[Coco] Linville's ramblings on assembly vs machine code

[Gene Heskett]

On Monday 10 July 2017 15:59:37 James Jones wrote:

> That's what disassemblers are for, save for programs that do tricks
> like modifying themselves.
>
I've seen quite a few disparaging remarks about self-modifying code pass 
by in this thread. Sloppy coding WILL bite you in the butt, so I got to 
tell you there is not a thing wrong with it if you have a good grip on 
what it is you are doing.  That code I wrote for the RCA 1802 had 6 or 7 
locations where the code was self modifying, otherwise I would have had 
to duplicate an identical string of 50+ bytes quite a few times.  The 
last thing that code did was to restore every modified location to the 
initial value.  It was dead stable, and I cannot recall there was ever 
an instance of miss-behavior after the first week it was in service.  It 
was out of service for a couple hours about a month later while I added 
a 6 volt gel-cell as a power outage UPS. The best one can say of it was 
that it "Just Worked", for 17 years that I  know of.  That was what I 
intended for it to do, Just Work.

> Were I teaching assembly language, I'd show the instruction formats,
> and then go into particular instructions as needed. Are you wondering
> whether the order of registers in the push/pull instructions makes a
> difference? Write assembly language that has them in different orders,
> and see whether the generated bytes for the two instructions as shown
> in the listing are the same. (If memory serves, they are.) About
> span-dependent instructions, I'd point out the different encodings and
> point interested students at the relevant paper from CACM back in the
> 70s, (The general problem is NP-complete, but there's an efficient
> algorithm that works for a family of displacements that includes all
> but the most pathological examples, and a good assembler ought to
> implement it.)
>
> James
>
> On Mon, Jul 10, 2017 at 12:38 PM, CoCo Demus <retrocanada76 at gmail.com>
>
> wrote:
> > Machine language is mandatory when you are debugging or
> > disassembling code


Cheers, Gene Heskett
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