In a recent note, Alan Altmark said:
> Date: Tue, 7 Nov 2006 00:04:23 -0500
>
> Best practice: MVC TARGET(6),=XL6'402021212121'
> Note the length modifer. This surfaces both the exact length of the
> source and the exact length of the move. People who omit the length
> modifer are sentenced to the 9th level of hell for all eternity. Or even
> longer.
>
I disagree strongly with the concept of such "surfacing". It
detracts from maintainability and requires the coder to bear in
mind microscopic details throughout the program.
Better:
SOURCE DC XL6'402021212121'
...
MVC TARGET(L'SOURCE),SOURCE
Eschew pleonastic use of self-defining terms. (And a conscientious
assembler (HLASM isn't) would warn if the source item overruns the
size of the target field.)
I once was assigned to ugrade a program where the original
coder had conscientiously coded such as:
NROWS EQU 16 Number of rows in matrix
Then tragically spoiled it by repeatedly:
MH =Y(NROWS) Multiply by 16
Apparently satisfying a "house rule" that every statement must
bear a comment. The verification seems to have been mechanical:
some of the comments were nonsensical, even just a single '.'
Etc. Guess what? I needed to increase NROWS, and where I might
have been able to change a single EQU if my predecessor had been
responsible, I had to ferret out all the affected comments and
change those also. (Generally, I deleted them; no "house rule".)
The same program also contained:
FIFTYSIX EQU 56
... apparently satisfying a "house rule" that all self-defining
terms be equated.
Good design of OS/360 would have had the sole declaration is
a SYS1.MACLIB member:
DSNLEN EQU 44 Length of a data set name
And applied occurrences such as:
SAMPLES DC CL(DSNLEN)'SYS1.SAMPLIB' IBM Samples Library
... and it would be possible to increase the length of supported
data set names simply by changing that one EQU and reassembling
z/OS. (Yah, I know there are concerns of base register reach,
etc. And existing media would need to be converted. But still ...)
-- gil
--
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