Of the top of my head
plugging in a cc mask
plugging in a register
modifying an opcode
--
Shmuel (Seymour J.) Metz
http://mason.gmu.edu/~smetz3
עַם יִשְׂרָאֵל חַי
נֵ֣צַח יִשְׂרָאֵ֔ל לֹ֥א יְשַׁקֵּ֖ר
________________________________________
From: IBM Mainframe Assembler List <[email protected]> on behalf
of Binyamin Dissen <[email protected]>
Sent: Sunday, August 3, 2025 7:07 AM
To: [email protected] <[email protected]>
Subject: Re: Execute-Type Instructions
External Message: Use Caution
On Fri, 1 Aug 2025 23:38:36 -0400 Dan Greiner <[email protected]> wrote:
:>As is well known to subscribers of the Assembler List, the EXECUTE
instruction (EX) - and the (relatively) new variant of EXECUTE RELATIVE LONG
(EXRL) - provide an extremely powerful means of altering the behavior of a
target instruction by ORing the contents of a the rightmost bits of the first
operand of the execute-type instruction into bits 8-15 of the target
instruction. Common uses include modifying the length(s) of an SS-format
instruction, the register(s), mask, or immediate field of RR, RX, SI, and many
other formats of instructions (it is also sufficiently complex that it drives
CPU designers slightly nuts).
:>One reason an execute-type instruction is particularly tricky is that certain
instruction formats contain part of the operation code in bits 8-15, thus the
actual target instruction executed may not be that which appears in the memory.
This can occur when the target instruction format is IE, RI, RIL, RRD, RRE,
RRF, S, SIL, SSE, and SSF.
:>My question is (aside from IBM diagnostics) does anybody actually exploit
this sort of chicanery/guile/subterfuge in their code?
Out of curiosity, which instructions (that do not alter length that is being
altered would benefit from EX over straight forward coding?)
--
Binyamin Dissen <[email protected]>
http://secure-web.cisco.com/15LikfxURFI5YBAADdp-pZBQNlAks6ZeqjIX3bK-6v-glWt21OG_0uVXyB7jCb2e3YOWRWvfevBx2hjZKe9bGUbOmGhptOp5vok06ntdcJI5DCXc8RzGJtQ7jep8bj3envuiiXHi4KiGv6SLJ_Q5Hk9gdx3lv_KSwd5OmXepA97bh4OmCPLc7uFxQB-gsB9kRdBSJLGTumHJ1B_DtXLWBvQRoOK-24yS93a3u2oJM-hIJq-GWYKF11tgJsLMT2TBPzNuPhN1iTtcN82qu5X_7mJBzSk0IRQh7fIuZK3ejJE-ch2mAWK5DFbf-HEj43reDHr1nzDTg__smBffBmaNuEYrHOLvM_3jwlYGAh2PvDv9TahOQNVjkitsx4KELil46pGy94CIC4Sh1rXb0q6b1usxmT_mAj5McJ2dpbdISpEw/http%3A%2F%2Fwww.dissensoftware.com
Director, Dissen Software, Bar & Grill - Israel
