[deliverable/binutils-gdb.git] / sim / mips / README.Cygnus

This directory contains two very different simulators:

	o	gencode (old)

		Gencode.c outputs a single monolithic file that is
		#included by interp.c

	o	igen (new)

		The *.igen files are used as inputs to ../igen/igen.
		A number of separate, fairly modula files, are created.

The new simulator has a number of advantages:

	o	builtin support for multi-simming (single simulator
		image supporting a number of different instruction
		set architectures).

	o	Easier maintenance. The input files are not confused
		by an intermixing with the generator code.

gencode continues to exist so that old architectures can be emulated.
*.igen should be used when adding new architectures or adding
instructions to an existing ISA.

Known bugs?

A mips16 simulator cannot be built using igen.  A custom mips16
engine.c needs to be written.

In mips.igen, the semantics for many of the instructions were created
using code generated by gencode.  Those semantic segments could be
greatly simplified.


----

Old README.Cygnus ...

> README.Cygnus
-------------------------------------------------------------------------------

The following are the main reasons for constructing the simulator as a
generator:

1) Avoid large fixed decode source file, with lots of #ifs controlling
   the compilation. i.e. keep the source cleaner, smaller and easier
   to parse.

2) Allow optimum code to be created, without run-time checks on
   instruction types. Ensure that the simulator engine only includes
   code for the architecture being targetted. e.g. This avoids
   run-time checks on ISA conformance, aswell as increasing
   throughput.

3) Allow updates to the instruction sets to be added quickly. Having a
   table means that the information is together, and is easier to
   manipulate. Having the table generate the engine, rather than the
   run-time parse the table gives higher performance at simulation
   time.

4) Keep all the similar simulation code together. i.e. have a single
   place where, for example, the addition code is held. This ensures that
   updates to the simulation are not spread over a large flat source
   file maintained by the developer.

-------------------------------------------------------------------------------

To keep the simulator simple (and to avoid the slight chance of
mis-matched files) the manifests describing an engine, and the
simulator engine itself, are held in the same source file.

This means that the engine must be included twice, with the first pass
controlled by the SIM_MANIFESTS definition.

-------------------------------------------------------------------------------
> EOF README.Cygnus
Commit	Line	Data
13151a93 AC	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	A mips16 simulator cannot be built using igen. A custom mips16
	29	engine.c needs to be written.
	30
	31	In mips.igen, the semantics for many of the instructions were created
	32	using code generated by gencode. Those semantic segments could be
	33	greatly simplified.
	34
	35
	36	----
	37
	38	Old README.Cygnus ...
	39
8ad57737 JSC	40	> README.Cygnus
	41	-------------------------------------------------------------------------------
	42
	43	The following are the main reasons for constructing the simulator as a
	44	generator:
	45
	46	1) Avoid large fixed decode source file, with lots of #ifs controlling
	47	the compilation. i.e. keep the source cleaner, smaller and easier
	48	to parse.
	49
	50	2) Allow optimum code to be created, without run-time checks on
	51	instruction types. Ensure that the simulator engine only includes
	52	code for the architecture being targetted. e.g. This avoids
	53	run-time checks on ISA conformance, aswell as increasing
	54	throughput.
	55
	56	3) Allow updates to the instruction sets to be added quickly. Having a
	57	table means that the information is together, and is easier to
	58	manipulate. Having the table generate the engine, rather than the
	59	run-time parse the table gives higher performance at simulation
	60	time.
	61
	62	4) Keep all the similar simulation code together. i.e. have a single
	63	place where, for example, the addition code is held. This ensures that
	64	updates to the simulation are not spread over a large flat source
	65	file maintained by the developer.
	66
	67	-------------------------------------------------------------------------------
	68
	69	To keep the simulator simple (and to avoid the slight chance of
	70	mis-matched files) the manifests describing an engine, and the
	71	simulator engine itself, are held in the same source file.
	72
	73	This means that the engine must be included twice, with the first pass
	74	controlled by the SIM_MANIFESTS definition.
	75
	76	-------------------------------------------------------------------------------
	77	> EOF README.Cygnus