The intent of my post was that it would be a lead-in to a more generalized discussion as to why it might be that System/360 was designed with a set of registers that contain heterogeneous control information, sometimes within a single register and yet provides instructions for loading and storing their values in a whole register at a time fashion. Having 16 of every type of register promotes a more orthogonal design aesthetic but I wonder if the designers got a little carried away with the concept. It allows for a smaller instruction set I suppose since you only need LCTL and STCTL for reading and updating values in one or more Control Registers but wholesale loading of control registers can involve interdependencies that add significant overhead. The existence of instructions such as ESEA that load specific bits of Control Registers without incurring overhead due to interdependencies suggests to me that the 16 of every type of register design aesthetic may have been over-extended a tad.
As for using the ESEA instruction? I see no issue with doing so provided steps are taken to ensure that changes to the EAX are encapsulated (such as when a PC routine whose ETE specifies a non-zero EAX value is executed). Like all privileged instructions its use requires care and attention to detail. BTW, I have not been able to find any documentation that corroborates Jim's assertion that ESEA is only for operating system use. Is there a list of OS-only instructions that I should be aware of? I guess I'd like to make sure it's okay for me to be using SCKC and SPX (just kidding). Keven -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Jim Mulder Sent: Tuesday, November 22, 2011 7:57 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Re: Conditional assembly for COBOL? IBM Mainframe Assembler List <ASSEMBLER-LIST@listserv.uga.edu> wrote on 11/22/2011 05:26:36 PM: > I feel the same way about the ESEA instruction. > Trigraphs are the wrong > answer to a question that should never have been asked. (IMO, only) > the correct answer is UTF-8. I don't know of any conditions under which you should be contemplating using the ESEA instruction. ESEA is intended for use by the operating system. Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY
