MCPU is a minimal cpu aimed to fit into a 32 Macrocell CPLD – one of the smallest available programmable logic devices. While this CPU is not powerful enough for real world applications it has proven itself as a valuable educational tool. The source code is just a single page and easily understood. Both VHDL and Verilog versions are supplied. The package comes with assembler, emulator and extensive documentation.
You can download the code and documentation from the opencores site. (One time registration is required to download their code, but its free and provides access to loads of other projects and code samples.)
Read the MCPU PDF describing the project. It can be found in the web-uploads subdirectory of the extracted folder.
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