Speciality Engines

TPM,. there's also a couple of aspects of zIIPS zAAPs and IFLs that you missed.

1) They run at full engine speed. So unless you're one of IBMs very large customers, then the chances are your z10 et al will probably be configured with reduced speed engines. So switching work to Specialities not only saves money, but will probably increase capacity of your machine..

2) You only pay once for them .. so when you upgrade to the latest shiny z(whatever) with a different colour strip on the door they stay with you and get transferred to the new box at no cost.. but.. the new box will have faster engines.. so you get "free" upgrade

Well those rules apply for the moment... as you say things are likely to change.

ps SRB = Service Request Block

@AC Dodgy Hack?

Yes the the Speciality engines are normal CPs, but they don't have any restrictions on them in microcode to restrict workload. (ok so there is one instruction that is different.. but that's only used by the OS not application code). It's the OS that prevents/controls work running on them. But yes I agree they are (were??) a sales tactic.. If you were to write your own OS, then you could happily use them.

As for microcode hack.. I don't think so.. have you tried to load/change z microcode from a zOS system?? Its probably not a OS hack either - changing the zOS dispatcher is not the sort of thing you want to be hacking around as an ISV... but there is no need.. as the bloke from Neon said, the dispatcher just assigns the work .. the hard bit is creating the SRB environment.. (if that's what they do). And doing that is smart software.