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Optional Instructions…!

One great thing about the development of the IBM mainframe hardware of the last few years has been the addition of lots of nice new, shiny instructions especially those that allow for the use of all or part of the ‘grande’, I.E. 64 bit, registers. For example, to do a locked update on a double word (8 bytes) in the past you would need to use two pairs of registers, for example, zero and one and fourteen and fifteen and use a Compare Double and Swap (CDS) instruction to do the update. Oh, and those pairs of registers had to be even-odd  as well. Now you can just use two 64 bit registers and use a Compares and Swap Grande (CSG) instruction to do the update and the registers do not need to be an even-odd pair either.

Basically, code that used to take several instructions and registers can often be simplified into fewer instructions and use less registers by using the newer instructions from the instruction set. The benefit is less CPU time consumed and faster execution.

This is great and I’ve been doing this sort of thing for a good few years now. However, if you read the POP (Principles of Operation) you may notice next to some instructions under the ‘Program Exceptions’ heading something along the lines of:

  • Operation (if …… facility is not installed)

What this means is that this instruction is ‘optional! What that means is that it may not be physically implemented on the machine and what that means is that if you try to execute it, you will get an operation exception (S0C1 abend).

There are fields in the PSA you can test to see if a facility is installed before attempting to use an instruction or feature of the hardware but to be honest, in all my forty some years of coding, I have never had to do that…..Until now.

Well, actually, not even now which I will explain in a minute.

I wrote some code recently and wanted to do a divide. Because of the size of the numbers, I used the grande version of the divide instruction. Before doing the divide I needed to clear the high word of the registers so I used the SHHHR Subtract High instruction.

This worked great on my test machine but failed on another machine with an S0C1 abend. Yep, that other machine did not have the high-word feature installed.

It was easy enough to replace the instruction with another (The ICMH version of Insert Character under Mask) which is not feature based and so should be there on all hardware but this is the first time in all my years of coding that I’ve ever had to even consider to availability of a feature.

I’ve always thought of the instruction set in the machine as being like an engine in a car, every car has an engine, but I’ve never really considered that not all car engines are the same. If you want more power, it typically costs more to buy a car with a more powerful engine. A more powerful car will (speed limits aside) get you from A to B more quickly and if time is money, spending money up front to get a more powerful car will save you time (and money) later on. In the computer world, you can pay more up front to buy a machine with more features (better instruction set) and get faster execution which save CPU time and thus chargeable MSUs or you can pay less and pay the money when you consume the extra MSUs due to the smaller and thus slower instruction set available to code running on the machine.

In my case, rather than test to see if the feature (instruction) was available before using it I just replaced it with an instruction that I know will always be there. This reason for this is that this is very low use code and it was two instances of one instruction. It would take longer to execute the code to test for the feature and branch to alternate code than it would to just always just execute an alternative instruction. That test and branch also adds slightly more complexity to the code which adds to the possibility of errors and makes it just that bit harder to maintain.

Had the block of code been high use and/or very complex it may have been worth while to test for the feature and use the faster instructions/simpler code when available but that was not the case here. Simple is good in my book!


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