====================================
010: Converting Verilog to BLIF page
====================================

Installation
============

Yosys written in C++ (using features from C++11) and is tested on modern
Linux. It should compile fine on most UNIX systems with a C++11
compiler. The README file contains useful information on building Yosys
and its prerequisites.

Yosys is a large and feature-rich program with a couple of dependencies.
It is, however, possible to deactivate some of the dependencies in the
Makefile, resulting in features in Yosys becoming unavailable. When
problems with building Yosys are encountered, a user who is only
interested in the features of Yosys that are discussed in this
Application Note may deactivate TCL, Qt and MiniSAT support in the
Makefile and may opt against building yosys-abc.

This Application Note is based on `Yosys GIT`_ `Rev. e216e0e`_  from 2013-11-23.
The Verilog sources used for the examples are taken from `yosys-bigsim`_, a
collection of real-world designs used for regression testing Yosys.

.. _Yosys GIT: https://github.com/YosysHQ/yosys

.. _Rev. e216e0e: https://github.com/YosysHQ/yosys/tree/e216e0e

.. _yosys-bigsim: https://github.com/YosysHQ/yosys-bigsim

Getting started
===============

We start our tour with the Navré processor from yosys-bigsim. The `Navré
processor`_ is an Open Source AVR clone. It is a single module (softusb_navre)
in a single design file (softusb_navre.v). It also is using only features that
map nicely to the BLIF format, for example it only uses synchronous resets.

.. _Navré processor: http://opencores.org/projects/navre

Converting softusb_navre.v to softusb_navre.blif could not be easier:

.. code:: sh

   yosys -o softusb_navre.blif -S softusb_navre.v

Behind the scenes Yosys is controlled by synthesis scripts that execute
commands that operate on Yosys' internal state. For example, the -o
softusb_navre.blif option just adds the command write_blif
softusb_navre.blif to the end of the script. Likewise a file on the
command line – softusb_navre.v in this case – adds the command
read_verilog softusb_navre.v to the beginning of the synthesis script.
In both cases the file type is detected from the file extension.

Finally the option -S instantiates a built-in default synthesis script.
Instead of using -S one could also specify the synthesis commands for
the script on the command line using the -p option, either using
individual options for each command or by passing one big command string
with a semicolon-separated list of commands. But in most cases it is
more convenient to use an actual script file.

Using a synthesis script
========================

With a script file we have better control over Yosys. The following
script file replicates what the command from the last section did:

.. code:: yoscrypt

   read_verilog softusb_navre.v
   hierarchy
   proc; opt; memory; opt; techmap; opt
   write_blif softusb_navre.blif

The first and last line obviously read the Verilog file and write the
BLIF file.

The 2nd line checks the design hierarchy and instantiates parametrized
versions of the modules in the design, if necessary. In the case of this
simple design this is a no-op. However, as a general rule a synthesis
script should always contain this command as first command after reading
the input files.

The 3rd line does most of the actual work:

-  The command opt is the Yosys' built-in optimizer. It can perform some
   simple optimizations such as const-folding and removing unconnected
   parts of the design. It is common practice to call opt after each
   major step in the synthesis procedure. In cases where too much
   optimization is not appreciated (for example when analyzing a
   design), it is recommended to call clean instead of opt.

-  The command proc converts processes (Yosys' internal representation
   of Verilog always- and initial-blocks) to circuits of multiplexers
   and storage elements (various types of flip-flops).

-  The command memory converts Yosys' internal representations of arrays
   and array accesses to multi-port block memories, and then maps this
   block memories to address decoders and flip-flops, unless the option
   -nomap is used, in which case the multi-port block memories stay in
   the design and can then be mapped to architecture-specific memory
   primitives using other commands.

-  The command techmap turns a high-level circuit with coarse grain
   cells such as wide adders and multipliers to a fine-grain circuit of
   simple logic primitives and single-bit storage elements. The command
   does that by substituting the complex cells by circuits of simpler
   cells. It is possible to provide a custom set of rules for this
   process in the form of a Verilog source file, as we will see in the
   next section.

Now Yosys can be run with the filename of the synthesis script as
argument:

.. code:: sh

   yosys softusb_navre.ys

Now that we are using a synthesis script we can easily modify how Yosys
synthesizes the design. The first thing we should customize is the call
to the hierarchy command:

Whenever it is known that there are no implicit blackboxes in the
design, i.e. modules that are referenced but are not defined, the
hierarchy command should be called with the -check option. This will
then cause synthesis to fail when implicit blackboxes are found in the
design.

The 2nd thing we can improve regarding the hierarchy command is that we
can tell it the name of the top level module of the design hierarchy. It
will then automatically remove all modules that are not referenced from
this top level module.

For many designs it is also desired to optimize the encodings for the
finite state machines (FSMs) in the design. The fsm command finds FSMs,
extracts them, performs some basic optimizations and then generate a
circuit from the extracted and optimized description. It would also be
possible to tell the fsm command to leave the FSMs in their extracted
form, so they can be further processed using custom commands. But in
this case we don't want that.

So now we have the final synthesis script for generating a BLIF file for
the Navré CPU:

.. code:: yoscrypt

   read_verilog softusb_navre.v
   hierarchy -check -top softusb_navre
   proc; opt; memory; opt; fsm; opt; techmap; opt
   write_blif softusb_navre.blif

Advanced example: The Amber23 ARMv2a CPU
========================================

Our 2nd example is the `Amber23 ARMv2a CPU`_. Once again we base our example on
the Verilog code that is included in `yosys-bigsim`_.

.. _Amber23 ARMv2a CPU: http://opencores.org/projects/amber

.. code-block:: yoscrypt
   :caption: `amber23.ys`
   :name: amber23.ys

   read_verilog a23_alu.v
   read_verilog a23_barrel_shift_fpga.v
   read_verilog a23_barrel_shift.v
   read_verilog a23_cache.v
   read_verilog a23_coprocessor.v
   read_verilog a23_core.v
   read_verilog a23_decode.v
   read_verilog a23_execute.v
   read_verilog a23_fetch.v
   read_verilog a23_multiply.v
   read_verilog a23_ram_register_bank.v
   read_verilog a23_register_bank.v
   read_verilog a23_wishbone.v
   read_verilog generic_sram_byte_en.v
   read_verilog generic_sram_line_en.v
   hierarchy -check -top a23_core
   add -global_input globrst 1
   proc -global_arst globrst
   techmap -map adff2dff.v
   opt; memory; opt; fsm; opt; techmap
   write_blif amber23.blif

The problem with this core is that it contains no dedicated reset logic. Instead
the coding techniques shown in :numref:`glob_arst` are used to define reset
values for the global asynchronous reset in an FPGA implementation. This design
can not be expressed in BLIF as it is. Instead we need to use a synthesis script
that transforms this form to synchronous resets that can be expressed in BLIF.

(Note that there is no problem if this coding techniques are used to model ROM,
where the register is initialized using this syntax but is never updated
otherwise.)

:numref:`amber23.ys` shows the synthesis script for the Amber23 core. In line 17
the add command is used to add a 1-bit wide global input signal with the name
``globrst``. That means that an input with that name is added to each module in the
design hierarchy and then all module instantiations are altered so that this new
signal is connected throughout the whole design hierarchy.

.. code-block:: verilog
   :caption: Implicit coding of global asynchronous resets
   :name: glob_arst

   reg [7:0] a = 13, b;
   initial b = 37;

.. code-block:: verilog
   :caption: `adff2dff.v`
   :name: adff2dff.v

   (* techmap_celltype = "$adff" *)
   module adff2dff (CLK, ARST, D, Q);

   parameter WIDTH = 1;
   parameter CLK_POLARITY = 1;
   parameter ARST_POLARITY = 1;
   parameter ARST_VALUE = 0;

   input CLK, ARST;
   input [WIDTH-1:0] D;
   output reg [WIDTH-1:0] Q;

   wire [1023:0] _TECHMAP_DO_ = "proc";

   wire _TECHMAP_FAIL_ =
       !CLK_POLARITY || !ARST_POLARITY;

   always @(posedge CLK)
           if (ARST)
                   Q <= ARST_VALUE;
           else
                   Q <= D;

   endmodule

In line 18 the :cmd:ref:`proc` command is called. But in this script the signal
name globrst is passed to the command as a global reset signal for resetting the
registers to their assigned initial values.

Finally in line 19 the techmap command is used to replace all instances of
flip-flops with asynchronous resets with flip-flops with synchronous resets. The
map file used for this is shown in :numref:`adff2dff.v`. Note how the
``techmap_celltype`` attribute is used in line 1 to tell the techmap command
which cells to replace in the design, how the ``_TECHMAP_FAIL_`` wire in lines
15 and 16 (which evaluates to a constant value) determines if the parameter set
is compatible with this replacement circuit, and how the ``_TECHMAP_DO_`` wire
in line 13 provides a mini synthesis-script to be used to process this cell.

.. code-block:: c
   :caption: Test program for the Amber23 CPU (Sieve of Eratosthenes). Compiled 
             using GCC 4.6.3 for ARM with ``-Os -marm -march=armv2a 
	     -mno-thumb-interwork -ffreestanding``, linked with ``--fix-v4bx`` 
	     set and booted with a custom setup routine written in ARM assembler.
   :name: sieve

   #include <stdint.h>
   #include <stdbool.h>

   #define BITMAP_SIZE 64
   #define OUTPORT 0x10000000

   static uint32_t bitmap[BITMAP_SIZE/32];

   static void bitmap_set(uint32_t idx) { bitmap[idx/32] |= 1 << (idx % 32); }
   static bool bitmap_get(uint32_t idx) { return (bitmap[idx/32] & (1 << (idx % 32))) != 0; }
   static void output(uint32_t val) { *((volatile uint32_t*)OUTPORT) = val; }

   int main() {
       uint32_t i, j, k;
       output(2);
       for (i = 0; i < BITMAP_SIZE; i++) {
           if (bitmap_get(i)) continue;
           output(3+2*i);
           for (j = 2*(3+2*i);; j += 3+2*i) {
               if (j%2 == 0) continue;
               k = (j-3)/2;
               if (k >= BITMAP_SIZE) break;
               bitmap_set(k);
           }
       }
       output(0);
       return 0;
   }

Verification of the Amber23 CPU
===============================

The BLIF file for the Amber23 core, generated using :numref:`amber23.ys` and
:numref:`adff2dff.v` and the version of the Amber23 RTL source that is bundled
with yosys-bigsim, was verified using the test-bench from yosys-bigsim. It
successfully executed the program shown in :numref:`sieve` in the test-bench.

For simulation the BLIF file was converted back to Verilog using `ABC`_. So this
test includes the successful transformation of the BLIF file into ABC's internal
format as well.

.. _ABC: https://github.com/berkeley-abc/abc

The only thing left to write about the simulation itself is that it probably was
one of the most energy inefficient and time consuming ways of successfully
calculating the first 31 primes the author has ever conducted.

Limitations
===========

At the time of this writing Yosys does not support multi-dimensional memories,
does not support writing to individual bits of array elements, does not support
initialization of arrays with ``$readmemb`` and ``$readmemh``, and has only
limited support for tristate logic, to name just a few limitations.

That being said, Yosys can synthesize an overwhelming majority of real-world
Verilog RTL code. The remaining cases can usually be modified to be compatible
with Yosys quite easily.

The various designs in yosys-bigsim are a good place to look for examples of
what is within the capabilities of Yosys.

Conclusion
==========

Yosys is a feature-rich Verilog-2005 synthesis tool. It has many uses, but one
is to provide an easy gateway from high-level Verilog code to low-level logic
circuits.

The command line option ``-S`` can be used to quickly synthesize Verilog code to
BLIF files without a hassle.

With custom synthesis scripts it becomes possible to easily perform high-level
optimizations, such as re-encoding FSMs. In some extreme cases, such as the
Amber23 ARMv2 CPU, the more advanced Yosys features can be used to change a
design to fit a certain need without actually touching the RTL code.
