[RC5] Re: rc5-digest V1 #176
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
+----------- Joseph Zbiciak ----------+
| - - - - j-zbiciak1 at ti.com - - - - | Ignorance is the
|- http://www.primenet.com/~im14u2c/ -| Mother of Devotion.
| - - -Texas Instruments, Dallas- - - | -- Robert Burton
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