NOTE: This driver has not been ported from
4.4BSD yet.
The
en interface provides access to a 3 Mb/s Ethernet network. Due to limitations in the hardware, DMA transfers to and from the network must take place in the lower 64K bytes of the UNIBUS address space, and thus this must be among the first UNIBUS devices enabled after boot.
Each of the host's network addresses is specified at boot time with an
SIOCSIFADDR ioctl(2). The station address is discovered by probing the on-board Ethernet address register, and is used to verify the protocol addresses. No packets will be sent or accepted until a network address is supplied.
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 mask (this is actually the two's complement of the value).
4.
Use the value calculated in step 3 to delay before retransmitting the packet.
The interface handles both Internet and NS protocol families. It 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).