nasm2009.txt

Well, here we are, ten years later.

What brought me back here was the fact that no one uses Turbo Assembler
anymore.  I'm not even sure if Borland is around anymore.  And I started
thinking, well, I use NASM these days; maybe I should translate the 8086
assembly version of Shelta into NASM.  It's free, and tinkers like free.
Plus Ben recommended it, way back when it was something like version 0.98.
I said I'd wait until it was past version 1.0.  Well, it's 2.mumble now, 
so it's high time, right?

So I started translating, and I discovered just how much more explicit
NASM is.  I was aiming to reproduce the same SHELTA86.COM file, or at
least one of the same length with the labels in the same places.  I had
to go through some lengths to stop NASM from inserting redundant ds:
segment references, and from padding the start of the data segment to a
word boundary (I just left out the data segment directive entirely.)

But then, when I got it all nice and translated -- Shock!  Horror!
I discovered the awful truth: shelta86.com cannot compile sheltas.she.

Where did I get the nerve to say that I had bootstrapped a half-kilobyte
compiler?  Misleading at best!  I had bootstrapped a probably about 555-
byte compiler.  I then butchered the language it compiled -- removing
three instruction forms -- so that I could shove that compiler into half
a kilobyte.  At no time did I actually bootstrap the <512-byte version.
No, that would have required rewriting shelta.she to have a lot of blocks
with temporary names that were only pushed once, and other garbage like
that, so that the stripped-down compiler wouldn't choke on it.  One of
the nice things about (the full) Shelta, I think, is that while it is
small, it doesn't force you to wallow in garbage.  At least not a lot.

So I screwed up my courage, cracked my knuckles, and tried to live up to
my own hype.  I re-instated the string (`) and push-pointer-anonymously
(]) functions, which bumped the size back up to around 555 bytes.  I
didn't bother with the push-named-pointer (]Name) form, because it's not
used in sheltas.she.  OK, so neither are strings, but a lot of the other
example Shelta programs use strings, so I thought they would be good to
have.

I then proceeded to squish the living daylights out of the new, NASM-
language shelta86.s.  Mostly this involved long, hard looks at the logic
and detailed liveness analysis (done by hand, of course.)  There were a
few small tweaks that were easily done; for example, removing one or two
instructions that were completely unnecessary, and replacing the jmp
in the handler dispatch with a call (several ret statements take up less
space than several jmps back to the top of the loop.  Who cares about
wasting space on the stack?)  The most significant savings, though, came
from factoring out some code to write a push instruction and calling an
existing routine for it instead, and from shuffling registers to keep dx
free long enough so that it, instead of a memory location, could be used
to store one of the crucial computed pointers.  The result: a 509-byte
executable which did all that the old shelta86.com could do *and* enough
more to actually compile sheltas.she!

The old shelta86.com is still in the distribution, for comparison, or
nostalgia, or completeness, or whatever.  The new executable is called
sheltan.com (for Shelta in NASM, I suppose.)  The bootstrapping and
driver scripts have been changed slightly to accomodate this newfangle-
ness.  I haven't touched the other documentation, which is now slightly
inaccurate but still quite useful.

Happy bootstrapping!
Chris Pressey
March 7, 2009
Bellevue, WA