Information Technology Reference
In-Depth Information
Latch
Processor
address
bus
System
address
bus
Address
control signal
Cache controller
DRAM
memory
Processor
Data
control
signal
SRAM
control signals
SRAM
cache memory
System
data
bus
Processor
data
bus
Data
transceiver
Figure A.9
Look-through cache
Second-level caches
A L1-cache (first-level cache) provides a relatively small on-chip cache, where a L2-cache
(second-level cache) provides an external, on-board, cache which provides a cache memory
of between 128 and 512 KB. The processor looks in its own L1-cache for a cache hit, if none
is found then it searches in the on-board L2-cache. A cache hit in the L1-cache will obvi-
ously be faster than the off-chip cache.
An L2-cache for the 486 has a maximum 512 KB memory size and is typically available
as 128 KB, 256 KB or 512 KB.
A.4 Pentium/Pentium Pro
Intel has gradually developed their range of processors from the original 16-bit 8086 proces-
sor to the 64-bit Pentium III processor. The original 8086 had just 29 000 transistors and op-
erated at a clock speed of 8 MHz. It had an external 20-bit bus and could thus only access up
to 1 MB of memory. Compare this with the Pentium III which can operate at 500 MHz, con-
tains over 8 ,000 ,000 transistors and can access up to 64 GB of physical memory. Table A.3
contrasts the Intel processor range. It can also be seen from the table that the Pentium II
processor is nearly a thousand times more powerful than an 8086 processor.
