Static instruction objects

This section describes the static instruction objects returned by the decoder. The definitive documentation for these objects is associated with the source code (particularly in static_inst.hh). The purpose of this section is to provide an overview of the static instruction class hierarchy and how it is used to factor the common portions of instruction descriptions.

The instruction objects returned by the decoder during simulation are instances of classes derived from the C++ class StaticInst, defined in static_inst.hh. Because some aspects of StaticInst depend on the ISA, such as the maximum number of source registers, StaticInst is defined as a template class. The ISA-specific characteristics are determined by a class template parameter.

The following code snippet is from Alpha's decoder function defined in decoder.cc. Lda is a class derived from class StaticInst, and defined in decoder.hh. Both of these files are generated from ISA description language by ISA parser, so they are under subdirectory build, not src.

StaticInstPtr
AlphaISA::decodeInst(AlphaISA::ExtMachInst machInst)
{
    using namespace AlphaISAInst;
    switch (OPCODE) {
    case 0x8:  
      // LoadAddress::lda([' Ra = Rb + disp; '],{})      
      return new Lda(machInst);
...

StaticInst base class

The information contained in a StaticInst object includes:

• a vector of flags that indicate the instruction's basic properties, such as whether it's a memory reference;
• the instruction's operation class, used to assign the instruction to an execution unit in a detailed CPU mode;
• the number of source and destination registers
• the number of destination registers that are integer and floating point, respectively
• the register indices of sources and destinations
• a function to generate the text disassembly of the instruction
• functions to emulate the execution of the instruction under various CPU models
• additional information specific to particular classes of instructions, such as target addresses for branches or effective addresses for memory references.

Some of these items, such as the number of source and destination registers, are simply data fields in the base class. When decoding a machine instruction, the decoder simply initializes these fields with appropriate values. Other items, such as the functions that emulate execution, are best defined as virtual functions, with a different implementation for each opcode. As a result, the decoder typically returns an instance of a different derived class for each opcode, where the execution virtual functions are defined appropriately. Still other features, such as disassembly, are best implemented in a hybrid fashion. Virtual functions are used to provide a small set of different disassembly formats. Instructions that share similar disassembly formats (e.g., integer immediate operations) share a single disassembly function via inheritance.

StaticInst class hierarchy

The class hierarchy is not visible outside the decoder; only the base StaticInst class definition is exported to the rest of the simulator. Thus the structure of the class hierarchy is entirely up to the designer of the decoder module. At one extreme, a single class could be used, containing a superset of the required data fields and code. The execution and disassembly functions of this class would then examine the opcode stored in the class instance to perform the appropriate action, effectively re-decoding the instruction. However, this design would run counter to the goal of paying decode overhead only once.

In the end, the structure of the class hierarchy is driven by practical considerations, and has no set relationship to opcodes or to the instruction categories defined by the ISA. For example, in the Alpha ISA, an "addq" instruction could generate an instance of one of three different classes:

1. A typical register-plus-register add generates an instance of the Addq class, which is derived directly from the base AlphaStaticInst class.
2. A register-plus-immediate add generates an instance of AddqImm, which derives from the intermediate IntegerImm class. IntegerImm derives directly from AlphaStaticInst, and is the base class of all integer immediate instructions. It adds an 'imm' data field to store the immediate value extracted from the machine instruction, and overrides the disassembly function to format the immediate value appropriately.
3. An "addq" instruction that specifies r31 (the Alpha zero register) as its destination is a no-op, and returns an instance of the Nop class, regardless of whether it uses a reg-reg or reg-imm format.