Parallel random access machines (PRAM) are a model of computation used in theoretical computer science. They consist of several independent processors (random access machines) communicating with each other by sharing the same memory.
PRAMs offer a simple model to teach and understand parallel algorithms but they are impractical to implement in hardware. Aside from the assumption of unbounded resources they obviously require a memory block with a large number of IO buses. Since memory blocks typically have a single input and/or output bus, applications where parallel access to the same memory block is required are forced to use workarounds. These include running memory logic at a higher clock frequency or adding several copies of the memory block that are kept in sync, which correspond to a lower general speed and higher die space respectively.
The model we implement in this project offers a compromise between the simplicity of PRAMs and the practical usability of general-purpose processors. We add one memory block per core and allow them to be either kept in sync for simple communication or used separately to maximize data storage per unit of die area. To achieve this we extend memory write operations to include a spread parameter besides the usual address and data. Processor cores are arranged in a binary tree and spread controls how far the memory write operation spreads in this tree. For instance, if core 0 writes 17 to memory address 8 with a spread of 2 then 17 is written to address 8 in the memory blocks of cores 0, 1, 2 and 3. This logic can be turned into a combinatorial circuit between processor cores and memory blocks that we call the memory mesh.
This repository contains a Verilog implementation of the TreePRAM model. It targets the OpenLane flow for the Skywater 130nm process and is an entry for the Google/Efabless MPW-3 shuttle. Source files live in verilog/rtl while the user area of the hardened Caravel chip is available under gds.