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  2. MI400
  3. March 2010

Re: QPROCT and QPRODT

QPROCT hashes the 8-letter MI opcodes into 563 linked lists. All those
x'FFFF's you observes in the QPROCT space are linked list tombstones.

The hash algorithm is remarkable: view the 8-letter opcode as two 4-byte
signed integers; exclusive-OR them together; divide by 563 and keep the
remainder; if the remainder is not positive, add 563. The result is a
number in the range 1 to 563, which is your index into the hash anchor
array. Here is MI code a disassembler can use to get your hash number:

DCL DD OPCODE CHAR(8);
DCL DD INDEX BIN(4);

XOR INDEX, OPCODE(1:4), OPCODE(5:4);
REM(SB) INDEX, 563 / NPOS(SKIP);
ADDN(S) INDEX, 563;

SKIP:

And this begs the question:

Why 563?



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