Re: bootstrapping from ECHO (SPOILERS)

Going back down the stack a bit, the BCD "aam" and "aad" instructions  
not only allow one to mess around with nibbles, but also (thanks to  
linearity) can be used to pun the decoder.

For instance, the following sequence decodes an ascii nibble, not  
only mapping "A..F" to 0xA-0xF, but also "a..f" and even (compatibly  
with some of my early kludges) ":..?":

0C 20	or al,20
D4 10	aam 10
D5 03	aad 3
2C 09	sub al, 9

-Dave

>> Before I started, this bootstrappy way of thinking tied my brain in
>> knots with even relatively simple problems.  Now it's starting to  
>> make
>> sense.  And I like the way you can get boxed in a corner and just
>> imagine some magic to unblock you, and if it's simple enough magic  
>> and
>> it works, you feel like you're getting away with something that
>> shouldn't be possible, like you're using your first wish to ask the
>> genie for 5 more wishes.
>
> That's awesome.  Inspirational, even.  It's like the "law of  
> attraction"
> with less bullshit.

magic, or leverage?  Archimedes[0] would say there's a conservation  
law for genies, so it's possible to exchange one wish for 5  
fractional wishes at a lesser hypothetical advantage...

(martial arts have a similar concept: the key to having a mechanical  
advantage is to arrange for the engagement to occur much closer to  
your center of gravity than that of your opponent -- win first, then  
hit)

[0] Archimedes:
> δῶς μοι πᾶ στῶ καὶ τὰν γᾶν κινάσω

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