MicroTESK

h1. MMU Description

_~By Alexander Kamkin, Taya Sergeeva, and Andrei Tatarnikov~_

{{toc}}

A _memory management unit_ (_MMU_) is known to be one of the most complex and error-prone components of a modern microprocessor. MicroTESK has a special subsystem, called _MMU subsystem_, intended for (1) specifying memory devices and (2) deriving testing knowledge from such specifications. The subsystem provides unified facilities for describing memory buffers (like _L1_ and _L2 caches_, _translation look-aside buffers_ (_TLBs_), etc.) as well as a means for connecting several buffers into a memory hierarchy.

h2. Grammar

<pre>

startRule

    : declaration* EOF!

    ;

declaration

    : address

    | segment

    | buffer

    | mmu

    ;

</pre>

The expression syntax is derived from nML/Sim-nML (see [[Sim-nML Language Reference]]).

h2. Address Description (address)

A buffer is accessed by an _address_, which is typically a _bit vector_ of a fixed length (width). Different buffers are allowed to have a common address space (e.g., L1 and L2 are usually both addressed by physical addresses). However, in general case, each buffer has its own domain.

An address space is described using a keyword @address@. The description includes the address type _identifier_ and the address _width_. The latter is specified in brackets. Its value should be non-negative (zero-length addresses are permitted).

h3. Grammar

<pre>

address

    : ''address'' addressTypeID ''('' expr '')''

    ;

</pre>

h3. Examples

<pre>// A 64-bit virtual address (VA).

address VA(64)</pre>

<pre>// A 36-bit physical address (PA).

address PA(36)</pre>

h2. Address Space Segment Description (segment)

An address space segment is specified using the @segment@ keyword. A segment is associated with a specific address type. It is possible to specify any number (≥ 0) of segments (with different names) for one address type. Each segment is characterized by its _identifier_ and _address range_. Different segments should have different names, but address ranges are allowed to overlap, and moreover, to be the same.

h3. Grammar

<pre>

segment

    : ''segment'' segmentID ''('' argumentID '':'' addressTypeID '')''

        ''range'' ''='' ''('' expr '','' expr '')''

    ;

</pre>

h3. Examples

<pre>

segment USEG (va: VA)

  range = (0x0000000000000000, 0x000000007fffffff)

</pre>

h2. Buffer Description (buffer)

A buffer is described using a keyword @buffer@. The description specifies a set of parameters, including @ways@, @sets@, @entry@, @index@, @match@ and @policy@. All of the parameters except @index@ (if @sets = 1@) and @policy@ are obligatory.

h3. Grammar

<pre>

buffer

    : ''buffer'' bufferTypeID ''('' addressArgID '':'' addressTypeID '')''

        (bufferParameter)*

    ;

bufferParameter

    : ways

    | sets

    | entry

    | index

    | match

    | policy

    ;

</pre>

h3. Buffer Associativity (ways)

The @ways@ parameter specifies the buffer _associativity_ (the number of lines in a set). The parameter is obligatory; its value should be positive.

h4. Grammar

<pre>

ways

    : ''ways'' ''='' expr

    ;

</pre>

h3. Buffer Length (sets)

The @sets@ parameter specifies the buffer _length_ (the number of sets a cache). The parameter is obligatory; its value should be positive.

h4. Grammar

<pre>

sets

    : ''sets'' ''='' expr

    ;

</pre>

h3. Buffer Line Format (entry)

The @entry@ parameter specifies the buffer _line format_ (a number of named fields). A field has three attributes: a name, a width and, optionally, an initial value.

h4. Grammar

<pre>

format

    : ''entry'' ''='' ''('' field ('','' field)* '')''

    ;

field

    : fieldID '':'' expr (''='' expr)?

    ;

</pre>

h3. Buffer Index Function (index)

The @index@ parameter specifies the _address-to-index function_, which maps an address into the set index. The function may be omitted if the number of sets is @1@.

h4. Grammar

<pre>

index

    : ''index'' ''='' expr

    ;

</pre>

h3. Buffer Match Predicate (match)

The @match@ parameter specifies the _address-line match predicate_, which checks if an address matches a line. The parameter is obligatory.

h4. Grammar

<pre>

index

    : ''match'' ''='' expr

    ;

</pre>

h3. Buffer Data Replacement Policy (policy)

The @policy@ parameters specifies the _data replacement_ (_eviction_) _policy_. The parameter is optional. The list of supported policies includes: @RANDOM@, @FIFO@, @PLRU@ and @LRU@.

h4. Grammar

<pre>

policy

    : ''policy'' ''='' policyID

    ;

</pre>

h3. Examples

<pre>

// A 4-way set associative cache (L1) addressed by physical addresses (PA).

buffer L1(addr: PA)

  // The cache associativity.

  ways = 4

  // The number of sets.

  sets = 128

  // The line format.

  entry = (

    V    : 1 = 0, // The validity flag (by default, the line is invalid).

    TAG  : 24,    // The tag (the <35..12> address bits).

    DATA : 256    // The data (4 double words).

  )

  // The address-to-index function (example: using address fields).

  index = addr.INDEX

  // The address-line predicate (example: using address bits).

  match = addr<35..12> == TAG

  // The data replacement policy (example: using predefined policy LRU - Least Recently Used).

  policy = LRU

</pre>

h2. MMU Description (mmu)

Memory management unit logic is described using the @mmu@ keyword. The description includes two obligatory parameters @read@ and @write@ that describe the semantics of memory read and memory write actions respectively.

h3. Grammar

<pre>

mmu

    : ''mmu'' memoryTypeID ''('' addressArgID '':'' addressTypeID '')'' = dataArgID

        (mmuVariable)*

        (mmuParameter)*

    ;

mmuVariable

    : ''var'' variableID '':'' variableTypeID (''.'' ''entry'')? '';''

    ;

mmuParameter

    : read

    | write

    ;

</pre>

h3. Memory Read Action (read)

The @read@ parameter specifies the _read action_, which is a sequence of statements describing how the read operation is to be performed (by means of data transfers between buffers). The parameter is obligatory.

h4. Grammar

<pre>

read

    : ''read'' ''='' ''{'' sequence ''}''

    ;

</pre>

h3. Memory Write Action (write)

The @write@ parameter specifies the _read action_, which is a sequence of statements describing how the write operation is to be performed (by means of data transfers between buffers). The parameter is obligatory.

h4. Grammar

<pre>

write

    : ''write'' ''='' ''{'' sequence ''}''

    ;

</pre>

h3. Examples

<pre>

// A memory unit addressed by virtual addresses (VA).

mmu Memory(addr: VA) = data

  // The read action.

  read = {

    // Some statements.

    ...

  }

  // The write action.

  write = {

    // Some statements.

    ...

  }

</pre>