Converting an Actual Track Address to a Relative Track Address

Convert an actual track address to a relative track address by using a resident system conversion routine that can be called in 24 or 31 bit mode.

The conversion routine has two entry points. One, at the address in the CVTPRLTV field of the communication vector table (CVT), is intended for data sets with up to 65535 tracks. The other, at offset +12 from the address in CVTPRLTV, is intended for any data set. Call the conversion routine with one of the following instructions:

BALR 14,15 Call +0 entry point for track conversion
BASR 14,15 Call +0 entry point for track conversion
BAL 14,12(,15) Call +12 entry point for track conversion
BAS 14,12(,15) Call +12 entry point for track conversion

IBM does not provide a macro for this conversion routine's invocations.

The conversion routine will return either TTR0 (if entered at CVTPRLTV+0) or TTTR (if entered at CVTPRLTV+12), where TTR0 and TTTR are as described in Table 1.

The address of the CVT is in storage location 16 (field FLCCVT of the PSA data area, mapped by macro IHAPSA).

The conversion routine does all its work in general registers. Load registers 1, 2, 14, and 15 with input to the routine. Register usage is as follows:

Table 1. Registers and Their Use for Converting Actual to Relative
Register	Use
0	Loaded with the resulting TTR0 or TTTR to be passed back to the caller. These two formats are as described for register 0 in Table 1.
1	Must be loaded with the address of the data extent block of the data set. Each DEB resides below 16 MB. When you call the +0 entry point, the called routine clears the high order byte of this register. When you call the +12 entry point, this byte must contain X'00'.
2	Must be loaded with the address of an 8-byte area containing the actual address to be converted to a TTR0 or TTTR. The actual address is of the form MBBCCHHR.
3–8	Not used by the conversion routine.
9–13	Used by the conversion routine and not restored.
14	Must be loaded with the address to which control is to be returned after execution of the conversion routine.
15	Used by the conversion routine as a base register and must be loaded with the content of field CVTPRLTV of the CVT.