Month: January 2020

In the last post I suggested we take the challenge that John Ehrman posed in problem 41.7.1 of his well-known and used assembler book:

Programming Problem 41.3.(3)+ Write a program with a program interruption exit that generates each of the 15 possible interruption types in turn. For each interruption, generate a message describing the interruption type and the address of the interrupted instruction. Then, return to the mainline program to generate the next interruption.

David Staudacher contributed a number of interesting ways to abend, but I especially liked BCTR R15,R15 for a protection exception. Melvyn Maltz had some interesting comments about the whole issue and suggested he gets S0C5s whenever he uses a bad index into a branch table. You can see all their contributions in the comments on the last article.

I was surprised (after posting) to discover that John himself had already weighed in on this challenge in the Solutions part of his book. Here is what he published (page 1275 of his book):

41.7.1. Some possible but unverified instruction sequences are:

(01)  DC H’0′ Invalid operation

(02)   LPSW 0,8(0,0) Privileged operation

(03)  EX 0,* Execute

(04)  MVI 0,0 Protection

(05)  LGHI 1,-1

IC 1,0(,1) Addressing

(06)   DC X’1D11′ DR 1,1: Specification

(07)  AP *,* Decimal Data

(08)  LA 1,1

SLA 1,33 Fixed-point overflow

(09)   SRDL 0,63

DR 0,0 Fixed-point divide

(0A)   AP A,A Decimal Overflow

A DC P’6′

(0B)   DP A,=P’0′ Decimal Divide

(0C)   LE 0,=E’1E72′ Exponent overflow

MER 0,0

(0D)  LE 0,=E’1E-72′ Exponent underflow

MER 0,0

(0E) LE 0,=X’41000001′ Significance

SU 0,=X’41000001′

(0F)  LE 0,=E’1′ Floating-pointdivide

D 0,=E’0′

I tried running the code above and found an assembly error on (02). John said the code suggestions were unverified! Still he was on the right track. LPSW is a type S instruction and expects a simple base/displacement address, so LPSW 0(8) will generate the error he intended.

You can also get away with SLA 1,32 on (08), but that’s nit-picking.

The only problem in the list was his suggestion for (05) which generated an S0C6 for me instead of the S0C5 we require.

So how do we get a S0C5 exception to occur? I’ve tried a number of bad branches – protection errors every one. After reading the Principles of Operation, I’m wondering if a garden-variety programmer can easily get a S0C5? I hope one of you will prove me wrong. Send me the code that generates a S0C5!

I never got to meet John Erhman but we corresponded a little.  He was kind enough to get me included on the program at a Share conference in Anaheim.  John wrote an excellent Assembler reference titled  Assembler Language Programming for IBM z System Servers . Even a seasoned assembler programmer can learn a lot from this book – it’s packed with many interesting assembler goodies. I keep a link to it nearby and you should, too. I was browsing through it today and came across a suggestion he had for a program to write:

The program suggestion comes after a section of the book that covers how to handle program interruptions using ESPIE. It wasn’t too hard to get a basic version of the program going. You do have to write some “bad” code to generate the 15 possible program interruptions. As I began to write this code, it struck me as funny that this would be the worst program ever. That’s when I thought it would be fun to enlist your help. Many of the interruptions are easy to generate – we’ve all sinned. But I did scratch my head on how to generate a S0C5. I can’t remember ever getting that interruption, but then my memory isn’t as good as it used to be.

Here’s how you can help: If you have a favorite way to generate one of the 15 possible program interrupts in just a few lines (the fewer the better) send it in. I’ll pick the ones I like best and include them in my program and give you credit. The program will appear in a future post or article on recovering from program interrupts. The program I’m writing does have an input file (FILEIN) and an output file (FILEOUT). That might help you with some of the interrupts. Be sure to include the definitions of any fields you need. As a small tribute to John, we’ll commit every possible error. Help me write the worst program ever!