About myself

I'm Dr. Gergő Érdi (Érdi Gergő in the original Hungarian order of surname first), born in Budapest and living in Singapore since 2011.

I graduated from Semmelweis University of Medicine with an MD in 2005. Meanwhile, in 2003 I also started studying at the Computer Science faculty of Eötvös Loránd University, and got my CS master's degree in 2011.

Between 2005 and 2011, I've worked at Intentional Software. Since 2011, I'm currently at Standard Chartered Bank.

Very high-level simulation of a CλaSH CPU

15 September 2018 (programming haskell fpga electronics retrochallenge)

Initially, I wanted to talk this week about how I plan to structure the CλaSH description of the CHIP-8 CPU. However, I'm postponing that for now, because I ran into what seems like a CλaSH bug, and I want to see my design run on real hardware before I describe it in too much detail. So instead, here's a post on how I am testing in software.

CPUs as Mealy machines

After stripping away all the nice abstractions that I am using in my description of the CPU, what remains is a Mealy machine, which simply means it is described by a state transition and output function s -> i -> (s, o). If that looks familiar, that is not a coincidence: this is, of course, just one argument flip away from the Kleisli category of the State s monad. Just think of it as being either this or that, depending on which one you have more intuition about. A lot more on this in my upcoming blogpost.

My CHIP-8 CPU is currently described by a Mealy machine over these types:

data CPUIn = CPUIn
    { cpuInMem :: Word8
    , cpuInFB :: Bit
    , cpuInKeys :: KeypadState
    , cpuInKeyEvent :: Maybe (Bool, Key)
    , cpuInVBlank :: Bool

data Phase
    = Init
    | Fetch1
    | Exec
    | StoreReg Reg
    | LoadReg Reg
    | ClearFB (VidX, VidY)
    | Draw DrawPhase (VidX, VidY) Nybble (Index 8)
    | WaitKeyPress Reg

data CPUState = CPUState
    { opHi, opLo :: Word8
    , pc, ptr :: Addr
    , registers :: Vec 16 Word8
    , stack :: Vec 24 Addr
    , sp :: Index 24
    , phase :: Phase
    , timer :: Word8

data CPUOut = CPUOut
    { cpuOutMemAddr :: Addr
    , cpuOutMemWrite :: Maybe Word8
    , cpuOutFBAddr :: (VidX, VidY)
    , cpuOutFBWrite :: Maybe Bit

cpu :: CPUIn -> State CPUState CPUOut

Running the CPU directly

Note that all the types involved are pure: signal inputs are turned into pure input by CλaSH's mealy function, and the pure output is similarly turned into a signal output. But what if we didn't use mealy, and ran cpu directly, completely sidestepping CλaSH, yet still running the exact same implementation?

That is exactly what I am doing for testing the CPU. By running its Mealy function directly, I can feed it a CPUIn and consume its CPUOut result while interacting with the world — completely outside the simulation! The main structure of the code that implements the above looks like this:

stateful :: (MonadIO m) => s -> (i -> State s o) -> IO (m i -> (o -> m a) -> m a)
stateful s0 step = do
    state <- newIORef s0
    return $ \mkInput applyOutput -> do
        inp <- mkInput
        out <- liftIO $ do
            s <- readIORef state
            let (out, s') = runState (step inp) s
            writeIORef state s'
            return out
        applyOutput out

Hooking it up to SDL

I hooked up the main RAM and the framebuffer signals to IOArrays, and wrote some code that renders the framebuffer's contents into an SDL surface and translates keypress events. And, voilà: you can run the CHIP-8 computer, interactively, even allowing you to use good old trace-based debugging (which is thankfully removed by CλaSH during VHDL generation so can even leave them in). The below screencap shows this in action: :main is run from clashi and starts the interactive SDL program, with no Signal types involved.

Older entries

My software

These days, I'm mostly just writing small programs for fun, or throw-away code relevant to some theoretical subjects I happen to get interested in in the field of functional programming, and then push them to my GitHub repos or write about them in my blog. Ones you might find interesting include:

Full list (including older stuff)


Compositional type checking

Pattern synonyms

Syntax-generic programming


These are slides from some talks I've done over the years at various meetup groups.

Bits and pieces from the ELTE CS course