Clash.Prelude.BlockRam

BlockRAM primitives

Using RAMs

We will show a rather elaborate example on how you can, and why you might want to use blockRams. We will build a "small" CPU+Memory+Program ROM where we will slowly evolve to using blockRams. Note that the code is not meant as a de-facto standard on how to do CPU design in Clash.

We start with the definition of the Instructions, Register names and machine codes:

{-# LANGUAGE RecordWildCards, TupleSections, DeriveAnyClass, TypeApplications #-}

module CPU where

import Clash.Prelude

type InstrAddr = Unsigned 8
type MemAddr   = Unsigned 5
type Value     = Signed 8

data Instruction
  = Compute Operator Reg Reg Reg
  | Branch Reg Value
  | Jump Value
  | Load MemAddr Reg
  | Store Reg MemAddr
  | Nop
  deriving (Eq, Show)

data Reg
  = Zero
  | PC
  | RegA
  | RegB
  | RegC
  | RegD
  | RegE
  deriving (Eq, Show, Enum, Generic, NFDataX)

data Operator = Add | Sub | Incr | Imm | CmpGt
  deriving (Eq, Show)

data MachCode
  = MachCode
  { inputX  :: Reg
  , inputY  :: Reg
  , result  :: Reg
  , aluCode :: Operator
  , ldReg   :: Reg
  , rdAddr  :: MemAddr
  , wrAddrM :: Maybe MemAddr
  , jmpM    :: Maybe Value
  }

nullCode =
  MachCode
    { inputX = Zero
    , inputY = Zero
    , result = Zero
    , aluCode = Imm
    , ldReg = Zero
    , rdAddr = 0
    , wrAddrM = Nothing
    , jmpM = Nothing
    }

Next we define the CPU and its ALU:

cpu :: Vec 7 Value          -- ^ Register bank
    -> (Value,Instruction)  -- ^ (Memory output, Current instruction)
    -> ( Vec 7 Value
       , (MemAddr, Maybe (MemAddr,Value), InstrAddr)
       )
cpu regbank (memOut, instr) =
  (regbank', (rdAddr, (,aluOut) <$> wrAddrM, fromIntegral ipntr))
 where
  -- Current instruction pointer
  ipntr = regbank !! PC

  -- Decoder
  (MachCode {..}) = case instr of
    Compute op rx ry res -> nullCode {inputX=rx,inputY=ry,result=res,aluCode=op}
    Branch cr a          -> nullCode {inputX=cr,jmpM=Just a}
    Jump a               -> nullCode {aluCode=Incr,jmpM=Just a}
    Load a r             -> nullCode {ldReg=r,rdAddr=a}
    Store r a            -> nullCode {inputX=r,wrAddrM=Just a}
    Nop                  -> nullCode

  -- ALU
  regX   = regbank !! inputX
  regY   = regbank !! inputY
  aluOut = alu aluCode regX regY

  -- next instruction
  nextPC =
    case jmpM of
      Just a | aluOut /= 0 -> ipntr + a
      _                    -> ipntr + 1

  -- update registers
  regbank' = replace Zero   0
           $ replace PC     nextPC
           $ replace result aluOut
           $ replace ldReg  memOut
           $ regbank

alu Add   x y = x + y
alu Sub   x y = x - y
alu Incr  x _ = x + 1
alu Imm   x _ = x
alu CmpGt x y = if x > y then 1 else 0

We initially create a memory out of simple registers:

dataMem
  :: HiddenClockResetEnable dom
  => Signal dom MemAddr
  -- ^ Read address
  -> Signal dom (Maybe (MemAddr,Value))
  -- ^ (write address, data in)
  -> Signal dom Value
  -- ^ data out
dataMem rd wrM =
  mealy dataMemT (replicate d32 0) (bundle (rd,wrM))
 where
  dataMemT mem (rd,wrM) = (mem',dout)
    where
      dout = mem !! rd
      mem' =
        case wrM of
          Just (wr,din) -> replace wr din mem
          _             -> mem

And then connect everything:

system
  :: ( KnownNat n
     , HiddenClockResetEnable dom
     )
  => Vec n Instruction
  -> Signal dom Value
system instrs = memOut
  where
    memOut = dataMem rdAddr dout
    (rdAddr, dout, ipntr) = mealyB cpu (replicate d7 0) (memOut,instr)
    instr  = asyncRom instrs <$> ipntr

Create a simple program that calculates the GCD of 4 and 6:

-- Compute GCD of 4 and 6
prog = -- 0 := 4
       Compute Incr Zero RegA RegA :>
       replicate d3 (Compute Incr RegA Zero RegA) ++
       Store RegA 0 :>
       -- 1 := 6
       Compute Incr Zero RegA RegA :>
       replicate d5 (Compute Incr RegA Zero RegA) ++
       Store RegA 1 :>
       -- A := 4
       Load 0 RegA :>
       -- B := 6
       Load 1 RegB :>
       -- start
       Compute CmpGt RegA RegB RegC :>
       Branch RegC 4 :>
       Compute CmpGt RegB RegA RegC :>
       Branch RegC 4 :>
       Jump 5 :>
       -- (a > b)
       Compute Sub RegA RegB RegA :>
       Jump (-6) :>
       -- (b > a)
       Compute Sub RegB RegA RegB :>
       Jump (-8) :>
       -- end
       Store RegA 2 :>
       Load 2 RegC :>
       Nil

And test our system:

>>> sampleN @System 32 (system prog)
[0,0,0,0,0,0,4,4,4,4,4,4,4,4,6,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,2]

to see that our system indeed calculates that the GCD of 6 and 4 is 2.

Improvement 1: using asyncRam

As you can see, it's fairly straightforward to build a memory using registers and read (!!) and write (replace) logic. This might however not result in the most efficient hardware structure, especially when building an ASIC.

Instead it is preferable to use the asyncRam function which has the potential to be translated to a more efficient structure:

system2
  :: ( KnownNat n
     , HiddenClockResetEnable dom  )
  => Vec n Instruction
  -> Signal dom Value
system2 instrs = memOut
  where
    memOut = asyncRam d32 rdAddr dout
    (rdAddr,dout,ipntr) = mealyB cpu (replicate d7 0) (memOut,instr)
    instr  = asyncRom instrs <$> ipntr

Again, we can simulate our system and see that it works. This time however, we need to disregard the first few output samples, because the initial content of an asyncRam is undefined, and consequently, the first few output samples are also undefined. We use the utility function printX to conveniently filter out the undefinedness and replace it with the string X in the few leading outputs.

>>> printX $ sampleN @System 32 (system2 prog)
[X,X,X,X,X,X,4,4,4,4,4,4,4,4,6,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,2]

Improvement 2: using blockRam

Finally we get to using blockRam. On FPGAs, asyncRam will be implemented in terms of LUTs, and therefore take up logic resources. FPGAs also have large(r) memory structures called Block RAMs, which are preferred, especially as the memories we need for our application get bigger. The blockRam function will be translated to such a Block RAM.

One important aspect of Block RAMs have a synchronous read port, meaning that, unlike the behavior of asyncRam, given a read address r at time t, the value v in the RAM at address r is only available at time t+1.

For us that means we need to change the design of our CPU. Right now, upon a load instruction we generate a read address for the memory, and the value at that read address is immediately available to be put in the register bank. Because we will be using a BlockRAM, the value is delayed until the next cycle. We hence need to also delay the register address to which the memory address is loaded:

cpu2
  :: (Vec 7 Value,Reg)
  -- ^ (Register bank, Load reg addr)
  -> (Value, Instruction)
  -- ^ (Memory output, Current instruction)
  -> ( (Vec 7 Value,Reg)
     , (MemAddr, Maybe (MemAddr, Value), InstrAddr)
     )
cpu2 (regbank,ldRegD) (memOut,instr) =
  ((regbank', ldRegD'), (rdAddr, (,aluOut) <$> wrAddrM, fromIntegral ipntr))
 where
  -- Current instruction pointer
  ipntr = regbank !! PC

  -- Decoder
  (MachCode {..}) = case instr of
    Compute op rx ry res -> nullCode {inputX=rx,inputY=ry,result=res,aluCode=op}
    Branch cr a          -> nullCode {inputX=cr,jmpM=Just a}
    Jump a               -> nullCode {aluCode=Incr,jmpM=Just a}
    Load a r             -> nullCode {ldReg=r,rdAddr=a}
    Store r a            -> nullCode {inputX=r,wrAddrM=Just a}
    Nop                  -> nullCode

  -- ALU
  regX   = regbank !! inputX
  regY   = regbank !! inputY
  aluOut = alu aluCode regX regY

  -- next instruction
  nextPC =
    case jmpM of
      Just a | aluOut /= 0 -> ipntr + a
      _                    -> ipntr + 1

  -- update registers
  ldRegD'  = ldReg  -- Delay the ldReg by 1 cycle
  regbank' = replace Zero   0
           $ replace PC     nextPC
           $ replace result aluOut
           $ replace ldRegD memOut
           $ regbank

We can now finally instantiate our system with a blockRam:

system3
  :: (KnownNat n
     , HiddenClockResetEnable dom  )
  => Vec n Instruction
  -> Signal dom Value
system3 instrs = memOut
  where
    memOut = blockRam (replicate d32 0) rdAddr dout
    (rdAddr,dout,ipntr) = mealyB cpu2 ((replicate d7 0),Zero) (memOut,instr)
    instr  = asyncRom instrs <$> ipntr

We are, however, not done. We will also need to update our program. The reason being that values that we try to load in our registers won't be loaded into the register until the next cycle. This is a problem when the next instruction immediately depended on this memory value. In our case, this was only the case when the loaded the value 6, which was stored at address 1, into RegB. Our updated program is thus:

prog2 = -- 0 := 4
       Compute Incr Zero RegA RegA :>
       replicate d3 (Compute Incr RegA Zero RegA) ++
       Store RegA 0 :>
       -- 1 := 6
       Compute Incr Zero RegA RegA :>
       replicate d5 (Compute Incr RegA Zero RegA) ++
       Store RegA 1 :>
       -- A := 4
       Load 0 RegA :>
       -- B := 6
       Load 1 RegB :>
       Nop :> -- Extra NOP
       -- start
       Compute CmpGt RegA RegB RegC :>
       Branch RegC 4 :>
       Compute CmpGt RegB RegA RegC :>
       Branch RegC 4 :>
       Jump 5 :>
       -- (a > b)
       Compute Sub RegA RegB RegA :>
       Jump (-6) :>
       -- (b > a)
       Compute Sub RegB RegA RegB :>
       Jump (-8) :>
       -- end
       Store RegA 2 :>
       Load 2 RegC :>
       Nil

When we simulate our system we see that it works. This time again, we need to disregard the first sample, because the initial output of a blockRam is undefined. We use the utility function printX to conveniently filter out the undefinedness and replace it with the string X.

>>> printX $ sampleN @System 34 (system3 prog2)
[X,0,0,0,0,0,0,4,4,4,4,4,4,4,4,6,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,2]

This concludes the short introduction to using blockRam.

BlockRAM synchronized to the system clock

blockRam

Arguments

:: (HasCallStack, HiddenClock dom, HiddenEnable dom, NFDataX a, Enum addr)

=> Vec n a

Initial content of the BRAM, also determines the size, n, of the BRAM.

NB: MUST be a constant.

-> Signal dom addr

Read address r

-> Signal dom (Maybe (addr, a))

(write address w, value to write)

-> Signal dom a

Value of the blockRAM at address r from the previous clock cycle

Create a blockRAM with space for n elements.

• NB: Read value is delayed by 1 cycle
• NB: Initial output value is undefined

bram40
  :: HiddenClock dom
  => Signal dom (Unsigned 6)
  -> Signal dom (Maybe (Unsigned 6, Bit))
  -> Signal dom Bit
bram40 = blockRam (replicate d40 1)

Additional helpful information:

• See Clash.Prelude.BlockRam for more information on how to use a Block RAM.
• Use the adapter readNew for obtaining write-before-read semantics like this: readNew (blockRam inits) rd wrM.

blockRamPow2

Arguments

:: (HasCallStack, HiddenClock dom, HiddenEnable dom, NFDataX a, KnownNat n)

=> Vec (2 ^ n) a

Initial content of the BRAM, also determines the size, 2^n, of the BRAM.

NB: MUST be a constant.

-> Signal dom (Unsigned n)

Read address r

-> Signal dom (Maybe (Unsigned n, a))

(write address w, value to write)

-> Signal dom a

Value of the blockRAM at address r from the previous clock cycle

Create a blockRAM with space for 2^n elements

• NB: Read value is delayed by 1 cycle
• NB: Initial output value is undefined

bram32
  :: HiddenClock dom
  => Signal dom (Unsigned 5)
  -> Signal dom (Maybe (Unsigned 5, Bit))
  -> Signal dom Bit
bram32 = blockRamPow2 (replicate d32 1)

Additional helpful information:

• See Clash.Prelude.BlockRam for more information on how to use a Block RAM.
• Use the adapter readNew for obtaining write-before-read semantics like this: readNew (blockRamPow2 inits) rd wrM.

blockRamU

Arguments

:: (HasCallStack, HiddenClockResetEnable dom, NFDataX a, Enum addr, 1 <= n)

=> ResetStrategy r

Whether to clear BRAM on asserted reset (ClearOnReset) or not (NoClearOnReset). Reset needs to be asserted at least n cycles to clear the BRAM.

-> SNat n

Number of elements in BRAM

-> (Index n -> a)

If applicable (see first argument), reset BRAM using this function.

-> Signal dom addr

Read address r

-> Signal dom (Maybe (addr, a))

(write address w, value to write)

-> Signal dom a

Value of the blockRAM at address r from the previous clock cycle

Version of blockram that has no default values set. May be cleared to a arbitrary state using a reset function.

blockRam1

Arguments

:: (HasCallStack, HiddenClockResetEnable dom, NFDataX a, Enum addr, 1 <= n)

=> ResetStrategy r

Whether to clear BRAM on asserted reset (ClearOnReset) or not (NoClearOnReset). Reset needs to be asserted at least n cycles to clear the BRAM.

-> SNat n

Number of elements in BRAM

-> a

Initial content of the BRAM (replicated n times)

-> Signal dom addr

Read address r

-> Signal dom (Maybe (addr, a))

(write address w, value to write)

-> Signal dom a

Value of the blockRAM at address r from the previous clock cycle

Version of blockram that is initialized with the same value on all memory positions.

data ResetStrategy (r :: Bool) where

Constructors

ClearOnReset :: ResetStrategy True

NoClearOnReset :: ResetStrategy False

Read/Write conflict resolution

readNew

Arguments

:: (HiddenClockResetEnable dom, NFDataX a, Eq addr)

=> (Signal dom addr -> Signal dom (Maybe (addr, a)) -> Signal dom a)

The ram component

-> Signal dom addr

Read address r

-> Signal dom (Maybe (addr, a))

(Write address w, value to write)

-> Signal dom a

Value of the ram at address r from the previous clock cycle

Create read-after-write blockRAM from a read-before-write one (synchronized to system clock)

>>> import Clash.Prelude
>>> :t readNew (blockRam (0 :> 1 :> Nil))
readNew (blockRam (0 :> 1 :> Nil))
  :: ...
     ...
     ...
     ...
     ... =>
     Signal dom addr -> Signal dom (Maybe (addr, a)) -> Signal dom a