Resource name hash routine

The Fast Path Resource Name Hash routine generates the hash value used by the IRLM.

You can specify the name of such a routine with the UHASH= startup parameter, but it is ignored. IMS always sets the value of UHASH to DBFLHSH0.

One technique used by the IMS-supplied Fast Path Resource Name Hash routine (DBFLHSH0) increases the range of values implicit with the relative CI numbers by combining parts of the 31-bit CI number with values derived from a database's DMCB number and its area number as follows: Bits 11 through 15 of DMCB number are XOR'd with bits 7, 6, 5, 4, 3 of the area number to give a combination 5-bit position number. (Using the area number's bits in reverse order helps make both DMCB number and area number vary the combination value.)

For the relative CI number (bits 0 through 15 are not used):

• Bits 16 through 20 are XOR'd with the combination value.
• Bits 21 through 25 are XOR'd with the combination value.
• Bits 26 through 29 are used unchanged.
• Bits 30 and 31 are not used—thus a hashed CI number used as a GHT entry represents four CIs.

For the hashed resource name:

• Bits 16 through 29 of the hashed relative CI become bits 18 through 31 of the hash value that is passed to the IRLM.
• Bits 18 through 26 of the hash value are used as the displacement into the resource hash table (RHT).
• Bits 18 through 31 are used as the displacement into the GHT.