Re: An introduction to MI by example II (part B)...

[Blair Wyman <wyman@xxxxxxxxxxxx>]

Excerpts from mi400: 22-Nov-99 Re: An introduction to MI b.. Francesco
Scafi@tin.it (1042*) 

> which are the reasons for the closing to user use of W-code?  

I don't know of any "official" reasons (or price tags ;-), but would
*guess* that one basic reason is that if W-code were externalized then
it would have to work...  in its entirety...  every single combination
of opcodes, etc....  forever...  That implies a significant expenditure
of resource to "harden" and "exhaustively test" the W-code... 

Now, compilers typically use a subset of a given representation's
possible opcode permutations, and frankly I'd be surprised if W-code
"plugged all the holes" that an enterprising assembler programmer could
find.  Compilers, given a sequence of HLL statements, are predictable. 
Programmers are less predictable, at least in my experience.  <g>  So,
if something goes wrong with a W-code stream today, it's an APAR to the
compiler developer - if something went wrong with a hand-coded W-code
program there would be no "safety net" to catch the hapless developer. 

>From a pragmatic standpoint, you certainly would _not_ want to try to
manually code W-code.  W-code is positioned as an
automatically-generated intermediate representation of a program or
module, created by a compiler front-end and consumed by a compiler
back-end/code-generator.  In that respect, it is very much like the MI. 
However, the fundamental model of W-code is that of a "stack" machine,
as opposed to MI's "storage-to-storage" model. 

I know a lot of programmers, but I don't know anybody who hand codes
W-code -- even the programming masochists shy away from it, preferring
alternate esoteric fare (such as MI, for example  - <g>). 

  ___   _           Blair Wyman                  IBM Rochester 
 ( /_)  /  _  ' _   (507)253-2891            blairw@us.ibm.com 
__/__)_/_<_/_/_/_'  Opinions expressed may not be those of IBM 



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