[RC5] Re: rc5-digest V1 #176

Unrau, Trevor UnrauT at aecl.ca
Mon Mar 23 14:35:24 EST 1998

> ----------
> From: 	Joe Zbiciak[SMTP:j-zbiciak1 at ti.com]
> Sent: 	Sunday, March 22, 1998 12:53 AM
> To: 	rc5 at llamas.net
> Subject: 	Re: [RC5] Re: rc5-digest V1 #176
> The Atari's have an additional problem, though.  I believe these
> machines had video circuitry which performed heavy cycle stealing from
> the CPU.  From what I recall, the Atari 2600's TIA stole about 50% of
> the CPU cycles from the 6502, and the CPU wasn't even running at 
> a full 1MHz, to boot!
> In contrast, the Apple ]['s video circuitry performed no cycle
> stealing.  I'm not sure about the Commodore computers, but I seem to
> recall that the Vic 20 could run faster if you blanked the display.  I
> think it would run its 6502A at a full 2MHz with no "wait states" in
> such a mode.  The speed up may have been from the lack of horizontal
> retrace interrupts, though... not sure.
Actually, the Vic 20 only ran at 1MHz, and no CPU cycles were stolen by
the video circuit because video's RAM access clock was phase-shifted
from CPU clock (90 deg. I think).  The C-64 had the same setup and
speed, but sometimes the video clocks ran long and would "steal" CPU
clocks (for graphics-intensive operations), so the C-64 ran slower than
the Vic 20.  Blanking the display eliminated the CPU clock stealing,
allowing the C-64 to run at its full clock rate.  BTW, the only 8-bit
machines Commodore made that ran >1MHz were C-16, Plus/4, C-128(D), and

> Then there's even weirder computers, such as the TRS-80 CoCo 2.  It
> used a 6800-series processor, and ran it at 0.9MHz, unless you poked
> the right value into a certain location, and then the CPU would run at
> twice the clock rate.  (All of the programs I ever wrote for it
> included that poke on around line 1.  ;-)  I never found out if I was
> clocking it beyond its safe operating limits, but I don't think I was.
> I think the slow clock rate was for backwards compatibility with older
> games/programs.
CoCo used the 6809E, which was only rated for 1MHz.  Running at the
double clock might burn out the CPU.

> Regards,
> --Joe
> -- 
>  +----------- Joseph Zbiciak ----------+
>  | - - - -  j-zbiciak1 at ti.com  - - - - |       Ignorance is the
>  |- http://www.primenet.com/~im14u2c/ -|       Mother of Devotion.
>  | - - -Texas Instruments, Dallas- - - |          -- Robert Burton
>  +-----#include "std_disclaimer.h"-----+
> --
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None of the above seem appropriate for RC5 cracking.  Machines are too
slow and combersome to connect to the 'net.

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