This is a VHDL model of 4-bit CPU called 4004 with a test bench. It is
suitable for simulation and probably for synthesis. The CPU was not
made to be useful in itself, but to demonstrate some HDL coding and
test bench techniques. For HDL coding this demonstrates the Two
process method as described here
http:https://www.gaisler.com/doc/vhdl2proc.pdf. For test bench it
demonstrates the use of a test sequencer and the use of bus functional
models.
 
Directory structure

./src
All source files are located here.

./sim
All files used for simulation are located here.

Compilation
The project can be compiled with a VHDL simulator or a logic
synthesizer. 

GHDL
An auto generated Makefile is present in ./sim. To compile the project
do:
$ cd sim
$ make all
$ ghdl -e --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c cpu_4004_tb
The last command may fail due to some part of the design being
obsoleted by some other part of the project. This is because the
Makefile is not capable of determine the compilation order.

The correct order is
$ ghdl -a --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c ../src/sim_pkg.vhd
$ ghdl -a --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c ../src/cpu_4004_pkg.vhd
$ ghdl -a --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c ../src/cpu_4004.vhd
$ ghdl -a --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c ../src/cpu_4004_tb.vhd
$ ghdl -e --workdir=work --work=work --ieee=synopsys -fexplicit --std=93c cpu_4004_tb

Simulation

GHDL
$ ghdl -r cpu_4004_tb --wave=cpu_4004_wave.ghw
The simulations writes simulation text output to the file
cpu_4004_sim.out. The signals can be inspected in the file
cpu_4004_wave.ghw with gtkwave.
$ cat cpu_4004_sim.out
.
.
.
-------------------------------------------------------------------------------
infos: 32
notes: 0
warnings: 0
errors: 0
failures: 0
Suppressed messged: 0
-------------------------------------------------------------------------------
Result: PASS
-------------------------------------------------------------------------------
$ gtkwave cpu_4004_wave.ghw

TODO:
Create the necessary interface logic to interface the CPU to switches,
buttons, displays and LEDs. Synthesize the CPU for a FPGA and run it
on a development board. Create logic to input a program using
switches and buttons. Implement logic to step through the program
executing.