Ref: 19140001
Title: Memory Mapping and Etherlink II
Date: 10/2/89

Copyright 3Com Corporation, 1991.  All rights reserved.

Memory mapping is a way for the PC to access the memory on the adapter
board.  The Etherlink II adapter contains 8 KB of RAM, used to temporarily
hold packets received from the network and to temporarily hold packets
waiting to be transmitted to the network.  To transmit, for example, a
1000-byte packet, the PC will copy the 1000 bytes to the Etherlink II's RAM
and then tell the Etherlink II hardware to start transmitting the packet.
A packet cannot be transmitted directly from the 640 KB of memory needed for
DOS; it must first be copied to the adapter board.

The Etherlink II adapter offers three methods for copying data to and
from RAM:  programmed IO, DMA, and memory mapped IO.  Of these three methods,
memory mapped IO is the fastest and should be used whenever possible.

Memory mapped IO takes advantage of the IBM PC's 128 KB of address space
allocated for memory expansion cards.  This memory space is from C000 to
DFFFF.

Using memory mapped IO, you may select any of four locations (via jumper 1
on the Etherlink II board) to place the Etherlink II adapter's 8 KB of RAM.
This area, of course, cannot be used by any other card in the system.

    C8000h to C9FFFh  (jumper setting C8000)
    CC000h to CDFFFh  (jumper setting CC000)
    D8000h to D9FFFh  (jumper setting D8000)
    DC000h to DDFFFh  (jumper setting DC000)

A fifth setting for jumper 1 is "Disable," the factory default setting.
If "Disable" is selected, an alternate method (either programmed IO or DMA)
will be used to access the Etherlink II adapter's RAM.

If you use the EtherStart PROM, you need to select one of the address
ranges, but the PROM is then mapped into this location and precludes its
use for packet buffering.

If you use the ETH503.SYS driver, you can improve the performance of
the adapter by setting the Memory Base Address option without installing the
start PROM.

For technical information on Etherlink II, refer to the Etherlink II
Technical Reference in the Developer's Guide to Network Adapters (3C576),
or to the installation guide that comes with the card.

