Content
12 Architecture Formats
This sections describes the format of the files used to describe an architecture. They includes description for:
microprocessor pipeline,
cache hierarchy,
memory address space.
12.1 Notation
These description files formats are based on XML. XML allows to represent structured documents with a textual representation organized as a tree of elements. Elements are delimited by tags and identifier between brackets, < and >, possibly containing attributes.
This document describes dialects of XML whose syntax is given with a special notation derived from EBNF.
<!-- NT ::= --> <mytag att="INT"?> <!-- NT1 --> (<!-- NT2 --> | <!-- NT3 -->) </mytag>
The example below allows to define the non-terminal symbol NT that starts and ends with tags mytag. It may have an attribute called att containing an integer. Its content start first with a an element NT1 (defined elsewhere) and followed either by an element of type NT2 or NT3.
Words in uppercase describe a terminal textual content while the tags are represented as-is.
Tags and attributes may be followed with one of this symbol:
* – item may be repeated zero or several times,
+ – item may be repeated one or several times,
? – item is optional.
The terminals may be:
INT – an integer (accepting C decimal, binary, octal or hexadecimal notation),
TEXT – any text supported by XML,
BOOL – one of true or false,
12.2 Microprocessor Pipeline
A microprocessor pipeline allows to describes the stages and the queue composing the pipeline. References between stages and queues is performed by assigning id attributes to the items and referencing them using ref attributes.
12.2.1 To Level
The top-level element is given below (notice that the class attribute is mandatory):
<!-- PIPELINE::= --> <processor class="otawa::hard::Processor"> <arch>TEXT</arch>? <model>TEXT</model>? <builder>TEXT</builder>? <frequency>INT</frequency>? <stages> <!-- STAGE -->* </stages>? <queues> <!-- QUEUE -->* </queues>? </processor>
The contained items have the following description:
<arch> – describe the Instruction-Set Architecture of the processor (this may be used to identify the loader plug-in used with this description),
<model> – processor model name as provided by the builder,
<builder> – builder name
<frequency> – frequency of the processor in Hz,
<stages> – list of stages,
<queues> – list of queues.
12.2.2 Stage Description
<!-- STAGE ::= --> <stage id="TEXT"?> <name>TEXT</name>? <width>INT</width>? <latency>INT</latency>? <pipelined>BOOL</pipelined>? <type>FETCH|LAZY|EXEC|COMMIT</type>? <ordered>BOOL</ordered>? <fus> <!-- FU -->* </fus>? <dispatch> <!-- INST -->* </dispatch>? </stage>
A pipeline stage may have <name> (for the human user), a <width> that gives the numer of instructions that may processed by the stage in one cycle and a <type>. <latency> gives the number of cycles an instruction spends in the stage while <pipelined> says if the instruction execution in the stage is pipelined, that is, overlapped.
The types have the following meaning:
FETCH – stage getting instruction from memory,
LAZY – do noting special (only spends time),
EXEC – stage performing execution of instructions,
COMMIT – stage where instructions are leaving the pipeline.
<ordered>, fus and dispatch elements are only used with EXEC type stages. ordered allows to know if the execution is in-order or not. FU gives the list of FU (Functional Units) while the <dispatch> allows to dispatch instructions to the different FUs. An EXEC stage dispatches instruction to FU for instruction only if the data dependencies are already fulfilled.
12.2.3 Functional Unit
A FU is responsible for performing the execution of an instruction.
<!-- FU ::= --> <fu id="TEXT"?> <name>TEXT</name> <width>INT</width> <latency>INT</latency> <pipelined>BOOL></pipelined> </fu>
The <name> is only for display to the human user and may be any text. In <width> element gives the number of instruction that may executed in parallel in each cycle with <latency> gives the number of cycles an instruction spends in the stage. With a latency of several cycles, the instruction traversal of the FU may be pipelined, element <pipelined>, that is, the different cycles may be overlapped.
12.2.4 <inst> Element
<inst> element allows to select which FU will execute an instruction.
<inst> <type>FLAGS</type> <fu ref="TEXT"/>
The <fu> element gives the FU that will execute an instruction matching the flags in the <type> element.
The flags are separated by | pipes and are a composition of identifier selecting the properties of the instruction (as provided by the loader). To be selected, an instruction must match all flags of the <type> element.
Known flags are derived from the otawa::Inst class kinds and includes:
IS_COND – conditional instruction,
IS_CONTROL – control instruction,
IS_CALL – sub-program call,
IS_RETURN – sub-program return,
IS_MEM – instruction performing memory access,
IS_LOAD – instruction reading in memory,
IS_STORE – instruction writing in memory,
IS_INT – instruction working on integers,
IS_FLOAT – instruction working on floats,
IS_ALU – computation instruction,
IS_MUL – multiplication,
IS_DIV – division,
IS_SHIFT – shift or rotation,
IS_TRAP – trap or exception instruction,
IS_INTERN – instruction performing internal work in hardware (not changing program behaviour),
IS_MULTI – performs multiple loads or stores,
IS_SPECIAL – instruction that can not be classified with other flags,
IS_INDIRECT – indirect control (branching on address obtained from a register).
12.2.5 Queue
A queue is a small memory containing instructions processed by the pipeline. It may be interleft between two stages (FIFO queue) or used as an instruction store for a stage (like a re-order buffer with out-of-order execution architecture)
<!-- QUEUE ::= --> <queue> <name>TEXT</name>? <size>INT</size>? <input ref="TEXT"/> <output ref="TEXT"/> <intern> <stage ref="TEXT"/> </intern>? </queue>
The <name> element is only used for human display while the <size> elements gives the number of instruction the queue can contain.
A queue must ever have an <input> stage (stage putting instructions in) and an <output> stage (stage getting instructions from). In addition, a queue may have several <intern> stages that use it as an instruction buffer. The referenced stages in an <intern> are usually of type EXEC and allows the instruction to leave the queue only if it has been executed.