in the adding circuitry. The intel 4004 is a 4 bit central processing unit cpu released by intel corporation in 1971.

That s because a microprocessor is an integrated circuit and way beyond the capabilities of even the most die hard hobbyist. I began construction of this processor with its memory by building a simple circuit to drive a 256 4 bit static ram chip 2112 that i picked up at a local electronics store. Bit 0 and 5 6 7 of microcode1 not used. So we will cheat and use a 4008 4 bit adder ic.

The alu is actually just a 4 bit ripple carry adder subtracter. The chip design implemented with the mos silicon gate technology started in april 1970 and was created by federico faggin who led the project from beginning to completion in 1971. The diagram at the bottom of this page provides an overview of my 4 bit contraption you can also click on any of the subsystem rectangles to link directly to the respective page or review some assembly instructions. At the top left of the schematic are the three chips pertaining to the execution of the current instruction.

Binary subtraction the theory. Microcode sequencer directly reads instruction from rom data output and produces necessary control signals. This 4 bit cpu was designed built and tested without the aid of a logic analyzer or any hardware simulation software. It has 4 bit i o and input port is connected to dip switch and output port is connected to leds.

This cpu has 16 instructions. Overview of the design. Unfortunately for the 4 bit alu it would be impractical to use discrete chips to create a 4 bit adder. The logic necessary to perform the invert consists of some multiplexers and inverters.

Clock speed switchable by 10hz or 1hz not mhz nor ghz just to make sure or manual clock which enables its operation step by step. Adapted from this image. 4008 4 bit full adder pinout. Also lda m and ldm a instructions not implemented in this published version.

As a hobbyist you could think about designing a 4 bit microprocessor but you won t be able to build one. The fetch register is a 377 an 8 bit register that holds the current instruction opcode in the high 4 bits and instruction or address data in the low 4 bits. I only used a logic probe and an oscilloscope. You can pick these up for a few dollars on ebay.

But you could. One drives the alu result onto the data bus which is necessary when storing data to ram or an out port.