6100 ADCCP Programming Manual

Application Tasks
Writing Applications that Use ADCCP
4–6 069225 Tandem Computers Incorporated
Flag Bits Meaning ADCCP Setting
.<4:5> Access mode Must be set to 0 for an ADDCP line to specify
READ/WRITE access.
.<10:11> Exclusion mode The values for this field are specified as follows:
0 = shared
1 = exclusive
2 = reserved for use by Tandem
3 = protected
Specify shared for a line opened concurrently by more
than one process pair or a process pair that opens a
line more than once.
.<12:15> Nowait depth Specifies the number of outstanding I/O operations.
For example, if the depth is 15, the application can
issue as many as 15 calls before it must call the
AWAITIO procedure.
The window size of a line can affect the choice of the nowait depth and determine
whether the application will open the line more than once. There are several
considerations:
ADCCP allows the number of write requests per station to be equal to the
window size. It allows up to eight read requests for all stations together. The
total number of requests can therefore be much greater than the window size.
An application can set a nowait depth greater than the window size, provided
that the application does not make too many read or write requests to any
station. (ADCCP rejects a request that exceeds the limits for a station.)
If many stations are on the link or if the window size is large, a nowait depth
of 15 (the maximum per OPEN call) may not be enough to send data over the
line. A solution to this problem is to open the line more than once. For
example, use one OPEN call for read requests and another for write requests,
or use separate opens for write requests to each remote station. As many as 15
OPEN calls can be issued to the same line. If each OPEN call has a depth of
15, there can be 225 outstanding requests.
sync-depth output
INT: value
determines whether the Guardian 90 operating system routes a new request
automatically to the backup process if the primary CP6100 process goes down.
(The alternative is to reject the request and have the application retry it.) You
should use a sync-depth of zero with 6100 ADCCP.