Physical Design of FabScalar Generated Cores

EE6052 Class Project

Wei Zhang

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Outline FabScalar Toolset Synthesis Physical design using IC Compiler – Problems

and solutions Things to do next

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FabScalar Toolset FabGen: This tool generates synthesizable RTL

of a superscalar core based on user-specified parameters:* Frontend superscalar width* Backend superscalar width* Pipeline depths of canonical pipeline stages* Issue queue size.

FabMem: This tool estimates read/write delays, read/write energies, and area of user-specified multi-ported RAMs/CAMs. It can also generate layouts of desired RAMs/CAMs.

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Synthesis FebGen-generated RTL is synthesizable. Before starting synthesis

Comment-out the verification components to remove unsynthesizable verilog needed for simulation only: VPIs (the verilog/C++ co-simulation environment) and performance counters.

Memory handling Use FabMem to generate custom designs of these critical

memory structures, including full layouts, SPICE netlists, and LEFs.

During synthesis, remove the behavioral modules of the memory structures and constrain their input-port and output-port timing paths based on timing numbers from SPICE simulation.

For place-and-route, use the LEFs to represent the memory structures.

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Physical Design – Problems Encountered ICC reports errors when importing netlist generated from

synthesis. Error message examples from ICC

Error: Can not create instance master 'SRAM_4R8W' in FRAM view. (MWDC-001)

Error: Can not create instance master 'CAM_4R4W' in FRAM view. (MWDC-001)

Warning message examples from synthesis Warning: Unable to resolve reference 'SRAM_4R8W' in

'InstructionBuffer'. (LINK-5) Warning: Unable to resolve reference 'CAM_4R4W' in 'IssueQueue'.

(LINK-5) Cause of the errors

No corresponding memory cells in the Milkyway library. FabMem uses 45nm FreePDK library and our design uses the

90nm EDK library.

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Physical Design – Problem Solutions Solution – Adapt the memory blocks generated

from the FabMem tool to the 90nm technology. The FabMem tool could generate layouts of

memories, from which we can get the GDSII files. To adapt these layouts to the 90nm technology, we

need to modify the GDSII files to1. Double the size of each physical component in the

layout.2. Map each layer in the FreePDK library to these layers in

the 90nm technology library. Import these memory blocks to the Milkyway

Library using the Milkyway Environment tool and the new GDSII memory files.

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Physical Design – Library Preparation Before using IC Compiler, logical and physical libraries

must be created that accurately reflect the characteristics of the available technology and cells that will be used to fabricate the chip.

FabScalar generated cores use a lot of RAM/CAM blocks, most of which are multi-ported.

The standard cells are already provided in the 90nm physical library, but macros like memory blocks are not provided.

The memory blocks must be created in the physical library. This can be done in the Milkyway Environment.

Layout data of these memory blocks are provided by FabMem. (GDS and LEF)

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Physical Library Preparation – Memory

Generate layout of memory blocks (using

FabMem tool)

Extract GDSII files and double the size of memory blocks

Import GDSII files into Milkyway Environment and create CEL view

Create FRAM view from the CEL view and get physical library of

memory blocks

Create Milkyway library in ICC and

reference the physical library of memory

blocks

Layer mapping file

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Library Preparation Flow in the Milkyway Environment

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CEL View – SRAM_4R4W

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FRAM View – SRAM_4R4W

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Errors Before Adding Memory Physical Library

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Errors After Adding Memory Physical Library

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New Problems Still missing a lot of instances in the physical library

1ICache PreDecode_PISA_3 Decode_PISA_3 SRAM_4R8W SRAM_4R1W Select_3 ForwardCheck_7

The “SRAM_4R8W”and “SRAM_4R1W”could not be generated from FabMem (only able to generate memories with XRXW and 2XRXW ports)

Most of the other instances are very similar to memories, but they are not standard memory blocks and could not be generated from FabMem.

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Problems Remaining Still could not generate the remaining

instances. Could not move forward without these

instances in the physical library.