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75 lines
2.5 KiB
Plaintext
75 lines
2.5 KiB
Plaintext
This directory contains two very different simulators:
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o gencode (old)
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Gencode.c outputs a single monolithic file that is
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#included by interp.c
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o igen (new)
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The *.igen files are used as inputs to ../igen/igen.
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A number of separate, fairly modula files, are created.
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The new simulator has a number of advantages:
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o builtin support for multi-simming (single simulator
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image supporting a number of different instruction
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set architectures).
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o Easier maintenance. The input files are not confused
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by an intermixing with the generator code.
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gencode continues to exist so that old architectures can be emulated.
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*.igen should be used when adding new architectures or adding
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instructions to an existing ISA.
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Known bugs?
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In mips.igen, the semantics for many of the instructions were created
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using code generated by gencode. Those semantic segments could be
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greatly simplified.
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----
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Old README.Cygnus ...
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> README.Cygnus
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-------------------------------------------------------------------------------
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The following are the main reasons for constructing the simulator as a
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generator:
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1) Avoid large fixed decode source file, with lots of #ifs controlling
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the compilation. i.e. keep the source cleaner, smaller and easier
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to parse.
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2) Allow optimum code to be created, without run-time checks on
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instruction types. Ensure that the simulator engine only includes
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code for the architecture being targetted. e.g. This avoids
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run-time checks on ISA conformance, aswell as increasing
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throughput.
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3) Allow updates to the instruction sets to be added quickly. Having a
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table means that the information is together, and is easier to
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manipulate. Having the table generate the engine, rather than the
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run-time parse the table gives higher performance at simulation
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time.
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4) Keep all the similar simulation code together. i.e. have a single
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place where, for example, the addition code is held. This ensures that
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updates to the simulation are not spread over a large flat source
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file maintained by the developer.
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-------------------------------------------------------------------------------
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To keep the simulator simple (and to avoid the slight chance of
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mis-matched files) the manifests describing an engine, and the
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simulator engine itself, are held in the same source file.
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This means that the engine must be included twice, with the first pass
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controlled by the SIM_MANIFESTS definition.
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-------------------------------------------------------------------------------
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> EOF README.Cygnus
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