[Coco] 1bit hsync nasties....

[Simon Jonassen]

Well John...

We have a nice litte vector (secondary) at $10c

Which usually holds a jmp $xxxx (somewhere in basic on a cold boot)....

If we hook the hsync (which is 15750 Hz (15625 for pal)) then that jmp $xxxx
gets executed very fast

So that jmp (4 cycles) gets executed 312 / 262 lines (per frame) 

312 * 4cycles = 1248 cycles (you do the math for NTSC)

What "IF" we "dared" to totally trash the $100 page by directly inserting
the irq code at $10c with no regard to consequence ??? Well on a coco III it
would fook up the firq vector at $10f... (coco II uses this for cart/rs232)

"IF" we dared to overwrite this, we would be saving 1200+ cycles.....

If we then directed the DP to $1 (as in $10c) we could also save cycles....

If the DP irq now involved "self modifing" code we would be up on speed..
right ????
 

Like i said... Alot of nasties going on here.....

/Simon :-)


-----Oprindelig meddelelse-----
Fra: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] På
vegne af John W. Linville
Sendt: 22. januar 2014 22:21
Til: CoCoList for Color Computer Enthusiasts
Emne: Re: [Coco] 1bit hsync nasties....

On Wed, Jan 22, 2014 at 08:45:08PM +0100, Simon Jonassen wrote:
> SO...
> 
> who would like to know how the HSYNC irq music thing did it's nasty tricks
?
> 
> /Simon :-)

I'm sure we would all love a good story of heroic feats and daring... ;-)

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.

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