HLASM - SSK = Set Storage Key

The opcode of the SSK instruction is X'08'.


  1. Set the storage key of a page of real storage.


  1. Register holding a storage key.
  2. Register containing a page address.


  1. The processor sets the storage key of the 2K or 4K page addressed by the second argument. It is set to the value specified in the low-order byte of the first argument.
  2. The condition code does not change.

Special Cases

  1. If the page address is not a fullword address the instruction fails. The page address does not have to be on a page-boundary, however; any fullword address within a page addresses the page in its entirety.

Related Instructions

  1. ISK was intended to obtain storage keys.
  2. SSKE is now used to set the storage key of a page of real storage.


  1. The SSK instruction was part of the original S/360 instruction set. However, it is no longer supported; it has been replaced by the SSKE instruction. Support for the ISK instruction has been dropped when the 370/XA architecture was introduced.


  1. Independent of Amode or DAT, the page address is always a 24-bit real address.
  2. The storage key was taken from the low-order 8 bits of the 32-bit register specified as the second argument in the following format:
    1. The first 4 bits (0 through 3) were the actual storage key.
    2. If fetch protect was installed (an optional feature on early machines), the fetch-protect bit was taken from bit 4.
    3. If DAT was installed the Change and Reference bits were taken from bits 5 and 6. I don't know in which order.
  3. Initially the SSK instruction worked on 2K pages only. When 4K pages were introduced, the SSK instruction was modified to address either a 2K page or a 4K page, depending on the address specified and the way that specific page was configured.


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