HLASM - STH = STore Halfword

The opcode of the STH instruction is X'40'.

Usage

  1. Store a halfword from a register into storage.

Arguments

  1. Register containing value to be stored.
  2. Address where halfword is to be stored.

Function

  1. The low-order halfword in the register is stored at the address.
  2. The condition code does not change.

Special Cases

  1. None.

Related Instructions

  1. STHY stores a halfword using a 20-bit displacement, instead of the 12-bit displacement STH uses.
  2. ST stores a fullword.
  3. STC stores a single byte.
  4. STCM stores a set of bytes.
  5. LH loads a halfword from storage into a register.

Hardware

  1. All hardware supports the STH instruction.

Remarks

  1. On older hardware the storage location had to be an even address; using an odd address caused a S0C6 abend.
  2. Currently, an assembler diagnostic is still generated, but execution is fine; a non-aligned storage operand, however, may cause performance degradation.
  3. When coding a table with halfword entries, it is good coding practice to put them - whenever feasible - on halfword but not fullword boundaries. When loading or storing, if a L or ST is used incorrectly, an assembler diagnostic will be generated.

Examples

Notes on the example below:

  1. R14 is used as an index into the table.
  2. MVI would normally be used to initialize a single byte field, but MVI does not use an index register, and thus cannot be used here.
         YREGS                          * Define register names
         ...
* This is a routine to initialize a table
         LA    R15,0                    * Load table entry number (start=0)
         SR    R14,R14                  * R14 indexes into the table
         LA    R0,10                    * Load number of table entries
         LA    R1,C' '                  * Char flag for the table entry
INITLOOP STH   R15,TABLE#(R14)          * Store entry number
         STC   R1,FLAG(R14)             * Store char flag
         LA    R14,12(R14)              * Bump index into the table
         LA    R15,1(R15)               * Bump entry number
         BCT   R0,INITLOOP              * Loop through all 10 table entries
         ...
         DS    0F                       * Insure table has fullword alignment
TABLE    DS    0CL12                    * Indicate each entry is 12 bytes
         DC    F'0'                     *
FLAG     DC    C' '                     * Character flag
         DC    X'00'                    * Bit type flag
ENTRY#   DC    H'0'                     * Table entry number
         DC    A(0)                     *
         DC    9XL12'00'                * Rest of the table
         ...

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