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**82495DX/490DX DX CPU-Cache Chip Set <Sep91
***Notes:...
***Info:
The 50 MHz Intel486 DX CPU-Cache Chip Set provides a high performance
solution for servers and high-end desktop systems. This binary
compatible solution has been optimized to provide 50 MHz, zero wait
state performance. The CPU-Cache chip set combines the 50 MHz Intel486
Microprocessor with the 82495DX/82490DX cache subsystem. It delivers
integer performance of 41 V1.1 Dhrystone MlPs and a SPEC integer
rating of 27.9. The cache subsystem features the 82495DX Cache
Controller and the 82490DX Dual Ported Data RAM. Dual ported buffers
and registers of the 82490DX allow the 82495DX Cache Controller to
concurrently handle CPU bus, memory bus, and internal cache operations
for maximum performance.

The CPU-Cache Chip Set offers many features that are ideal for multi-
processor based systems. The Write-Back feature provides efficient
memory bus utilization by reducing bus traffic through eliminating
unnecessary writes to main memory. The CPU-Cache chip set also
supports MESI protocol and monitors the memory bus to guarantee cache
coherency.

The 50 MHz Intel486 DX CPU and 82495DX/82490DX Cache subsystem are
produced on Intel's latest CHMOS V process which features submicron
technology and triple layer metal.

3.0 ARCHITECTURAL OVERVIEW
3.1 Introduction
The Intel486 CPU-cache chip set provides a tightly coupled processing
engine based on the Intel486 microprocessor and a cache subsystem
comprised of the 82495DX cache controller and multiple 82490DX cache
components. Figure 3.1 [see datasheet] diagrams the basic config-
uration.

The cache subsystem provides a gateway between the CPU and the memory
bus. All CPU accesses that can be serviced locally are transparent to
the memory bus and serve to avoid bus traffic. As a result, the cache
chip set reduces memory bus bandwidth to both increase Intel486
processor performance and support efficient multiprocessor systems.
The cache subsystem also decouples the CPU from the memory bus to
provide zero-wait-state operation at high clock frequencies while
allowing relatively slow and inexpensive memories.

The CPU-cache chip set prevents latency and bandwidth bottlenecks
across a variety of uniprocessor and multiprocessor designs. The
processor’s on-chip cache supports a very wide CPU data bus and
high-speed data movement. The second-level cache greatly extends the
capabilities of the on-chip cache resources, enabling a larger portion
of memory cycles to be satisfied independently of the memory bus.

3.2 CPU-Cache Chip Set Description
The chip set is comprised of three functional blocks:

3.2.1 CPU
The chip set includes a special version of the Intel486DX micropro-
cessor at 50 MHz. The Intel486DX Microprocessor Data Sheet provides
complete component specifications.

3.2.2 CACHE CONTROLLER
The 82495DX cache controller is the main control element for the chip
set. providing tags and line states. and determining cache hits and
misses. The 82495DX executes all CPU bus requests and coordinates all
main memory accesses with the memory bus controller (MBC).

The 82495DX controls the data paths of the 82490DX cache components
for cache hits and misses and furnishes the CPU with needed data. The
controller dynamically adds wait states as needed using the most
recently used (MRU) prediction algorithm.

The 82495DX also performs memory bus snoop operations in shared memory
systems and drives the cycle address and other attributes during
memory bus accesses. Figure 3.2 [see datasheet] diagrams the 82495DX.

3.2.3 CACHE SRAM

Multiple 82490DX cache components provide the cache SRAM and data
path. Each component includes the latches, muxes and logic needed to
work in lock step with the 82495DX to efficiently serve both hit and
miss accesses. The 82490DX components take full advantage of VLSI
silicon flexibility to exceed the capabilities of discrete
implementations. The 82490DX components support zero-wait-state hit
accesses and concurrent CPU and memory bus accesses, and they
replicate MRU bits for autonomous way prediction. During memory bus
cycles. the 82490DX components act as a gateway between CPU and memory
buses. Figure 3.3 [see datasheet] diagrams an 82490DX cache component.

3.3 Secondary Cache Features

The 82495DX cache controller and 82490DX cache components provide a
unified, software transparent secondary data and instruction cache.
The cache enables a highspeed processor core that provides efficient
performance even when paired with a significantly slower memory bus.

The secondary cache interprets CPU bus cycles and can service most
memory read and write cycles without accessing main memory. I/O and
other special cycles are passed directly to the memory bus. The cache
has a dual-port structure that permits concurrent CPU and memory bus
operation.

The 82495DX cache controller contains the 8K tag entries and logic
needed to support a cache as large as 256K. Combinations of between 4
and 9 82490DX cache SRAMs are used to create caches ranging from 128K
to 256K, with or without data parity.

The MBC provides logic needed to interface the CPU, 82495DX and
82490DX to the memory bus. Because the MBC also affects system
performance. its design can be the basis of product differentiation.

***Configurations:...
***Features:...
**82495XP/490XP Cache Controller / Cache RAM (for i860) 06/05/91...
**82496/491 Cache Controller / Cache RAM (for P5 Pentium) 03/22/93...
**82497/492 Cache Controller / Cache RAM (for P54 Pentium) <Nov94...
**82498/493 Cache Controller / Cache RAM (for P54 Pentium) <Nov94...
**
**Later chipsets (basic spec):
**440 series:...
**450NX (?) 06/29/98:...
**????? (Profusion) c:99...
**800 series...
*Headland/G2...
**HTK340 "Shasta" 486 Chip Set c:Jun92
***Notes:...
***Info:

The HTK340 chip set is a two chip, high performance, cost-effective
solution for the 80486SX DX, and DX2 processing environments. In its
minimum configuration, this highly integrated chip set requires only
four external TTL devices to implement a fully compatible IBM PC/AT
system at speeds up to 33MHZ.

The HTK340 is based upon Headland's HTK320 Bus Architecture and
consists of the HT321-ISA Bus Controller and the HT342-Memory Control
Unit (MCU). Both chips are packaged in 184 pin plastic quad flat
packs.

The HTK340 is unique in that it provides performance approximating
that of large secondary cache systems, including the highest
performance write back cache architectures, without any external
cache. Secondary cache solutions should be considered in applications
that make use of multi-tasking and large model operating systems. The
Headland HT44 secondary cache was designed to meet the cost and
performance objectives for these applications. The key to this level
of performance is the 4-level deep write buffer, which includes byte
gathering for up to 32-bit DRAM writes.

Due to the effectiveness of the primary cache internal to the 80486
most of the bus activity in a PC/AT compatible environment consists of
writes. Indeed, this write activity consists almost exclusively of
writes of either bytes or Words (16 bit entities). In addition, much
of this write activity is into sequential memory locations. The byte
gathering feature of the buffer has the effect of reducing the number
of memory accesses required. Since the 80486 can always write into the
buffer with zero wait states (assuming the buffer is not full), and
the buffer can empty faster than it can be filled for most write
activity, the net effect is that the writes from the CPU never cause a
wait state.

The HTK340 can support Peripheral Devices such as VGA or SCSI on the
local processor bus, or any other devices that are designed to work
within the 80486 bus protocol and timing. By eliminating the ISA
backplane bottleneck, system designers can greatly improve the
performance of functions such as graphics generation and disk access.

The HTK340 supports up to 4 banks of DRAM, configurable as 1-4
banks. This flexible memory architecture allows for any memory type,
from 256K to 16M devices, in any bank. Maximum system performance is
achieved from the DRAM banks through various means, including
interleaving of memory banks and/or paging, and CAS before RAS
refresh. The memory can also be tuned to maximum potential through the
use of extensive DRAM timing control registers. These controls
include: precharge time, access time on reads, active time on writes,
as well as CAS and RAS delays. In addition, further system perfor-
mance is gained by separate timing parameters on the read and write
cycles which allow system designers to take maximum advantage of the
pipelined structure of the chip set.

The HTK340 also supports extensive mapping registers, which allow
system designers to take maximum advantage of system memory. The chip
set supports Shadow/Remap in 16K blocks between the 640K and 1M
boundaries, and eliminates the requirement for external decoding logic
by supporting 26 programmable non-cache regions. Devices which meet
HTK340 local bus requirements may be implemented without external
TTL. The mapping structure of the HTK340 provides for a single 8-bit
EPROM to be used for both the System and Video BIOS, further reducing
system chip count and cost.

***Configurations:...
***Features:...
**Support Chips:
**HT44 Secondary Cache c:Jun92...
**Other:...
*HMC (Hulon Microelectronics)...
*Logicstar...
*Motorola...
*OPTi...
*PC CHIPS/Amptron/Atrend/ECS/Elpina/etc...
**Later Chipsets:
Info sourced from the very useful plasma-online.de:
http://www.plasma-online.de/index.html?content=http%3A//www.plasma-online.de/english/identify/picture/pcchips.html

*SIS...
*Symphony...
*TI (Texas Instruments)...
*UMC...
*Unresearched:...
*VIA...
*VLSI...
*Western Digital...
*Winbond...
*ZyMOS...
*General Sources:...