7.0 Packet Reception
(Continued)
Buffer Ring Overflow
If the Buffer Ring has been filled and the DMA reaches the
Boundary Pointer Address, reception of the incoming pack-
et will be aborted by the ST-NIC. Thus, the packets previ-
ously received and still contained in the Ring will not be
destroyed.
In heavily loaded network which cause overflows of the Re-
ceive Buffer Ring, the ST-NIC may disable the local DMA
and suspend further receptions even if the Boundary regis-
ter is advanced beyond the Current register. To guarantee
this will not happen, a software reset must be issued during
all Receive Buffer Ring overflows (indicated by the OVW bit
in the Interrupt Status Register). The following procedure is
required to recover from a Receiver Buffer Ring Overflow.
If this routine is not adhered to, the ST-NIC may act in an
unpredictable manner. It should also be noted that it is not
permissible to service an overflow interrupt by continuing to
empty packets from the receive buffer without implementing
the prescribed overflow routine. A flow chart of the ST-NIC’s
overflow routine follows.
Note:
It is necessary to define a variable in the driver, which will be called
‘‘Resend’’.
1. Read and store the value of the TXP bit in the ST-NIC
Command Register.
2. Issue the STOP command to the ST-NIC. This is accom-
plished by setting the STP bit in the ST-NIC’s Command
Register. Writing 21H to the Command Register will stop
the ST-NIC.
3. Wait for at least 1.6 ms. Since the ST-NIC will complete
any transmission or reception that is in progress, it is
necessary to time out for the maximum possible duration
of an Ethernet transmission or reception. By waiting
1.6 ms this is achieved with some guard band added.
Previously, it was recommended that the RST bit of the
Interrupt Status Register be polled to insure that the
pending transmission or reception is completed. This bit
is not a reliable indicator and subsequently should be
ignored.
4. Clear the ST-NIC’s Remote Byte Count registers
(RBCR0 and RBCR1).
5. Read the stored value of the TXP bit from step 1, above.
If this value is a 0, set the ‘‘Resend’’ variable to a 0 and
jump to step 6.
If this value is a 1, read the ST-NIC’s Interrupt Status
Register. If either the Packet Transmitted bit (PTX) or
Transmit Error bit (TXE) is set to a 1, set the ‘‘Resend’’
variable to a 0 and jump to step 6. If neither of these bits
is set, place a 1 in the ‘‘Resend’’ variable and jump to
step 6.
This step determines if there was a transmission in prog-
ress when the stop command was issued in step 2. If
there was a transmission in progress, the ST-NIC’s ISR
is read to determine whether or not the packet was rec-
ognized by the ST-NIC. If neither the PTX nor TXE bit
was set, then the packet will essentially be lost and re-
transmitted only after a time-out takes place in the upper
level software. By determining that the packet was lost at
the driver level, a transmit command can be reissued to
the ST-NIC once the overflow routine is completed (as in
step 11). Also, it is possible for the ST-NIC to defer indef-
initely, when it is stopped on a busy network. Step 5 also
alleviates this problem. Step 5 is essential and should
not be omitted from the overflow routine, in order for the
ST-NIC to operate correctly.
Overflow Routine Flow Chart
TL/F/11157 – 57
17
Obsolete
