NOTE: This driver has not been ported from
4.4BSD yet.
The
ec interface provides access to a 10 Mb/s Ethernet network through a 3Com controller.
The hardware has 32 kilobytes of dual-ported memory on the UNIBUS. This memory is used for internal buffering by the board, and the interface code reads the buffer contents directly through the UNIBUS. The address of this memory is given in the
flags field in the configuration file. The first interface normally has its memory at UNIBUS address 0.
Each of the host's network addresses is specified at boot time with an
SIOCSIFADDR ioctl(2). The
ec interface employs the address resolution protocol described in
arp(4) to dynamically map between Internet and Ethernet addresses on the local network.
The interface normally tries to use a “trailer” encapsulation to minimize copying data on input and output. The use of trailers is negotiated with ARP. This negotiation may be disabled, on a per-interface basis, by setting the
IFF_NOTRAILERS flag with an
SIOCSIFFLAGS ioctl(2).
The interface software implements an exponential backoff algorithm when notified of a collision on the cable. This algorithm uses a 16-bit mask and the VAX-11's interval timer in calculating a series of random backoff values. The algorithm is as follows:
1.
Initialize the mask to be all 1's.
2.
If the mask is zero, 16 retries have been made and we give up.
3.
Shift the mask left one bit and formulate a backoff by masking the interval timer with the smaller of the complement of this mask and a 5-bit mask, resulting in a pseudo-random number between 0 and 31. This produces the number of slot times to delay, where a slot is 51 microseconds.
4.
Use the value calculated in step 3 to delay before retransmitting the packet. The delay is done in a software busy loop.