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Design Goals for SixtyPical
===========================

(draft)

The intent of SixtyPical is to have a very low-level language that
benefits from abstract interpretation.

"Very low-level" means, on a comparable level of abstraction as
assembly language.

In the original vision for SixtyPical, SixtyPical instructions mapped
nearly 1:1 to 6502 instructions.  However, many times when programming
in 6502 you're using idioms (e.g. adding a 16-bit constant to a 16-bit
value stored in 2 bytes) and it's just massively easier to analyze such
actions when they are represented by a single instruction.

So SixtyPical instructions are similar to, inspired by, and have
analogous restrictions as 6502 instructions, but in many ways, they
are more abstract.  For example, `copy`.

The intent is that programming in SixtyPical is a lot like programming
in 6052 assembler, but it's harder to make a stupid error that you have
to spend a lot of time debugging.

The intent is not to make it absolutely impossible to make such errors,
just harder.

### Things it will Not Do ###

To emphasize the point, the intent is not to make it impossible to make
data-usage (and other) errors, just harder.

Here are some things SixtyPical will not try to detect or prevent you
from doing:

*   Check that a vector is initialized before it's called.
*   Check that the stack has enough room on it.
*   Prevent bad things happening (e.g. clobbering a static storage
    location) because of a recursive call.  (You can always recursively
    call yourself through a vector.)
*   Check that reads and writes to a buffer are in bounds.  (This may
    happen someday, but it's difficult.  It's more likely that this
    will happen for tables, than for buffers.)

At one point I wanted to do a call-tree analysis to find sets of
routines that would never be called together (i.e. would never be on
the call stack at the same time) and allow any static storage locations
defined within them to occupy the same addresses, i.e. allow storage
to be re-used across these routines.  But, in the presence of vectors,
this becomes difficult (see "Prevent bad things happening", above.)
Also, it would usually only save a few bytes of storage space.

### Some Background ###

The ideas in SixtyPical came from a couple of places.

One major impetus was when I was working on [Shelta][], trying to cram
all that code for that compiler into 512 bytes.  This involved looking
at the x86 registers and thinking hard about which ones were preserved
when (and which ones weren't) and making the best use of that.  And
while doing that, one thing that came to mind was: I Bet The Assembler
Could Track This.

Another influence was around 2007 when "Typed Assembly Language" (and
"Proof Carrying Code") were all the rage.  I haven't heard about them
in a while, so I guess they turned out to be research fads?  But for a
while there, it was all Necula, Necula, Necula.  Anyway, I remember at
the time looking into TAL and expecting to find something that matched
the impression I had pre-formulated about what a "Typed Assembly"
might be like.  And finding that it didn't match my vision very well.

I don't actually remember what TAL seemed like to me at the time, but
what I had in mind was more like SixtyPical.

(I'll also write something about abstract interpretation here at some
point, hopefully.)