Dissecting ARM's Machine Readable Specification

ARM logo Last week, ARM released their Machine Readable Architecture Specification and I wrote about what you can do with it. But before you can do anything with the specification, you need to extract the useful bits from the release so I thought I would try for myself and describe what I found out (and release some scripts that demonstrate/test what I am saying).

So what exactly is in the release?

  • It contains a lot of html files (see Milosch Meriac’s online v8-A html)

    $ tar ztf A64_v82A_ISA_xml_00bet3.1.tar.gz | grep xhtml
    ... 1300 more files omitted

    These are the human readable version of the specification — all nicely hyperlinked. This is the best way to navigate the ARM specification.

  • It contains some pdf files

    $ tar ztf A64_v82A_ISA_xml_00bet3.1.tar.gz | grep pdf

    These contain the same information as the html files above. The “OPT” versions are equivalent to the standard version but they have had some simplifying optimisations applied to make them more readable. The “OPT_diff” versions show the difference between the standard version and the OPT version — the best way of understanding what the OPT version does.

  • It contains some xml files

    $ tar ztf A64_v82A_ISA_xml_00bet3.1.tar.gz | grep xml
    ... 3000 more files omitted

    This is the bit that we want! There are two files of particular interest:

    • notice.xml ARM’s legal notice
    • shared_pseudocode.xml hundreds of shared support functions used in the definition of instructions, page table walk, permission checking, exceptions/interrupts, etc.

    And the rest of the files are instruction encodings such as adc.xml.

  • And it contains files like a DTD file that defines the XML schema used by the XML.

Inside the Shared XML files

The Shared XML files contain all the type definitions, constants, variables and functions required by the instructions and system support code. Here is how a typical function is presented in XML:

<ps name="aarch32/debug/breakpoint/AArch32.BreakpointMatch"
   <pstext mayhavelinks="1"
       // AArch32.BreakpointMatch()
       // =========================
       // Breakpoint matching in an AArch32 translation regime.

       (boolean,boolean) <anchor link="AArch32.BreakpointMatch.3" hover="...">AArch32.BreakpointMatch</anchor>(integer n, bits(32) vaddress, integer size)
           assert <a link="impl-shared.ELUsingAArch32.1" file="shared_pseudocode.xml" hover="...">ELUsingAArch32</a>();
           ... more code

This shows a named “ps” section containing a small number of related definitions (just one in this case) and with the ASL implementation of the function enclosed in a “pstext” section. Almost all objects defined in the pstext section are tagged with “anchor” and almost all references to objects in the pstext section are tagged with “a”. (We will use these links later.) The rest of the XML attributes are mostly useful for generating HTML — I will ignore them.

As we process this ASL code, it will be useful to track what definitions each “ps” section contains and what the section depends on. Here is a Python class to represent this (the full code is here).

class ASL:
    '''Representation of ASL code consisting of the code, list of names it defines and list of dependencies'''

    def __init__(self, name, code, defs, deps):
        self.name = name
        self.code = code
        self.defs = defs
        self.deps = deps

    def __str__(self):
        return "ASL{"+", ".join([self.name, str(self.defs), str(self.deps)])+"}"

and here is some code to read the XML, extract dependencies and package it up as an ASL object.

Read pseudocode to extract ASL.
def readASL(ps):
    name = ps.attrib["name"]
    name = name.replace(".txt","")
    name = name.replace("/instrs","")
    name = name.replace("/Op_","/")
    chunk = ps.find("pstext")

    # list of things defined in this chunk
    defs = { x.attrib['link'] for x in chunk.findall('anchor') }

    # extract dependencies from hyperlinks in the XML
    deps = { x.attrib['link'] for x in chunk.findall('a') if not x.text.startswith("SEE") }

    # drop impl- prefixes in links
    deps = { re.sub('(impl-\w+\.)','',x) for x in deps }
    defs = { re.sub('(impl-\w+\.)','',x) for x in defs }

    # drop file references in links
    deps = { re.sub('([^#]+#)','',x) for x in deps }

    return ASL(name, ET.tostring(chunk, method="text").decode().rstrip()+"\n", defs, deps)

Read shared pseudocode files to extract ASL.
def readShared(files):
    asl = {}
    for f in files:
        xml = ET.parse(f)
        for ps in xml.findall('.//ps_section/ps'):
            r = readASL(ps)
            # Various workarounds omitted - see github repo
            asl[r.name] = r
    return asl

Inside the Instruction XML files

The ARM architecture often contains several different encodings for a single instruction. Each instruction shares some common ASL code to execute the instruction and (optionally) to perform part of the decoding. The “pstext” sections containing these are labelled “Execute” and “Postdecode”. Some instructions are just aliases for other instructions so they don’t contain an execute section — I will discard these instructions.

def readInstruction(xml):
    execs = xml.findall(".//pstext[@section='Execute']/..")
    posts = xml.findall(".//pstext[@section='Postdecode']/..")
    assert(len(posts) <= 1)
    assert(len(execs) <= 1)
    if not execs: return None # discard aliases

    exec = readASL(execs[0])
    post = readASL(posts[0]) if posts else None

These pieces of ASL can be shared by several different instruction encodings and each encoding is accompanied by a piece of ASL to interpret the fields of the encoding.

    # for each encoding, read instructions encoding, matching decode ASL and index
    encs = []
    for iclass in xml.findall('.//classes/iclass'):

Instruction encodings and register descriptions use a common section format called “regdiagram”. One of the key pieces of information about an instruction is which instruction set it belongs to. The XML uses four different tags: “T16” (Thumb-32 short encoding), “T32” (Thumb-32 long encoding), “A32” (ARM-32 encoding) and “A64” (ARM-64 encoding). The T16 encoding is 16-bits long and all the others are 32-bits long.

        encoding = iclass.find('regdiagram')
        isT16 = encoding.attrib['form'] == "16"
        insn_set = "T16" if isT16 else iclass.attrib['isa']

The “regdiagram” section contains a number of boxes corresponding to one or more contiguous bits within the encoding. The location of each box is specified by the width of the box and the highest bit position in the box. Awkwardly, the T16 encoding numbers its bits from 31 down to 16 instead of from 15 down to 0 so my script fixes that.

        fields = []
        for b in encoding.findall('box'):
            wd = int(b.attrib.get('width','1'))
            hi = int(b.attrib['hibit'])
            # normalise T16 encoding bit numbers
            if isT16: hi = hi-16
            lo = hi - wd + 1

Some boxes have an attribute ‘name’, I use “_” for any anonymous boxes.

            nm  = b.attrib.get('name', '_')

And some boxes have a constant bitvector made up of “0”, “1”, “x”, “(0)” or “(1)”. 0 and 1 should be obvious, x means “don’t care” and (0) and (1) mean “should be 0/1 and it is UNPREDICTABLE what happens if they are not.” I use a suitable number of “x”s for any field with no constant specified.

The constant can also take the form “!= 1111” meaning “must not equal 1111”. This check is always replicated in the ASL code so I discard that information for now.

            consts = ''.join([ c.text for c in b.findall('c') if c.text is not None ])

            # normalise constants: note that it discards != information
            # because it is better obtained elsewhere in spec
            if consts == "" or consts.startswith('!='):
                consts = 'x'*wd

Sometimes the XML splits a single field into two adjacent fields: typically one of the fields has a constant value. When this happens, the fields have a name like “reg<4:1>” and “reg<0>”. This is not very convenient for our purposes so I look for this pattern and merge them back into a single field called “reg”.

            # if adjacent entries are two parts of same field, join them
            # e.g., imm8<7:1> and imm8<0>
            m = re.match('^(\w+)<', nm)
            if m:
                nm = m.group(1)
                split = True
                if fields[-1][3] and fields[-1][2] == nm:
                    (hi1,lo1,_,_,c1) = fields.pop()
                    assert(lo1 == hi+1) # must be adjacent
                    hi = hi1
                    consts = c1+consts
                split = False


To finish off reading an encoding, we read the decode ASL and pick a good name for the instruction encoding.

        dec_asl = readASL(iclass.find('ps_section/ps'))

        name = dec_asl.name if insn_set in ["T16","T32","A32"] else encoding.attrib['psname']
        encs.append((name, insn_set, fields, dec_asl))

Finally, the collection of the encodings, the postdecode ASL and the execute ASL are packaged up as an instruction named after the shared execute ASL.

    return Instruction(exec.name, encs, post, exec)

And to read all the instructions in a directory, we use the following code:

instrs = []
for d in args.dir:
    for inf in glob.glob(os.path.join(d, '*.xml')):
        name = re.search('.*/(\S+).xml',inf).group(1)
        if name == "onebigfile": continue
        xml = ET.parse(inf)
        instr = readInstruction(xml)
        if instr is None: continue

Sorting the shared code

Once you have extracted all the code, you are going to want to process it in some way. This will probably be easier to do if we arrange the ASL type and function definitions so that definitions always occur before their first use. So my script uses the dependencies that we extracted from the ASL to perform a topological sort of the code.

There are several modes it can work in:

  • Sort all the code. This will include code used by instructions but also code used when an interrupt occurs or an external debugger is attached.
  • Extract all the code that is used by AArch64 instructions. That is, instructions using the A64 encodings.
  • Extract all the code that is used by AArch32 instructions. That is, instructions using the A32 encodings.
  • Extract all the code used by AArch64 or AArch32 instructions.


I hope this is useful for those who want to make use of ARM’s Machine Readable Architecture specification. The files are designed to meet many different purposes so it is not always obvious which parts of them are useful for your purpose. This is why I thought it would be a good idea to write some scripts that actually extract the code instead of just writing about how I believe you can do it.

At work, I have access to the raw files from which the XML files are built so it has been a while since I have tried to extract the specification from the XML and it has been interesting seeing how much easier it is to use the XML files than it was when I first started using the architecture specs. (But there were some issues that I had to work around as well — search the script for the word “workaround” for details.)

I would really welcome contributions from other people:

  • I have a fairly narrow focus in what I want to do with the XML so my tools discard potentially useful information.
  • I am also not a native Python speaker — you can probably tell.
  • Many of the people who want to use the machine readable specification are much more comfortable with functional languages — translations are most welcome.

So if you have a suggestion for improving the scripts or you want other scripts, feel free to implement your suggestion and send me a pull request.


p.s., unpacking the tarballs and extracting the code is just the beginning. I am working with Cambridge University’s REMS research group to convert the ASL to their SAIL language from which you can generate O’Caml, LEM and HOL versions of the spec (with more backends planned).

And if the SAIL version does not suit your needs, then you might want want to know how to lex, parse, typecheck and execute ASL code yourself. I will describe those in future posts.

Written on April 29, 2017.
The opinions expressed are my own views and not my employer's.