Building an Assembler in Haskell


Recently I started building an emulator for the MosTech 6502 Cpu, this post is about the initial stages of building an assembler for a simple assembly language that compiles to runnable 6502 machine code. I've created a repo and updated it as I wrote this post, so at the end of most sections that introduce new code I'll link to a commit which has the code up to that point.

You can see the repo here: The only pre-requisite is installing stack, once the project is cloned you can use stack install to install dependencies and build the project.

The Language

First, let's define what we want our assembly language to be able to do. To keep it simple, we only want to allow assignment and the definition of instructions for now:

  • We should be able to define instructions and their operands, e.g. LDA #$20. This says load the value $20 into the accumulator.
    • LDA means load date into the accumulator
    • # means this is immediate addressing so use the operand as a value not as an address.
    • $20 stands for 20 in hexadecimal
  • We should be able to assign values to variables, e.g. LOCATION = $2020.
    • Now anywhere we see LOCATION we can replace with $2020.

Now for a quick and dirty grammar for our simple language:

<expr>        := <instruction> | <assignment>
<assignment>  := <label> "=" <bytes>
<instruction> := [<label> ":"] <menmonic> [<label> | <operand>]
<operand>     := ["#"] <bytes>
<bytes>       := "$" <byte> [<byte>]
<label>       := ([A-Za-z]*[A-Za-z0-9]*)
<mnemonic>    := 3 * ([A-Z])
<byte>        := 2 * ([A-Fa-f0-9])

The above is a variation of EBNF (Extended Backus Naur Form) notation, we allow regular expressions (denoted by brackets () e.g. ([A-Z]) denotes a single upper case letter) for simplicity.

Here's a breakdown, from the bottom up:

  • <byte> is defined as 2 * ([A-Fa-f0-9]), this means two consecutive characters that are upper or lower case letters betwen A and F or digits, i.e. a two digit hexadecimal value.

    • e.g. 2F
  • <mnemonic> is a three letter string - all upper case.

    • e.g. LDA
  • <label> is an alphanumeric string of any size, which must start with a letter.

    • e.g. Stor3
    • Note that we should limit it's size, but lets leave it infinite for now!
  • <bytes> is a string which starts with a $ and must contain at least one <byte>, at most two.

    • e.g. $2F
  • <operand> starts with an optional # followed by <bytes>.

    • e.g. #$2f
  • <instruction> starts with an optional <label> which must be followed by a : if it exists, this is followed by a <mnemonic> and finally an optional <label> xor an <operand>. Some instructions use implicit addressing and require no operand or label, hence why this is optional.

    • e.g. LABEL LDA #$20
    • Note that the possible label at the end is only for certain instructions, for example BNE LABEL will jump to the address corresponding to LABEL if the zero flag is not set.
  • <assignment> simply an = with a <label> on the left hand side and a <bytes> on the right.

    • e.g. STORE = $2020
  • Finally an <expr> can be either an <instruction> or an <assignment>.

An example program would look as follows.

LDA #$01
CMP #$02
BNE notequal
STA $22
notequal: BRK

And the machine code this compiles to 1:

a9 01 c9 02 d0 02 85 22 00

Now that the language is somewhat spec'd out, we have an nice overview of how we can start building a parser for it. There are a lot of rules not defined in the above spec, for example a <label> cannot match a mnemonic - e.g. LDA cannot be a <label> - let's not worry about these for now.

Building Our Parser

Now that we have our grammar, we can start thinking about how we want to build our parser. Normally I would work out some types first along with some top-level functions and go from there, so lets do that.

module Assembler where

import qualified Data.Text as T -- from the "text" package
import Text.Megaparsec  -- from the "megaparsec" package

-- | Create a custom parser type. This is megaparsec specific, we will gloss over this in
-- this post.
type Parser = Parsec Dec T.Text

-- | A Label is just a Text value.
newtype Label = Label T.Text deriving Show

-- | Indicates whether an address/ value is preceeded by a "#".
newtype IsImmediate = IsImmediate Bool deriving Show

-- | An address/value of one or two bytes which may have a "#", meaning
-- immediate, before it.
data Operand = Operand IsImmediate T.Text deriving Show

-- | A three letter upper case string.
newtype Mnemonic = Mnemonic T.Text deriving Show

-- | A label, which should be assigned a value.
newtype Var = Var Label  deriving Show

-- | A value, which should be assigned to a Var.
newtype Val = Val T.Text deriving Show

-- | Either a label or an operand.
data LabelOrOperand = Lbl Label | Op Operand deriving Show

-- | Either an instruction or an assignment.
data Expr
  = Instruction (Maybe Label) Mnemonic (Maybe LabelOrOperand)
  | Assignment Var Val
  deriving Show

Most of the values will just be strings (Text types) so to distinguish between them we wrap Text in a newtype wrapper for each type we care about. For now we're not going to worry about constructing anything other than strings (I'll be using the type Text to denote strings instead of the built-in String in this post 2). Looking back at our grammar we have eight symbols, each one can be represented as a function which is itself a parser for some subset of the grammar. So the top-level functions in this case would be the symbols in our grammar - <expression>, <label> etc... We'll also add an extra function here for parsing label assignment - labels with a ":" after them as in the first part of <instruction> - let's call it labelAssign.

expression :: Parser Expr
expression = undefined

assignment :: Parser Expr
assignment = undefined

instruction :: Parser Expr
instruction = undefined

operand :: Parser Operand
operand = undefined

bytes :: Parser T.Text
bytes = undefined

labelAssign :: Parser Label
labelAssign = undefined

label :: Parser Label
label = undefined

mnemonic :: Parser Mnemonic
mnemonic = undefined

byte :: Parser T.Text
byte = undefined

All functions are undefined so the type checker will pass before we begin to implement the logic. With our grammar, we know what each symbol corresponds to, so we can use QuickCheck to write properties for each function that adhere to its specification in the grammar.

Property Driven Parser Development

To build the parser I'm going to use a parser combinator library called megaparsec. I won't go into much detail on megaparsec or parser combinators in this post, simply put, parser combinators are a way of building more complex parsers by combining parsers.

The simplest parser above would be byte, from our grammar this is just a two character hexadecimal string. Before we start implementing it, let's write a property which encodes what we expect it to do.

Generating Our Data - QuickCheck Arbitrary

Using QuickCheck to test parsers is really simple and quite powerful. It involves writing properties which encode expectations about the ouput of a function given some input.

To build a property for byte first we need to create an Arbitrary instance 3 for the data it expects - two character hexadecimal strings. Creating an instance of Arbitrary for a type allows random values of that type to be generated, by default QuickCheck will generate 100 random values of the type each test run. For byte this might look as follows.

-- | Wrapper for our two character hexadecimal strings.
newtype TwoCharHexString = TwoCharHexString T.Text deriving Show

instance Arbitrary TwoCharHexString where
  arbitrary = do
    upper <- choose ('A', 'F')
    lower <- choose ('a', 'f')
    num   <- choose ('0', '9')
    let vals = [upper, lower, num]
    x <- elements vals
    y <- elements vals
    pure $ TwoCharHexString (T.pack (x:[y]))

Here we define a newtype called TwoCharHexString, which is just a wrapper for Text. Then we create an Arbitrary instance for this type which builds two character hex string Text values. Let's run through the instance:

  • choose generates a random element in the given range, choose (1, 4) generates integers between 1 and 4 inclusive.
  • elements generates a single value from the given list.
  • With these functions we can generate x and y and build our two character string by building a two element list of characters made up of x and y - see x:[y] above, this is just a String which is a list of Char - we then pack this with T.pack to get our Text value.

Building Our Property

Next we need to write a property that defines what should happen when byte parses these string values.

prop_byte_parse (TwoCharHexString s) = parse byte "" s  `shouldParse` s

This is simply a function called prop_byte_parse which takes a value of type TwoCharHexString runs the parser byte with the megaparsec parse function 4 and checks that the result is as expected, in this case parsing a string s should return that same string. parse is a function from the megaparsec package which runs our parser on the supplied string.

Finally, shouldParse is a function from hspec-megaparsec - a library containing utility functions for testing parsers built with megaparsec. Here we are using it to say parse byte "" s should parse to the string s - meaning the byte parser run on string s should just give us back s.

Let's add this to our spec so the property check gets run when we launch stack test.

asmSpec = do
  describe "byte" $
    it "should parse two consecutive characters in the hex range into a two character string" $
      property prop_byte_parse

Running the tests with stack test will run this spec and check that the property prop_byte_parse holds when parsing the random values of TwoCharHexString that quickcheck produces - which we defined in our Arbitrary instance.

But wait! The byte function was undefined so the test should fail! Yip, it should give output similar to the following.

    should parse two consecutive characters in the hex range into a two character string FAILED [1]


  1) Assembler.byte should parse two consecutive characters in the hex range into a two character string
       uncaught exception: ErrorCall (Prelude.undefined
       CallStack (from HasCallStack):
         error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
         undefined, called at src/Assembler.hs:52:8 in emu-mos6502-asm-blog- (after 1 test)
       *** Failed! (after 1 test):
         CallStack (from HasCallStack):
           error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
           undefined, called at src/Assembler.hs:52:8 in emu-mos6502-asm-blog-
       TwoCharHexString "fF"

Randomized with seed 1913513661

Finished in 0.0023 seconds
1 example, 1 failure

Good, now we can implement byte. I've deliberatly left out some boiler plate such as dependencies and test setup, but you can view the full code up to this point, see Assembler.hs for the types and functions and AssemblerSpec.hs for the the property and Arbitrary instance.


So now that we have a property which defines what our byte function should do, we can implement it. hexDigitChar is a parser from megaparsec which parses a a hexadecimal digit. A byte is made up of two such digits so byte is simply a parser which tries to parse two hexadecimal chars.

byte :: Parser T.Text
byte = do
  high <- hexDigitChar
  low <- hexDigitChar
  pure $ T.pack [high,low]

Nice! We read a hex char and call it high and another called low and build a Text value.

The rest of the parsers can be implemented in a similar way - define Arbitrary instances for the data they should take, define properties for the expected output and implement!

I'll leave the rest of the implementation for another post. Running stack test now should give the following output.

    should parse two consecutive characters in the hex range into a two character string

Finished in 0.0016 seconds
1 example, 0 failures

Excellent, our implementation passed the property check! You can check out the code up to this point here.


This is where I'll leave this post. I think it's long enough! In the next post we'll create the rest of the parsers and their properties, and also run through megaparsec in some more detail. There are definitely quite a few improvements that can be added to the language, and plenty more features that would be useful to have which we can implement in the future.

There are also some limitations in the grammar, for example it currently does not allow X or Y indexed addressing 5 - e.g. LDA ($2020,X) - we can address these too.

This post outlines what I have done so far when building the assembler, and really just shows my own thought process around designing and implementing. Im actively working on the 6502 emulator, so I hope to do a post every week. The main goal is to outline my development process in implementing the project, hopefully I'll introduce some bugs or have some interesting issues along the way!


The reference I'm using for the emulator is mainly


Haskell String Types is a good post detailing the different string types.


See the documentation for Arbitrary, this StackOverflow answer is also good.


See the documentation for parse.


See Indexed Indirect, and Indirect Indexed addressing modes here.