Designing a simple CPU

[sci4me]

@Troncoso

Well, let’s see if we can get some stuff straight here. First of all, there really isn’t such thing as a “simple CPU” if you want something that actually functions. I recommend the book “Computer Systems - A Programmer’s Perspective 2nd Edition” It goes through everything you need to do to simulate the x86 architecture (albeit with not quite as many instructions).

Things to consider when designing the cpu:

It’s not as simple as reading a file and calling a function that represents each line.

You’ll want to separate the different stages, such as getting the instructions (fetching), determining what they do (decoding), executing them, making changes to memory as needed, and then setting up the registers for the next cycle.

You’ll want to look into pipe-lining (basically what I mentioned above). There are different methods of pipe-lining though, so it’s good to know your options.

You’ll need registers to store temporary data.

You’ll need some memory for longer term storage.

You need to know what instructions you want to support and assign them all op-codes and operands. op code being the unique ID of an instruction and the operands being the information needed by the instruction to carry out whatever it does.

Labels are accomplished with a symbol table. You need map for each label and the address it corresponds to. This means you need to read through your assembly code twice. Once to build the symbol table and a second time to execute the instructions. (After being loaded into your memory) Symbols are used with jump instructions. So, when you call a jump, it’s as simple as getting the address from the symbol table and setting the program counter to that.

Interrupts are exactly what they sound like. Though, what you don’t understand is that is something that can be implemented on the code level. In your assembly code, you just create a subfunction that acts as the interrupt code to run, then whenever it needs to be called, you jump to it, and at the end of the subfunction, you return back to where you were. Case in point, that’s not something you need to implement in your simulator.

This is really an extensive topic. I spent an entire semester learning nothing but how the CPU works and building my own simulator. It’s not something you are going to be able to tackle in a few days time.

Did you look at that github? The S80 class? If that doesnt qualify as a simulation of a custom designed cpu then I dont know what does. But yes, you’re still right. I don’t know all of what I need to know. I’ve only started to scratch the surface as far as learning how cpu’s actually function… but still…

[Riven]

@Troncoso

Labels are accomplished with a symbol table. You need map for each label and the address it corresponds to. This means you need to read through your assembly code twice. Once to build the symbol table and a second time to execute the instructions. (After being loaded into your memory) Symbols are used with jump instructions. So, when you call a jump, it’s as simple as getting the address from the symbol table and setting the program counter to that.

I think you confused compiler functionality and the execution phase. The compiler creates the symbol table, and uses it to calculate the absolute or relative jumps, storing the results directly as operands.

So an unconditional jump wouldn’t be:

program_counter = symbol_table[decoded_label_index]

as that level of indirection is already resolved by the compiler, so we’d simply do:

program_counter = decoded_target_address

[Roquen]

I’m not clear on the goal, but along the same lines as sproingie suggestion, instead of a CPU…implement forth-a-like instead is an interesting exercise.

[lcass]

1: for this I would say dont go complicated do something like the redpower2 computers, they are very effective and do pretty much what you are asking for.
2: probably dont need to worry about bits.
3: To make your life a heck of a lot easier write an interpreter for the incoming code.
4: if you want make an xml that holds your commands.
5: yes as some people have said do something similar to a fouth language, people know it it’s a lot easier to code and its commands are simple.

Anyway hope this goes well. Just dont try to simulate full computer functions that will not only destroy your tick and frame rate but also turn into inception and when you dont have a crazy guy who jumps into dreams then gets everyone killed with his dead wife and a train well, its not gonna work.