There is a starting circuit on Canvas. It is based on the simple MIPS machine. The circuit includes a main and 6 subcircuits: an ALU, Registers, DataMemory, Control, ALUControl, and InstructionSplitter. In the top level circuit, everything is complete except that the Control unit has not been connected. The 6 subcircuits are all empty shells. Their input and output ports have been created, but nothing else.
Your goal is to finish this simple MIPS machine.
You must use these starting materials. Submitting your own version is not acceptable. You may not modify the subcircuit interfaces.
A subcircuit interface includes its input and output pins and its appearance (how it looks when used in a bigger circuit). As mentioned on p. 259, the simple machine should be able to do:
lw (load word), sw (store word), BEQ, ADD, SUB, AND, OR, and SLT (set less than).
Here are a few hints:
Control Unit: You should read the control signals from a small ROM.
ALU: The ALU should do all possible calculations and let a mux choose the output.
Data Memory: You will need a RAM. They come with different interfaces, so under "Data Interface" you should choose "Separate load and store ports"
Registers: There are 32 registers. You will have to either hook up 32 different registers or if you prefer, you can create a sub-subcircuit. I recommend a component that manages 8 of the 32 registers. Then you can use 4 of those to implement the registers subcircuit.
Testing
I will post a couple of tests, but you should make up your own as well. To run the MIPS machine, you must:
load the program memory with machine instructions
load the data memory with your initial variable/constant data set the clock to off
set the PC to 0
initialise a base register as needed
To create test materials using MARS,
compile some test assembly code that uses only the approved instructions listed above under File, use Dump Memory to
first save the text memory segment (the code) then save the data memory segment (the data)
choose "Hexadecimal Text" as the format in each case now open each of the two files you just created and add v2.0 raw
as a new line at the top of the file. Logisim expects your files to start with that label. You can then use those files to initialize the program and data memories ("Load Image").
Memory
Your program memory and data memory should each be at least 256 bytes big and the addresses 0 through 255 must be usable. (We actually only uses address for words. Since words are aligned, all addresses will be multiples of 4.) MARS/Mips automatically loads instructions at 0x400000, but the code can easily be relocated, so we will load it at address 0x0 in program memory. Therefore the PC must initially be 0. MARS/Mips loads data starting at 0x10010000. We could place this data at address 0x0 in data memory, but that would suggest a base register value of 0. To give the base register a more realistic value, we will put data at some more interesting address in data memory. For the two tests programs released initially, the data dump has been modified to push the data to address 0x40.
Therefore the base register $at must be manually initialized to x40 to run the test programs. What to turn in? Your completed circuit will be a single Logisim .circ file.