collapseos/blk/651

This assembler works very much like Z80 assembler (B200) so
refer to this documentation first. Here, we document specifici-
ties.

All mnemonics in AVR have a single signature. Therefore, we
don't need any "argtype" suffixes.

Registers are referred to with consts R0-R31. There is
X, Y, Z, X+, Y+, Z+, X-, Y-, Z- for appropriate ops (LD, ST).
XL, XH, YL, YH, ZL, ZH are simple aliases to R26-R31.

Branching works differently. Instead of expecting a byte to be
written after the naked op, branching words expect a displace-
ment argument.

                                                        (cont.)