MMU description » History » Revision 103
Revision 102 (Andrei Tatarnikov, 01/29/2015 11:36 AM) → Revision 103/132 (Andrei Tatarnikov, 01/29/2015 11:37 AM)
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
: declaration* EOF!
;
declaration
: address
| segment
| buffer
| mmu
;
</pre>
The expression syntax is derived from nML/Sim-nML (see [[Sim-nML _MicroTESK Language Reference]]). 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 can specify two parameters: @width@ (obligatory) and @format@ (optional).
h3. Grammar
<pre>
address
: ''address'' ID
(addressParameter)*
;
addressParameter
: width
| segment
| 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 Space Segment (segment)
The @segment@ parameter specifies the address space _segment_. Any number (≥ 0) of segments (with different names) can be specified for one address. Each segment is characterized by its _name_ and _address range_. Different segments should have different names, but address ranges are allowed to overlap, and moreover, to be the same.
h4. Grammar
<pre>
segment
: ''segment'' segmentID ''='' ''('' expr '','' expr '')''
;
</pre>
h3. Address Format (format)
The @format@ parameter specifies the address _format_ (a number of named fields). Any number (≥ 0) of formats (with different names) can be specified for one address.
A field has three attributes: a _name_, a _width_ and, optionally, an _initial value_.
h4. Grammar
<pre>
format
: ''format'' formatID ''='' ''(''
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 PA_L1 = (
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 (buffer)
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). Any number (≥ 0) of formats (with different names) can be specified for one buffer.
A field has three attributes: a name, a width and, optionally, an initial value.
h4. Grammar
<pre>
format
: ''format'' formatID ''='' ''('' 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 LINE = (
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> == LINE.TAG
// The data replacement policy (example: using predefined policy LRU - Least Recently Used).
policy = LRU
</pre>
h2. Memory Description (memory)
A memory is described using a keyword @memory@. The description includes two obligatory parameters @read@ and @write@.
h3. Grammar
<pre>
memory
: ''memory'' memoryTypeID ''('' addressTypeID addressArgID '')''
(memoryParameter)*
;
memoryParameter
: 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).
memory Memory(VA addr)
// The read action.
read = {
// Some statements.
...
}
// The write action.
write = {
// Some statements.
...
}
</pre>