ISA description system

The purpose of the M5 ISA description system is to generate a decoder function that takes a binary machine instruction and returns a C++ object representing that instruction. The returned object encapsulates all of the information (data and code) needed by the M5 simulator related to that specific machine instruction. By making the object as specific as possible to the machine instruction, the decoding overhead is paid only once; throughout the rest of the simulator, the object makes the needed instruction characteristics accessible easily and with low overhead.

Because a typical commercial ISA has hundreds of instructions, specifying the properties of each individual instruction in detail is tedious and error prone. The ISA description (as specified by the programmer) must take advantage of the fact that large classes of instructions share common characteristics. However, real ISAs invariably have not only several different instruction classes, but also a number of oddball instructions that defy categorization. The goal of the M5 ISA description system is to allow a human-readable ISA description that captures commonalities across sets of instructions while providing the flexibility to specify instructions with arbitrary characteristics. For example, once the structure is in place to define integer ALU instructions, the specification of simple instructions in that class (e.g., add or subtract) should constitute a single, readable, non-redundant line of the description file.

M5 uses a custom, domain-specific language to achieve this goal. The language's syntax and features are specifically designed to make instruction definitions compact and readable while minimizing redundancy. The language's flexibility arises because the final code generation is performed by user-provided functions written in a general-purpose programming language (Python).

An ISA description written in this custom language is processed by a parser to generate several C++ files containing class definitions and the decode function. This file is in turn compiled into the M5 simulator.

The documentation for the ISA description system is divided into three pages:

• Static instruction objects provides a basic familiarity with the structure of these C++ objects, which are the final output of the decoding process. The goal of the ISA description system is to generate definitions for these objects, so you'll need to understand them before continuing.
• The M5 ISA description language covers the description language proper.
• Code parsing describes a library of Python classes and functions that automate the process of deducing an instruction's characteristics from a brief description of its operation. Although this library is not strictly part of the description language, it is used extensively by the Python code that generates the final C++ objects.