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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