Système Digital 2013-2014
Cours J.Vuillemin, TD T.Bourke

Alex AUVOLAT (Info 2013)


Introduction
============

Contents of the repository :
----------------------------

sched/
	A scheduler for netlists.
	Input : a netlist.
	Output : a netlist with topologically sorted operators
	 plus a dumbed-down version for input to the C circuit simulator
	This program is also capable of a few optimisation (usually reduces netlist
	size by 1/4 or 1/3).
	$ cd sched/
	$ ocamlbuild main.byte

csim/
	A circuit simulator written in C.
	This program does NOT do the scheduling.
	This program does NOT read a netlist, it reads a specifically formatted
	dumbed-down netlist, that is output by the scheduler.
	$ cd csim/
	$ make

camlsim/
	A circuit simulator written in OCaml.
	This program does NOT do the scheduling. The netlist must be passed through
	the scheduler first.
	This program is *highly incomplete* and not recomended for real use..
	$ cd camlsim/
	$ ocamlbuild simulator.byte

minijazz/
	The MiniJazz compiler (given by the teachers).

tests/
	Various test files.

*.pdf
	Documentation about the project.



REFERENCES
----------

  -	Computer organization and design : the hardware/software interface
	4th ed
	Chapters 2 to 4



INTERNALS
=========

Convention for binary values
----------------------------

/!\ This convention is contrary to the one used in the example file nadder.mj
	(Therefore I have modified that file...)

The bit array [a_0 a_1 a_2 ... a_n-1] represents the decimal number :
	a_0 + 2*a_1 + 4*a_2 + ... + 2^(n-1)*a_n-1

When represented in binary, we write the bits in the order :
	a_0 a_1 a_2 ... a_n-1
Even though the normal notation for a binary number is :
	a_n-1 a_n-2 ... a_0

/!\ BINARY NUMBERS ARE WRITTEN REVERSE !

What we call a shift-left does a left shift on the representation as a binary
number, but we use the reverse notation here. Therefore in the ALU, the shift-left
should actually right-shift the bits when they are seen in the order a0, a1, ...

The right shift is also inverted.
- left shift -> right shift
- right shift -> left shift
- left shift logical -> rigth shift logical


The dumbed-down netlist format (OLD FORMAT)
-------------------------------------------

This is the description of the old format, not used anymore.

The C simulator does NOT read a properly formatted netlist, but only a file
representing it written in a simple format, that can be parsed with only calls
to `scanf`. The file is organized as follows :

<number of variables>
<variable types and names *1>
<inputs *2>
<outputs *2>
<number of equations>
<equation list *3>

*1 : this section contains for each variable a line in the format :
<variable width> <variable name>

In the rest of the files, the variables are referenced by their position in the
list given in *1 (the first line gives variable #0, etc.)

*2 : these section contain one line :
<number of inputs/outputs> [<variable number> ...]

*3 : this section contains one line for each equation, in the following format :
<destination variable> <equation type> [<argument> ...]

Table of equation types :

Type#	Desc	Arguments
0		Arg		<arg>
1		Reg		<var#>
2		Not		<arg>
3		Binop	<op# *4> <arg> <arg>
4		Mux		<arg> <arg:when false> <arg:when true>
5		Rom		<addr_size> <word_size> <arg:read_addr>
6		Ram		<addr_size> <word_size> <arg:read_addr> <arg:write_enable>
					<arg:write_addr> <arg:data>
7		Concat	<arg> <arg>
8		Slice	<begin> <end> <arg>
9		Select	<id> <arg>

An argument (<arg> or <arg:*>) can be either a binary value, represented by the
bits in standard order defined above, either a reference to another variable,
with the syntax $<var_id>

*4 : The operators are :
0	OR
1	XOR
2	AND
3	NAND
	
This syntax is formalized by the reference implementation provided in the source
file csim/load.h.

To simplify the internals, we have created an intermediate format where <arg>
can be nothing but a reference to another variable. All constants will then have
to be assigned to a constant-type variable that can be counted among the inputs
(or whatever - anyway, no need to recalculate them at every iteration). This new
file format is the one described in the next section.


The dumbed-down netlist format (NEW FORMAT)
-------------------------------------------

This is the description of the format currently used by the C simulator.

<var count>
[for each variable]
	<var size> <var name>
<input list size> [for each input <input var id>]
<out list size> [for each input <output var id>]
<register list size>
[for each register]
	<register destination variable> <register source variable>
<ram list size>
[for each ram]
	<destination variable> <addr size> <word size> <read addr var>
		<write enable var> <write addr var> <data var>
<equation list size>
[for each equation]
	<destination variable> <equation type> <args...>

Equation types :
ID	DESCR		ARGS
--	-----		----
0	Copy		var_id
1	Not			var_id
2	Binop		op_id var_a_id var_b_id
3	Mux			var_a_id var_b_id
4	ROM			addr_size word_size write_addr_var_id
5	Concat		var_a var_b
6	Slice		begin end var_id
7	Select		number var_id

Operators :
0	OR
1	XOR
2	AND
3	NAND

Constant variables are standardized so that their name (eg. $00101)
gives the value of the constant, therefore there is no need to write a constant
list in the program file.


The Input/Output format
-----------------------

The C simulator is capable of reading inputs for each step from a file (by
default stdin, but can be redirected from a file using the -in option) and of
writing the outputs at each step to a file (by default stdout, but can be
redirected with -out).

The input format is extremely straightforward : at each step, read a line
containing for each input variable, *in the order given in the program file*,
either a binary value (represented by a series of bits given in the standard
order), either a decimal value prefixed with a slash (/), which is then
converted to binary according to the standard representation defined above.

The output format contains at each step one line for each output variable, in
the format :
<variable name>\t<binary representation>\t<decimal representation>
The output contains an empty line at the end of every cycle.


How ROMs are handled
--------------------

The MiniJazz language has many flaws, and one is the impossibility to specify
what is to be loaded in a ROM (it can be specified in the .mj file, but it won't
appear in the generated .net file).

To know what file to load in a ROM chip, we recognize a certain prefix in the
name of the variable holding the ROM. For example, if we have in a netlist the
following line :

	decode7_128 = ROM 4 7 _l_42_122

which is a possible output for the MiniJazz compiler, and if the simulator is
provided with the command-line argument :

	-rom decode7 path/to/decode7.rom
	
then the simulator will detect the prefix `decode7` in the variable name
decode7_128, and use the ROM data from the file specified on the command line.

The format of a ROM file is as follows :

<address width> <word size>
<data>

The data is composed of 2^(address width) values separated by spaces. If they
have no prefix, they are read as binary. If they have a / prefix, they are read
as decimal (this enables the use of whichever representation is preferred).



TODO
----

- More advanced commands for the simulator (cf Jonathan's simulator)
- RAM reads give result immediately

NEXT STEPS
----------

- Define convention for processor input/output
- Code UI
- How can we generate netlists with ocaml code ?
- Code processor