You will design a four-bit general purpose microprocessor. Since its a small processor, you dont need to use the pointers like stack pointer, global pointer, etc. The operation of a general purpose microprocessor involves three steps:

? In step one the control unit fetches an instruction from memory. The memory location to be fetched is determined by the content of the program counter (PC) register. The instruction fetched from memory is copied into the instruction register (IR). The PC is then incremented by 1. ? In step two, the instruction that is in the IR is decoded. The control unit checks the encoding of the instruction to determine what instruction it is. ? The control unit perfoms the third state by simply asserting the appropriate control signals for controlling the datapath to execute the given instruction.

Instructions for the program are usually stored in external memory. To keep the design simple, instead of having externel memory, we will include the memory as part of the datapath.

Write the VHDL code for the given ciruit. Write the VHDL test bench. Force the inputs of IR (which are otherwise fetched from the program memory) so as to mimic the normal operation and test all the stages of operation.