Pico CPU: Pico-Memory Unit

Our Pico-CPU memory system uses two separate registers, one as instruction memory, and one as data memory. we will describe the memories in behavioural VHDL. as we saw earlier, we have 16 bits instructions and 8 bit data. this implies that we have to either implement the instruction memory as 16 bit wide word or we can divide each instruction in two lines. If the later is the case, then we need to perform two reads from instruction memory for each fetch.
Memory Test
figure 1

Addressing Modes

Our Pico-CPU will only support immediate addressing meaning we will directly use the memory address which we would like to use in our instruction. This constraint has an impact on our memory size. If our address should fit in one instruction, it means that our address is bounded in 8bits. In other terms, we can at most address 256 memory spaces (0 to 255).

Testing Memory System

To test the Memory system we start reading data from a file into the memory in the first phase, and then read it back from memory and write to a file. In the end we have to check if these two files contain identical data. Our test setup should look like figure 2.
Memory Test
figure 2

Hints on VHDL file handeling

To read from a file in VHDL you have to first declare the input file and open it with the following commands:

file InputFile : text; file_open(InputFile, "Input_File.txt", read_mode);

to read from file, we can define a variable and read file line by line into the variable:

variable LINEVARIABLE : line; readline(InputFile, LINEVARIABLE);

To write into a file, the process is the same, you define and open a file in write mode:

file OutputFile : text; file_open(OutputFile, "Output_File.txt", write_mode);

And then you can write a line to the file:

writeline(OutputFile, LINEVARIABLE);

To make a line out of different signals you can use the write command as follows:

write(LINEVARIABLE, Signal_1, Signal_2,..., Signal_N);