MicroTESK

h1. MMU Description

_~By Alexander Kamkin and Taya Sergeeva~_

{{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 

    : bufferOrAddress*

    ;

bufferOrAddress

    : address

    | buffer

    ;

</pre>

The expression syntax is derived from nML/Sim-nML (see _MicroTESK Language Reference_).

h2. Address Description

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 can specify two parameters: @width@ (obligatory) and @format@ (optional).

h3. Grammar

<pre>

address

    : ''address'' ID

        (addressParameter)*

    ;

addressParameter

    : width

    | format

    ;

</pre>

h3. Address Width (width)

The @width@ parameter specifies the address _width_. The parameter is obligatory; its value should be non-negative (zero-length addresses are permitted).

h4. Grammar

<pre>

width

    : ''width'' ''='' expr

    ;

</pre>

h3. Address Format (format)

The @format@ parameter specifies the address _format_ (a number of named fields). The parameter is optional. By default, the address is unstructured (no fields are specified).

A field has three attributes: a _name_, a _width_ and, optionally, an _initial value_.

h4. Grammar

<pre>

format

    : ''format'' ''='' ''(''

        field ('','' field)*

      '')''

    ;

field

    : ID '':'' expr (''='' expr)?

    ;

</pre>

h2. Examples

<pre>

// The singleton.

address Void

  // The address width is zero (this is admissible for single-item buffers).

  width = 0

</pre>

<pre>

// An unstructured 64-bit virtual addresses (VA).

address VA

  // The address width.

  width = 64

</pre>

<pre>

// A stuctured 40-bit physical addresses (PA).

address PA

  // The address width.

  width = 40

  // The address format: (<39..36>, TAG=<35..12>, INDEX=<11..5>, LOCAL=<4..0>).

  format = (

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

    INDEX : 7,  // The set index (the <11..5> address bits).

    LOCAL : 5,  // The byte position (the <0..4> address bits).

  )

</pre>

h2. Buffer Description

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

h3. Grammar

<pre>

buffer

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

        (bufferParameter)*

    ;

bufferParameter

    : ways

    | sets

    | format

    | 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 (format)

The @format@ parameter specifies the buffer _line format_ (a number of named fields). The parameter is optional. By default, the buffer line is unstructured (no fields are specified).

A field has three attributes: a name, a width and, optionally, an initial value.

h4. Grammar

<pre>

format

    : ''format'' ''='' ''('' 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(PA addr)

  // The cache associativity.

  ways = 4

  // The number of sets.

  sets = 128

  // The line format.

  format = (

    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>