Sim-nML is an architecture description language (ADL) used in MicroTESK to describe the architecture of a microprocessor under verification. It is a flexible and easy-to-use language based on attribute grammar. Sim-nML was designed to provide a retargetable way to specify microprocessor architecture for various microprocessor-related software tools including instruction-set simulators, assemblers, disassemblers, compiler back-ends etc. It works at the instruction-set level concentrating on behavioral properties and hiding implementation details of the microprocessor. A Sim-nML specification represents a programmer's model of the microprocessor that covers the following aspects:
- supported data types;
- registers and memory;
- addressing modes;
- syntax and semantics of instructions.
Sim-nML uses a hierarchical tree-like structure to describe an instruction set. Such a structure facilitates grouping related instructions and sharing their common parts. An instruction is described as a path in the tree from the root node to a leaf node. The set of all possible paths represents an instruction set. A node describes a primitive operation responsible for some task within an instruction. Nodes have attributes that can be shared with their parents. Actions performed by instructions are described as operations with registers and memory represented by bit vectors of arbitrary size.
A specification in Sim-nML starts with definitions of types and constants. For example, a type definition for a 32-bit word looks as follows:
let WORD_SIZE = 32 type word = card(WORD_SIZE)
Type definitions and constants can be used to describe registers and memory. In addition to registers and memory, it is also possible to define temporary variables, internal abstractions provided by Sim-nML to store intermediate results of operations. They do not correspond to any data storage in real hardware and do not save their data across instruction calls. Also, there is often a need to specify some properties of the described model. For this purpose, special constants are used. For example, the code below defines general-purpose registers, memory and a temporary variable. Also, there are two special constants that specify endianness and establish a correspondence between the general purpose register number 15 and program counter. Here is the code:
reg GPR[32, word] mem M[2 ** 20, byte] var carry[1, bit] let byte_order = "little" let PC = "GPR"