Modes Of Operation - 3Com EtherLink/MC Technical Reference Manual

Et h erLi n k/ M C:
I n t r oduct i o n
1-2
ARBITER
NETWORK
COPROCESSOR
!
XCVR
II
ADAPTER ADDRESS/DATA BUS >
REGISTERS
RAM & ROM !4
<
Figure 1-1. EtherLink/MC Adapter Block Diagram
MICRO CHANNEL
INTERFACE
MICROCHANNEL
>
Modes of Operation
EtherLink/MC adapter has only one significant mode of operation: shared memory. Shared memory
allows the adapter to map a 16K window of its memory into the address space of its host. The
adapter's memory may be regarded as a specialized extension of the host system's memory.
Random reads and writes directed at the shared memory space may take place at any time. Be
aware, however, that the 82586 coprocessor also uses this memory and, although transparent to the
user, host memory cycles that address the adapter's memory may run slower than system memory
cycles because of the need to arbitrate between the host and the 82586.
The only communication path between the host system and the 82586 is through the shared memory.
Commands are set up in the shared memory by the host and responses from the 82586 are left in the
shared memory for the host to interpret. The host informs the 82586 of pending commands by
strobing Channel Attention (described in detail later). Similarly, the 82586 informs the host that it
has either completed a command or received a packet by generating an interrupt for the host to
respond to.
The EtherLink/MC adapter supports loopback within the 82586 as well as using the loopback
capabilities of the Manchester Encoder/Decoder. External loopbacks (functionally equivalent to
Encoder/Decoder loopbacks) may also be performed on short packets (18 bytes).