write_blif - determinize, encode and write an hnode to a blif file

Encodes, determinizes, and writes all tables in the current hnode to a BLIF file. With the '-l' or '-c' options, or when 'write_blif' is used with no options, only the current hnode is written out as a blif file. The '-R' option writes the entire hierarchy rooted at the current hnode to the blif file, assuming all tables are deterministic and all variables are boolean.

Encoding the Multi-Valued Variables:

First, all multi-valued variables in the hnode are binary encoded. Each multi-valued variable requires 'm' binary variables in its encoding, where m = log2(n). Here 'n' is the number of values in the domain of the multi-valued variable. If variable X has n binary variables in its encoding, these are called X0, X1, ... X. X0 is the least significant encoding variable. The value i of multi-valued variable X is encoded such that X0 + 2^1 * X1 + ... + 2^(n-1) * X = i. After encoding, each table in the hnode is written out to the blif file.

Determinizing Non-Deterministic Tables:

Non-deterministic tables are determinized by adding more binary variables to a particular variable's encoding. The variable that is chosen is the one that has the smallest number of binary variables in its encoding. Determinization is explained by means of an example:

Consider the BLIF-MV table, where each of a, b, and c can have 4 values. Each multi-valued variable has two binary variables in its encoding. These are called a0, a1, b0, b1, c0 and c1.

.model foo

.inputs a b

.outputs c

.mv a, b, c 4

.table a b ->c

2 1 (1,2)

(1,2) 1 3

.end

The first row of this table represents non-determinism, since for a given assignment of input values (i.e. a=2, b=1), the output can take on two values (1 or 2). To determinize this row, it is split into two rows, each with a unique singleton output value. A new binary variable is added to the encoding of a (or b, since in this case each of them has the same number of binary variables in its encoding). The new variable a2 is given separate values in each of the new rows. The resulting blif table looks like:

.model foo

.inputs a0 a1 a2 b0 b1

.outputs c0 c1

.names a0 a1 a2 b0 b1 ->c0 c1

0 1 0 1 0 1 0 (these two rows represent

0 1 1 1 0 0 1 row 1 of the MV table)

1 0 - 1 0 1 1 (these two rows represent

0 1 - 1 0 1 1 row 2 of the MV table)

.end

Note that this table is still not deterministic, since rows 1 and 4 have input minterm(s) in common, but the corresponding outputs are different. To resolve this, a new binary variable is added to the encoding for b (since b currently has the smallest encoding). For the conflicting rows, this variable b2 is assigned different values, and for all other rows, it is assigned a '-' value. After this change, rows 1 and 4 no longer have common input minterm(s). The intermediate table now looks like:

.model foo

.inputs a0 a1 a2 b0 b1 b2

.outputs c0 c1

.names a0 a1 a2 b0 b1 b2->c0 c1

0 1 0 1 0 1 1 0

0 1 1 1 0 - 0 1

1 0 - 1 0 - 1 1

0 1 - 1 0 0 1 1

.end

This process is continued until the table is determinized. In this example, the final blif file looks like:

.model foo

.inputs a0 a1 a2 a3 b0 b1 b2

.outputs c0 c1

.table a0 a1 a2 a3 b0 b1 b2 ->c0 c1

0 1 0 - 1 0 1 1 0

0 1 1 1 1 0 - 0 1

1 0 - - 1 0 - 1 1

0 1 - 0 1 0 0 1 1

.end

Blif allows only single output tables, so in reality the above table is split into two single output tables before being written out to the blif file. The new binary variables that are thus introduced are treated as primary inputs in the blif file.

We make no claim on the optimality of this process, just that it is simple. It may turn out that the number of new variables is much larger than the optimum one.

Interfacing Between Binary and Multi-Valued Variables:

In order for the SIS-optimized version of this file to be read back into VIS, we need to re-create the corresponding multi-valued variables. For this purpose, interface information (model name, input and output declarations for the hnode) is written out to a special encoding file (e.g. foo.enc). Additionally, binary->multi-valued encoding tables are written to the encoding file for each PO. In the above example, the binary->multi-valued table for variable c looks like

.table c0 c1 -> c

0 0 0

1 0 1

0 1 2

1 1 3

and this can be seen as a decoder of the binary variables. Similarly, multi-valued->binary encoding tables are written to the encoding file for each PI. The multi-valued->binary table for variable b in the above example is:

.table b -> b0 b1

0 0 0

1 1 0

2 0 1

3 1 1

and this can be seen as an encoder of the multi-valued variables. Similar tables are written to the encoding file for sub-circuit inputs and outputs. Likewise, such tables are written out for latch inputs and outputs. This is needed so that the original multi-valued latches can be reinstated while the blif file is being read back into VIS. These multi-valued latch declarations are written to the encoding file during the write_blif process.

The combination of this encoding file and the blif file results in a legal BLIF-MV hnode description.

If no file is specified, the blif file is written out to the standard output. If the encoding file is not specified, encoding information is written out to .enc.

Options:

The '-R' option writes the entire hierarchy rooted at the current hnode to the blif file, first checking that all tables are deterministic and all variables are boolean. Other options ('-c', '-l', or write_blif with no options) only write out the current hnode. Also, when the '-R' option is specified, no encoding file is written since none is needed.

Command options:

-c

Writes only combinational part to blif file.

-l

Writes out latches, making latch IOs primary outputs in the blif file.

-e <encoding_file_name>

If set, the program writes the encoding information into this file. The default is the model name for the current node extended by .enc.

-R

Recursively writes out the entire hierarchy rooted at the current hnode to the blif file. In this sub-hierarchy, all tables must be deterministic and all variables must be boolean. Either of the -c, -l or -e options cannot be used together with this option.

-h

Prints the command usage.

-v <verbosity_value>

Specifies verbosity level, an integer less than 3.

<file>

name of blif file to be written, default is standard output.

Note that if neither the -c or the -l options are chosen, then latches are written out, without making latch IOs primary outputs in the blif file. Currently the files written out using this option cannot be read back into VIS.