sim/mips/README.Cygnus

   1 This directory contains two very different simulators:
   2
   3         o       gencode (old)
   4
   5                 Gencode.c outputs a single monolithic file that is
   6                 #included by interp.c
   7
   8         o       igen (new)
   9
  10                 The *.igen files are used as inputs to ../igen/igen.
  11                 A number of separate, fairly modula files, are created.
  12
  13 The new simulator has a number of advantages:
  14
  15         o       builtin support for multi-simming (single simulator
  16                 image supporting a number of different instruction
  17                 set architectures).
  18
  19         o       Easier maintenance. The input files are not confused
  20                 by an intermixing with the generator code.
  21
  22 gencode continues to exist so that old architectures can be emulated.
  23 *.igen should be used when adding new architectures or adding
  24 instructions to an existing ISA.
  25
  26 Known bugs?
  27
  28 In mips.igen, the semantics for many of the instructions were created
  29 using code generated by gencode.  Those semantic segments could be
  30 greatly simplified.
  31
  32
  33 ----
  34
  35 Old README.Cygnus ...
  36
  37 > README.Cygnus
  38 -------------------------------------------------------------------------------
  39
  40 The following are the main reasons for constructing the simulator as a
  41 generator:
  42
  43 1) Avoid large fixed decode source file, with lots of #ifs controlling
  44    the compilation. i.e. keep the source cleaner, smaller and easier
  45    to parse.
  46
  47 2) Allow optimum code to be created, without run-time checks on
  48    instruction types. Ensure that the simulator engine only includes
  49    code for the architecture being targetted. e.g. This avoids
  50    run-time checks on ISA conformance, aswell as increasing
  51    throughput.
  52
  53 3) Allow updates to the instruction sets to be added quickly. Having a
  54    table means that the information is together, and is easier to
  55    manipulate. Having the table generate the engine, rather than the
  56    run-time parse the table gives higher performance at simulation
  57    time.
  58
  59 4) Keep all the similar simulation code together. i.e. have a single
  60    place where, for example, the addition code is held. This ensures that
  61    updates to the simulation are not spread over a large flat source
  62    file maintained by the developer.
  63
  64 -------------------------------------------------------------------------------
  65
  66 To keep the simulator simple (and to avoid the slight chance of
  67 mis-matched files) the manifests describing an engine, and the
  68 simulator engine itself, are held in the same source file.
  69
  70 This means that the engine must be included twice, with the first pass
  71 controlled by the SIM_MANIFESTS definition.
  72
  73 -------------------------------------------------------------------------------
  74 > EOF README.Cygnus