Rockwell Automation 1747-PCIS API Software - Open Controller - Manual (Page 51)

Publication 1747-UM002A-US-P - June 2000
Developing Applications 3-19
Any remaining shared memory can be allocated for host retentive
data, which is the portion of the dual port RAM that you can use to
store data in case power fails. If the application doesn’t need host
retentive data, set its size to 0. If the application needs host retentive
data, the application can determine the amount of memory available
by using the OCSEGMENTSIZELIMIT constant.
This constant specifies the total number of bytes available for the
three segment sizes. To calculate the maximum memory available for
the host retentive data, use this formula:
initinfo.HostRetentiveDataSize =
OCSEGMENTSIZELIMIt - initinfo.OutputImageSize - initinfo.InputImageSize;
If the I/O configuration changes and causes the image sizes to
change, the maximum memory available for Host Retentive Data will
change accordingly, and information stored in the Host Retentive Data
memory may be overwritten.