The B JIO routines allow a privileged program to access information that is on the disks but stored outside the normal file system. This includes such things as J*'s, backdoor files, and disk bit maps. In TSS, the JIO routines use the DRL JOUT interface that allows the user to create a PAT that points to any mass storage device on the system. In batch, the routines use PMME P.PVIO to access the device without using any PAT.
The program callable routines in the JIO package are
J.READ - Read from an arbitrary place on disk J.WRITE - Write to an arbitrary place on disk J.FREE - Deallocate the PAT GT.DSZ - Allow access to released devicesNormally, a program would only use J.READ, which will look after the details of allocating the PAT, filling in the appropriate values, and deallocating the PAT during wrapup. Because only one PAT can be allocated per user, it is important that the PAT be freed when it is no longer needed. In most circumstances, the JIO routines arrange that this is done during wrapup. However, if the program calls another subsystem, there is a possibility that the called subsystem will need to allocate a PAT; thus the calling program should call J.FREE to release its PAT before calling the subsystem.
On the rare occasions when it is necessary to write an arbitrary disk sector, J.WRITE can be called. Under very rare circumstances, it is useful to be able to access a released device. If this capability is required, the program must do an initializing call to GT.DSZ before calling either J.READ or J.WRITE. If this is done, J.READ and J.WRITE will ignore the fact that the device has been released. The user is warned that if this is done and the device is not actually functioning, TSS will hang attempting to do the I/O.
The disk address to be accessed is specified to J.READ or J.WRITE as device and sector. The device can be specified as either the three character BCD device name with zeroes in the upper 18 bits of the word (e.g. `st1`) or as the negative of the device's SCT address (UCI value) The sector is the absolute sector address on the disk. Note that GCOS does not allow access to the label llink (sectors 0-4).
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