AM79C960KCW datasheet(31/127 Pages) AMD

31 / 127 page

P R E L I M I N A R Y

AMD

1-373

Am79C960

Shared Memory Mode

System Interface

The Shared Memory mode is the other fundamental op-

erating mode available on the PCnet-ISA controller. The

PCnet-ISA controller uses the same descriptor and

buffer architecture as the LANCE, but these data struc-

tures are stored in static RAM controlled by the

PCnet-ISA controller. The static RAM is visible as a

memory resource to the PC. The other resources look

the same as in the Bus Master mode.

The Boot PROM is selected by an external device which

drives the Boot PROM Address Match (

BPAM) input to

the PCnet-ISA controller. The PCnet-ISA controller can

perform two 8-bit accesses from the 8-bit Boot PROM

and present16-bits of data. The shared memory works

the same way, with an external device generating

Shared Memory Address Match and the PCnet-ISA

controller performing the read or write and the 8 to 16-bit

data conversion.

Converting shared memory accesses from 8-bit cycles

to 16-bit cycles allows use of the much faster 16-bit cy-

cle timing while cutting the number of bus cycles in half.

This raises performance to more than 400% of what

could be achieved with 8-bit cycles. Converting boot

PROM accesses to 16-bit cycles allows the two memory

resources to be in the same 128 Kbyte block of memory

without a clash between two devices with different data

widths.

Note that the external address buffer must drive all the

bits of PRAB10-15 even if the static RAM is less than

64 Kbytes. The PCnet-ISA controller uses an internal

address comparator to perform SRAM prefetches on

the Private Data Bus; the PRAB0-15 signals are used in-

ternally to determine whether a SRAM read cycle

prefetch is a match or a miss.

Access to the Ethernet controller registers, board con-

figuration registers, and Address PROM is done with

on-chip address comparators.

Network Interface

The PCnet-ISA controller can be connected to an IEEE

802.3 network via one of two network interface ports.

The Attachment Unit Interface (AUI) provides an

IEEE 802.3 compliant differential interface to a remote

MAU or a transceiver on the system board. The

10BASE-T interface provides a twisted-pair Ethernet

port. The PCnet-ISA controller provides three modes of

network interface selection: automatic selection, soft-

ware selection, and jumper selection of AUI or

10BASE-T interface.

In the automatic selection mode, the PCnet-ISA control-

ler will select the interface that is connected to the

network by checking the Link Status state machine. If

both AUI and 10BASE-T interfaces are connected, the

10BASE-T interface is selected over AUI. If the

PCnet-ISA controller is initialized for software selection

of network interface, it will read the PORTSEL [1:0] bits

in the Mode register (CSR15.8 and CSR15.7) to deter-

mine which interface needs to be activated. For jumper

selection of the network interface, the MAUSEL pin is

used. When the XMAUSEL bit in ISACSR2 is set, a

HIGH on the pin will select the 10BASE-T interface, and

a LOW on the pin will select the AUI interface.

ISA

Bus

16-Bit System Data

System

Address

PCnet-ISA

Controller

16-Bit Private Address Bus

Ethernet

Address

PROM

8-Bit

SRAM

16907B-6

Address

Buffer

Address

Compare

Boot

PROM

8-Bit Private Data

ABOE BPAM SMAM

A0–X

D0–7

A0–X

D0–7

SRWE

A0–X

D0–7

PRAB0–15

PRDB0–7

BPCS

SROE

APCS

Shared Memory Block Diagram

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