Introduction; Introduction; Cache Memory; Cache Memory - Intel Embedded Intel486 Hardware Reference Manual

6.1

INTRODUCTION

Cache is an important means of improving overall system performance. The Intel486™ proces-
sors have an on-chip, unified code and data cache. The on-chip cache is used for both instruction
and data accesses and operates on physical addresses. The Intel486 processor and most variants
have an 8-Kbyte cache (the IntelDX4 processor has a 16-Kbyte cache) which is organized in a 4-
way set associative manner. To understand cache philosophy and the system advantages of a
cache, many issues must be considered.
This chapter discusses the following related cache issues:
•
Cache theory and the impact of cache on performance.
•
The relationship between cache size and hit rates when using a first-level cache.
•
Issues in mapping (or associativity) that arise when main memory is cached. Different
cache configurations including direct-mapped, set associative, and fully associative. They
are discussed along with the performance trade-offs inherent to each configuration.
•
The impact of cache line sizes and cache re-filling algorithms on performance.
•
Write-back and write-through methods for updating main memory. How each method
maintain cache consistency and the impact on external bus utilization.
•
Cache consistency issues that arise when a DMA occurs while the Intel486 processor's
cache is enabled. Methods that ensure cache and main memory consistency during cache
accesses.
•
Cache used in single versus multiple CPU systems.
6.2

CACHE MEMORY

Cache memory is high-speed memory that is placed between microprocessors and main memory.
Cache memory keeps copies of main memory that are currently in use to speed microprocessor
access to requested data and instructions. When properly implemented, cache access time can be
three to eight times faster than that of main memory, and thus can reduce the overall access time.
Cache also reduces the number of accesses to main memory DRAM, which is important to sys-
tems with multiple bus masters that all access that same memory. This section introduces the
cache concept and discusses memory performance benefits provided by a cache.
6.2.1

What is a Cache?

A cache memory is a smaller high-speed memory that fits between a CPU and slower main mem-
ory. Cache memory is important in increasing computer performance by reducing total memory
latency. A cache memory consists of a directory (or tag), and a data memory. Whenever the CPU
is required to read or write data, it first accesses the tag memory and determines if a cache hit has
CHAPTER 6
CACHE SUBSYSTEM
6-1