MMU Description¶

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.

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 construct address. A couple of examples are given below.

address Void { width = 0  }

address PA   { width = 40 }

The code above defines two address spaces: (1) a single-element space Void and (2) a space PA consisting of 40-bit addresses (PA usually stands for physical address).

Buffer Description¶

Buffer can be described by different parameters, such as the associativity, the number of sets, the tag computing function, the index computing function, the structure of data unit, the controlling bits, the strategies of data changing when ''miss'' occurs, and so on.

For instance, there is an example of the buffer below:

buffer L1 
{
    sets = 4;
    lines = 128;
    line = (tag:30, data:256);
    index(addr:PA) = addr<9..8>;
    match(addr:PA) = line.tag == addr<39..10>;
    policy = lru;
}

Description of each constructor in the buffer example is below:

address¶

  gives the width of the field occupied in bytes;
  _address_ has a name; ''PA''(Physical Address) in our case; it also can be virtual (VA);

buffer¶

  has a name, ''L1'' in pur example; it can have names ''L2'' and ''TLB'' also;
  _buffer_ can be described by different parameters, such _sets_, _lines_, _index_, _match_, _policy_, and so on, which number is infixed;

sets¶

  _sets_ is an associativity of a buffer; it returns the number of lines in a one set;

lines¶

  _lines_ is the number of sets in a given buffer;

line¶

  _line_ is an optional description of line''s fields;
  it designates each line of the cache; 
  _line_ includes its own parameters in the braces: _tag_ and _data_, each of them has an appropriate width of the fields kept in bytes;
  in our example _line_ has only two parameters, but in general case it can include more;
  it contains a 30-bit tag and a 256-bit data;

index¶

   _index_ is the function for index calculation;
   returns the initial and the final points of the field kept in bytes; they are marked in a three-cornered brackets, after _addr_; in our case index has 2 bits;
  _index_ depends on an _address_, which is ''physical'' (PA) in our case; the type of an address is set in the braces after _index_; 

match¶

  _match_ is a predicate checking whether the line and the address match each other or not;
  it returns ''true'' or ''false'' depending on if the data required is in the given line or not; 
  it returns ''true'' if there is a ''hit'' in the line, and returns ''false'' otherwise; if the set contains a line with the tag equal to the 30 upper bits of the physical address, this is a ''hit''; if the set does not contain the line, this is a ''miss'' situation;
  _match_ description contains the the initial and the final points of the address field in the triangle brackets after _addr_; 
  as _index_ in the round braces _match_ also has the type of the address used; ''PA'' in our case;

policy¶

  _policy_ is the strategy of data displacement; 
  sets a policy which will be applied to our buffer, ''lru'' (Least Recently Used) in our example; i.e. if the ''miss'' occured, the cache displaces the least-recently-used line of the set; 
  policy also can be ''plru'' (Pseudo LRU) and ''fifo'' (First Input First Out).

Code Structure¶

The MMU grammar is in ru.ispras.microtesk.translator.mmu.grammar folder. It contains Lexer, Parser and TreeWalker files. These files can be compiled by build.xml file (microtesk++/build.xml). The files generated (MMULexer.java, MMUParser.java, MMUTreeWalker.java) are in microtesk++.gen.ru.ispras.microtesk.translator.mmu.grammar folder.

The folders ru.ispras.microtesk.translator.mmu.ir.* contain the inner representation of the MMU hierarchy of one buffer.

MMU translator is in the ru.ispras.microtesk.translator.mmu.translator folder.

Files in ru.ispras.microtesk.model.api.mmu folder contain different policies of cache. Folder ru.ispras.microtesk.model.api.mmu.buffer contains the model of MMU - the files which describe Buffer, Set, Line, Address expressions.

After grammar files being generated the file ''BufferExample'' can be loaded to the translator.