[RC5] Re: rc5-digest V1 #176

Joe Zbiciak j-zbiciak1 at ti.com
Sat Mar 21 23:53:15 EST 1998


'Brice D. Fleckenstein' said previously:

|  I suspect that it would be possible, but an 8 bit Atari (or C64 or
| other comparable setup) is unlikely to produce more than about 1
| keyblock a MONTH out of ported C code - and hand-optimised assembly
| might manage one block a week. Main limitation is lack of RAM, forcing a
| LOT of I/O.

Actually, to crack a key doesn't take much RAM at all.  You only need
around 128 bytes to store all of the cracking state, and this would all
fit in the 6502's quick-to-access zero page.  The biggest limitation is
the small word-size of an 8-bit device, requiring many instructions to
do just a simple rotate or ADD.  (A 32-bit ADD or XOR is the simplest,
and both take four instructions.  A ROTL would require a loop.)  The 
only I/O would be for checkpointing or grabbing blocks.

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.

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.

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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