[RC5] Re: rc5-digest V1 #176
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
CoCo used the 6809E, which was only rated for 1MHz. Running at the
double clock might burn out the CPU.
> +----------- 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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