Converting a Relative Track Address to an Actual Track Address

Convert a relative track address to the actual 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 CVTPCNVT 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 CVTPCNVT, 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 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 0, 1, 2, 14, and 15 with input to the routine. Register usage is as follows:

Table 1. Registers and Their Use for Converting Relative to Actual
Register Use
0 Must be loaded with a 4-byte value of the relative track number.
If entered at CVTPCNVT+0, this value must be in the form TTRn, where:
TT
The track number relative to the beginning of the data set
R
The block identification on that track
n
If the DEB is for a partitioned concatenation, (the DSORG field in the DCB indicates PO), then supply the concatenation number for the data set. A value of 0 indicates the first data set, 1 indicates the second data set and so forth. If your partitioned concatenation includes PDSEs or z/OS® UNIX directories, each of them is represented by a dummy extent in the DEB. EXCP and EXCPVR are not valid for those extents. If the data set is not concatenated or the concatenation is not partitioned, set to 0.
If entered at CVTPCNVT+12, this value must be in the form TTTR, where:
TTT
The track number relative to the beginning of the data set
R
The block identification on that track
This entry point is intended for any type of data set.
1 Must be loaded with the address of the data extent block (DEB) 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'. Note that if the DEB31UCB bit is zero, the UCB address field is three bytes in DEBUCBA. If the DEB31UCB bit is one, the UCB address field is four bytes in DEBUCBAD.
2 Must be loaded with the address of an 8-byte area that is to receive the actual address of the block to be processed. The converted address is of the form MBBCCHHR, where:
M
Indicates which extent entry in the data extent block is associated with the direct access program. (0 indicates the first extent, 1 indicates the second, and so forth)
BB
Two bytes of zeros
CC
Low order 16 bits of the cylinder number. The cylinder number is 28 bits on all currently supported DASD.
HH
The actual track number in the low order four bits and the high order twelve bits of the cylinder number in the high order twelve bits.
R
The block number.
3–8 Not used by the conversion routine.
9–12 Used by the conversion routine and not restored.
13 Used by the conversion routine and not restored. If you call the +12 entry point, the high order three bytes must contain zero and the low order byte must be as set as follows:
  • If the DEB is for a partitioned concatenation, (the DSORG field in the DCB indicates PO), then supply the concatenation number for the data set. A value of 0 indicates the first data set, 1 indicates the second data set and so forth.
  • If the data set is not concatenated or the concatenation is not partitioned, set to 0.
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 address where the conversion routine is to receive control (from field CVTPCNVT of the CVT).