Dual-Mode Cache RAM for Compressed and Uncompressed Data
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Solution Overview
Problem
Existing cache compression techniques are either inflexible or wasteful, leading to inefficiencies in handling both compressed and uncompressed data, with separate caches required for uncompressible data causing latency, power consumption, and memory congestion.
Innovation Solution
A cache structure with a flexible RAM design that can dynamically switch between storing uncompressed and compressed cachelines, utilizing a second RAM to hold either tags or partial cachelines, allowing for efficient use of space and reducing latency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If data is cached in compressed form to reduce space usage, then the data RAM size is reduced saving silicon area, but separate caches are required for uncompressible data causing increased device complexity
Solution Approach 1:
The second RAM is designed to perform multiple functions: it can store tags for compressed cachelines or store remaining portions of uncompressed cachelines. This multi-functionality eliminates the need for separate caches while maintaining space efficiency for compressed data and full functionality for uncompressed data, thereby reducing device complexity without sacrificing area optimization
Solution Approach 2:
The cache structure dynamically adapts its mode of operation based on data compressibility. The control logic determines whether to store data in compressed or uncompressed form and configures the second RAM accordingly - either as tag storage or as additional data storage. This dynamic adaptability allows the same hardware structure to optimize for different data types without requiring separate fixed architectures
2Quantity of substance
If the second RAM is used to hold tags for multiple compressed cachelines, then space efficiency is improved, but access time increases due to additional retrieval steps
Solution Approach 1:
When data is cached in compressed form, the tag is stored in the second RAM along with compression metadata. Upon access, the tag is retrieved first to determine compressibility and locate the compressed data. This preliminary tagging action enables efficient subsequent retrieval by providing advance information about data location and format, reducing the overall access time penalty
Solution Approach 2:
The control logic acts as an intermediary that manages the trade-off between space efficiency and access time. It determines whether to retrieve data from the first RAM (uncompressed, faster) or second RAM (compressed, space-efficient) based on the access pattern and data characteristics, optimizing the balance between quantity of cached data and access speed
3Adaptability or versatility
If a fixed width data RAM is used to accommodate uncompressed cachelines, then all data can be cached without separate structures, but space is wasted when caching compressed data
Solution Approach 1:
The data storage functionality is segmented into two separate RAM structures: the first RAM for uncompressed cachelines and the second RAM for compressed cachelines or remaining data portions. This segmentation allows each RAM to be optimized for its specific purpose - the first RAM maintains full width for uncompressed data while the second RAM provides flexible storage for compressed data, eliminating space waste while maintaining adaptability
Solution Approach 2:
The cache structure changes its operational parameters based on data characteristics. When data is compressible, the system switches to compressed storage mode utilizing the second RAM for tag/remaining portion storage. When data is not compressible, it uses uncompressed storage mode in the first RAM. This parameter switching allows the system to adapt to different data types without permanently allocating excessive silicon area
Data Source
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AI summary
A device in which each field in a first RAM together with a respective field in a second RAM form a respective entry of a cache RAM. Caching circuitry is operable to select between applying a first mode and a second mode in at least one entry in the cache RAM. In the first mode, the respective field in the first RAM is used to hold a first portion of a single cacheline in a first format, and the respective field in the second RAM is used to hold the corresponding tag of the single cacheline and a remaining portion of the single cacheline. In the second mode, the first RAM is used to hold a plural cachelines in a second format shorter than the first format, and the corresponding entry in the second RAM is used to hold the corresponding tags of the plural cachelines.