MMU description » History » Revision 85
Revision 84 (Alexander Kamkin, 12/02/2014 07:29 AM) → Revision 85/132 (Alexander Kamkin, 12/02/2014 07:29 AM)
h1. MMU Description
_~By Alexander Kamkin and Taya Sergeeva~_
*UNDER CONSTRUCTION*
{{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>
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.
address VA {
// The address width.
width = 64;
}
</pre>
<pre>
// A stuctured 40-bit physical addresses.
address PA {
// The address width.
width = 40;
// The address format.
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>
The code above defines three address spaces: (1) a singleton @Void@; (2) a space @VA@ consisting of 64-bit addresses (_virtual addresses_) and (3) a space @PA@ consisting of 40-bit addresses (_physical addresses_), each being divided into for fields: @tag@ (24 bits), @l1Index@ (7 bits), @dwPosition@ (2 bits) and @bytePosition@ (3 bits).
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 ''{''
(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'' ''('' addressTypeID addressArgID '')'' ''='' 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'' ''('' addressTypeID addressArgID '')'' ''='' 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).
buffer L1 {
// The cache associativity.
ways = 4;
// The number of sets.
sets = 128;
// The line format.
format = (
V VAL : 1 = 0, // The validity flag (by default, the line is invalid).
TAG : 24, // The tag (the <35..12> address bits).
DATA ROW : 256 // The data (4 double words).
);
// The address-to-index function.
index(PA addr) = addr<11..5>;
// The address-line predicate.
match(PA addr) = addr<35..12> == TAG;
// The data replacement policy.
policy = LRU;
}
</pre>