I have to hide a store CPU ID instruction in my pre-retirement days. Keith
Sent from an iPhone > On Aug 1, 2025, at 23:34, Dan Greiner <dan_grei...@att.net> 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?
