IBM Software for System z For Dummies ®

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An ongoing list of z/Architecture assembler things that you forget if you don't use them often enough

This blog entry will be updated and reposted whenever I come across things that I should remember but my brain has forgotten due to infrequent contact with the item.

• TRT munches GPR2. That's why the subsequent compare failed miserably.
• Man, there are a lot of SVCs and PCs that won't work in AR mode. Make sure you switch out of AR mode before calling macros like WAIT.

Little bitty z/Architecture assembler tips and tricks not worthy of their own blog entry

This blog entry will be updated continually and reposted when I come up with little tips and techniques not worthy of their own blog entry.

High half register check

Can't use those bitchin' new high-half register instructions because you have customers not on the latest z196/z114? Need to check the upper half of a register for non-zero so your AMODE 64 doesn't run amok? Use the following:

CLG   R0,=X'00000000FFFFFFFF'
JH    THERE_IS_SOMETHING_IN_THE_HIGH_HALF_OF_THE_REGISTER

And you also have an example of a label longer than 8 characters.

Make sure your base register is clean in AR mode

LAE is a nice instruction for setting a base register if you aren't sure your code will be called in primary or AR mode. If the B2 field is 0, when you are in AR mode, 00000000 is placed into the corresponding R1 access register, so you can code 

LAE   R12,0(R15)

and get your entry point without the possibility of getting messed up by a stray ALET in AR12.

WAIT LONG=YES and baseless programming (z/OS, 1.12 and earlier)

If you've tried this combination before, you've gotten bit because the macro generates a BCR GPR8,0 instruction followed by an ICM of the second byte via an "*-1" operand. If you're baseless, this fails because you are baseless. However, there is no need to fret. The following code works and gets around this issue.

         IILH  GPR0,X'8000'

         IILL  GPR0,1

         WAIT  (0),ECB=MYECB

The macro generates an unnecessary LR GPR0,GPR0 followed by the appropriate code. If you are using an ECBLIST, just put the number of this-many-POSTed ECBs in GPR0; in this case, it might be easier to load GPR0 first, then use an OILH GPR0,X'8000' instruction.

As noted in the comments below, starting with 1.13, if you are SYSSTATE ARCHLVL=2, WAIT will generate appropriate code for baseless situations.

More to come as I think of them...