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I/O INTERFACING
82586 and the Intel386 DX microprocessor.
In
general, higher performance interfaces
(requiring less servicing time from the Intel386 DX microprocessor) are more expensive.
Four types of interfaces are described
in
this section:
• Dedicated CPU
•
Decoupled dual-port memory
• Coupled dual-port memory
• Shared bus
8.6.6.1 DEDICATED CPU
Dedicating a CPU to control the
82586
results in a high-performance, high-cost inter-
face. The CPU, typically an
80186,
an
80188,
or a microcontroller, executes the data link
layer (a functional division) of software and sometimes the network, transport, and
ses-
sion layers
as
well. (For definitions of these layers, see the Local Area Networking (LAN)
Components
User's
Manual). The dedicated CPU relieves the Inte1386 DX microproces-
sor of these layers and provides a high-level, message-oriented interface that can be
treated
in
software
as
a standard I/O device.
In
hardware, the interface
is
mapped into a
dual-port memory.
8.6.6.2 DECOU PLED DUAL-PORT MEMORY
A decoupled dual-port memory interface, shown
in
Figure 8-15, contains
two
sections of
memory:
• Inte1386 DX microprocessor core memory-typically DRAM that provides executable
memory space for the operating system
• 82586 communication channel memory-typically dual-ported SRAM that contains
the commands and buffers of the
82586
82502
LAN
231732;8·15
Figure 8·15. Decoupled Dual·Port Memory Interface
8-24