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
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]
<addr size> <word size>
<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
8 RAM Read ram_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)
NEXT STEPS
----------
- Define convention for processor input/output
- Code UI
- How can we generate netlists with ocaml code ?
- Code processor
