Self-modifying code

[David Goodenough]

>From article <225800044 at uxe.cso.uiuc.edu>, by mcdonald at uxe.cso.uiuc.edu:
> 
>> [discussion of execute-only code segments]
>> >	Besides, I thought that self-modifying code was (a) extremely difficult
>> >to write, and (b) considered poor programming practice.
>> 
>>   Yes, it is and it is.
>> 
>     Yes, it is and yes it is, but only by those who don't need it. There
> are good uses for self-modifying code: I have used it recently in
> two places (one on an IBMPC and the second on a PDP-11.

These are MY OPINIONS ONLY - you are free to agree and disagree and flame as
you see fit. I have used S-M-C only once when doing a section of code that
handled single stepping. The problem W/ the Z80 (comp.lang.c ??????) is that
it has conditional jumps, calls AND returns. Now I go and fetch an instruction
out of the code portion (i.e. where my PC is pointing to). It's 0xc2. Aha, I
have a conditional instruction. Now to figure out whether the condition is
met I have two choices:

	1. Decode the bits that determine which flag is being looked at, and
		whether the flag should be set or reset. Get the flags into
		some register where I can use them. Mask out the flag in
		question. Do a condional jump on the result of the mask
		and whether the flag shold be set or clear.

	2. Turn the instruction into a conditional jump (and with a mask
		then or with a mask - turns any condional (except the
		relative jumps) into a conditional jump). Drop the condional
		jump into a hole - get the flags and do the jump.

If someone wants to see the code that I produced for both of the above I can
e-mail. I ask you to take it on faith that 1 was about 40-50 lines, whereas
2 was 6 lines. Also BECAUSE I COMMENTED, it was possible to figure out what
was going on. My mark of good commenting is code that can be read a year later
and still understood. I agree that this is not for the faint at heart, but
it was faster, and to my mind easier to understand. Note also that in this
application speed was a premium: when I'm single stepping 1000 instructions
I want things to happen PDQ as I would like to see the program appear to run
as fast as possible. But then I'm a little insane, because who in their
right mind writes a single step utility for a dead micro like the Z80 :-)

Like everything  it has it's place: and is not to be misused. Misuse of
S-M-C *_IS_* a sin (well I think so), but where it is justified I will
use it.
-- 
	dg at lakart.UUCP - David Goodenough		+---+
							| +-+-+
	....... !harvard!cca!lakart!dg			+-+-+ |
						  	  +---+