McKown, John wrote:
IIRC, IBM has stated that an opcode of x'00' will __never__ be valid and
will __always__ produce a program check interrupt code 1 (S0C1 in
MVS-speak).
A Few years back I would have disagreed with you..
The ESA/390 POO states (6.5.2.24, programing note 2 of SA22-7201-04) :
//"The operation code 00, with a two-byte instruction format, currently
is not assigned. It is improbable that this operation code will ever be
assigned."//
(so it was not "__never__" and "__always__" !)
But they changed the wording now (and for a pedant like me, the meaning) :
The z/Architecture POO States (Page 6-27, left col, Programming note 2
of Operation Exception Program Interrupt of SA22-7832-06) :
//"Operation code 00 hex will never be assigned to an instruction
implemented in the CPU."//
So after 40+ years - they finally made up their mind !
--Ivan
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