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8. Combinational Circuit Simplifier: ONSET

ONSET (Optimizer for logic Networks Supported by Extension of Transduction method) is a powerful program for simplifying combinational logic circuits. It has the following functions.

• Synthesis of a multi-level logic circuit from the logic function given in the sum-of-product format

• Simplification of a multi-level logic circuit

8.1 Starting ONSET

onset [-f file_name] [-t value] [argument ...]

file_name : The name of a file containing an ONSET command script

value : Execution time limit to be set (unit: second)

argument : Any character string (word or "statement including spaces")

• Omission of '-f' option specifies "interactive mode" and receives commands entered from 'stdin.'

• Specification of '-f' option executes the content of the script file as an ONSET command string.

• Omission of '-t' option forces termination of ONSET execution at the default time limit (displayed at start). (Forced termination does not occur in the workstation version in which a default time limit is not set.)

• Selection of '-t' option forces termination of ONSET execution at the specified value or the default time limit whichever comes sooner (displayed at start):

The execution time limit here denotes the used CPU time in the workstation version while it denotes the actual elapsed time (including waiting time for command entry) in the MS-DOS version.

Character strings specified in argument are used in the order of $1, $2, $3, ... by the ONSET command script.

8.2 Examples of Using ONSET

ONSET operation depends on how the commands listed in Section 8.4 are provided. In PARTHENON, ONSET is usually called from OPT_MAP. In such a case, ONSET proceeds in the following order:

• Input of multi-level logic circuits in the BLIF format
• Application of various types of simplification
• Output of a netlist in the NLD format

If the script file of a:\par\com\onset.dir\b2n_area.scr contains the following:

	verbose $2
	in_blif $1.bli
	area.scr
	out_nld $1.nld

A>onset -f a:\par\com\onset.dir\b2n_area.scr ALU4 on

	verbose on  ........ Specifies information display mode
	in_blif ALU4.bli ... Inputs multi-level logic circuit ALU4.bli in the BLI format
	area.scr ........... Calls the script file for circuit simplification
	out_nld ALU4.nld ... Outputs netlist ALU4.nld in the NLD format

The script file name specified in the '-f' option must be specified by the correct full path name, such that it can be referenced from the current directory. On the other hand, if a script file is called in the interactive mode or from another script, the normal path name (as mentioned above) and a path name relative to %PARTHENON%\com\onset.dir are both searched for the file to be executed (with higher priority given to the former). If area.scr does not exist in the current directory in the above example, a:\par\com\onset.dir\area.scr is executed.

Practical example of using ONSET in the interactive mode

An example of executing commands in the interactive mode is given below. This example assumes that file ALU4.bli in the BLIF format has already been created in the current directory by the blif command of OPT_MAP.

A>onset
ONSET> in_blif ALU4.bli .... (1) Inputs multi-level logic circuit ALU4.bli in the BLIF format
ONSET> print_cost .......... (2) Displays various circuit cost information details
ONSET> prefact ............. (3) Performs factorization and local simplification
ONSET> sgs ................. (4) Performs simplification by replacing gates
ONSET> condis .............. (5) Performs simplification by adding or deleting connections 
ONSET> sgs ................. (6) Performs simplification by replacing gates
ONSET> gm .................. (7) Performs simplification by merging gates
ONSET> condisa ............. (8) Performs simplification by adding or deleting connections
ONSET> cpb 4 ............... (9) Re-synthesizes critical paths
ONSET> map ................ (10) Maps to a circuit with composite gates
ONSET> sgsm ............... (11) Performs simplification of the circuit with composite gates
ONSET> constant ........... (12) Treats constant output terminals
ONSET> print_cost ......... (13) Displays various circuit cost information details
ONSET> out_nld  ALU4.nld .. (14) Outputs netlist ALU4.nld in the NLD format
ONSET> !dir ............... (15) Executes DOS command dir
ONSET> bye ................ (16) Terminates ONSET
A>

The above yields a netlist in the NLD format of a multistage logic circuit after a series of simplification steps.

Figure 8.1 illustrates the various commands provided by ONSET and how they fit into the various steps of ONSET processing. The above example follows the flow in bold lines in the figure. However, the order of execution of various commands for simplification in items (3) to (8) and the number of repetitions can be arbitrary. A key point in simplification is the selection of an appropriate order for the commands and an appropriate number of repetitions according to the characteristics of the given logic circuit.

<Fig. 8.1> ONSET process flow

(Notes)

PLA : Sum-of-product logic expression format of PLA minimizer ESPRESSO

BLIF : Multi-level logic circuit expression format of UC Berkeley

NLIF : An extension of the netlist format of logic synthesis program SYLON of University of Illinois at Urbana-Champaign

8.3 Additional Information on Using ONSET

Details of logic circuit simplification are not provided here. For details, refer to documents on the transduction method.

Considerable time may be required in executing logic circuit simplification commands, logic function and netlist input commands, even for a relatively small circuit, depending on the nature of the given logic circuit.

The script examples 'area.scr' and 'delay.scr' in %PARTHENON%\com\onset.dir are expected to perform simplification with priority on area and delay time respectively, based on experience in designing numerous circuits. Changing the order or the number of repetitions of simplification commands in these scripts may yield better results depending on the given circuit.

If you want to execute various simplification commands after the execution of the 'map' command, you should first execute the 'unmap' command to convert the circuit back into one consisting of simple gates only. Repeating this may further simplify some circuits.

The 'resynthesis' command abandons all the previous circuit structure and re-synthesizes a circuit anew from the logic functions to be satisfied by the output terminals. This may greatly simplify some circuits. However, considerable execution time may be required depending on the characteristics of the circuit (logic functions).

Correct operation of ONSET cannot be assured if looped combinational logical circuits are included.

8.4 List of ONSET Commands

Table 8.1 provides a list of ONSET commands. The details on each command can be displayed with the on-line help function, which can be initiated by the 'help' command (MS-DOS's more.com is called from ONSET).

<Table 8.1> List of ONSET Commands

     ' $ ' and ' $0 ' are replaced with set module names.

     ' $1 ',' $2 ',........ are replaced with respective parameters specified at start. 

     '\' is used as an escape character.