With Revision’s deadlines looming and the party build-up starting tomorrow later today, my time budget for a potential size-coded Amiga release doesn’t look too good anymore… That’s life!

But at least I had some time to revisit my packer setup between my last work day before Easter and packing my luggage. Behold, the magic of known register contents at boot time across Kickstart versions!

	Kickstart 1.2	Kickstart 1.3	Kickstart 2.04	Kickstart 3.1
d0	00000000	00000000	00000000	00000809
d1	8000ffff	8000ffff	0000ffff	00000808
d2	00000001	00000001	00000001	00000001
d3	00000000	00000000	0000d688	0000ad48
d4	00000000	00000000	00001468	00001520
d5	00000000	00000000	00000000	00000000
d6	ffffffff	ffffffff	ffffffff	ffffffff
d7	00000000	00000000	00000000	00000000
a0	00001958	00001958	0000dfb8	0000ed38
a1	00c014e6	00c014e2	0000d54c	0000e2c4
a2	00c0185c	ffffffff	0000d530	0000e2a8
a3	00fe8b3a	00fe86ee	0000d54c	0000e2c4
a4	00001558	00001558	00003e4c	00002708
a5	00c014ba	00c014b6	00000a44	00000a74
a6	00c00276	00c00276	00001468	00001520
a7	00c014b6	00c014b2	00003e10	000026cc
pc	00001564	00001564	0000dfc4	0000ed44

Note: Of course, this is not an exhaustive list; these are merely samples of the most common configuations: Amiga 500 with 512+512K RAM for 1.2 and 1.3, Amiga 500+ for 2.04, vanilla Amiga 1200 for 3.1

Warning: The values for d6 and d7 will differ if the bootblock is run after a guru meditation! For example, after this:

…d6 and d7 will contain 00000004 and 00c01570 instead of -1 and 0. Thanks to platon42 for the hint!

This gives us:

Register	Contents
d2	Constant value 00000001
d5	Constant value 00000000
d6	Constant value ffffffff – see warning above!
d7	Constant value 00000000 – see warning above!
a1	IORequest, as specified by the OS
a6	exec.library, as specified by the OS
pc	12 bytes after the bootblock content as it was loaded from disk (i. e. 'DOS',0 followed by checksum and rootblock longwords)

Nifty! Together with some register shuffling, we can then use these values to omit some constant initializations in the depacker code.

Shrinkler (source) – 4 bytes saved:

ShrinklerDecompress:
        ; Init range decoder state
        moveq.l #0,d2           ; omit, use d5 instead
        moveq.l #1,d3           ; omit, use d2 instead
        ...

ZX0 (source) – 2 bytes saved:

zx0_decompress:
        moveq.l #-128,d1        ; initialize empty bit queue
        moveq.l #-1,d2          ; omit, use d6 instead
        ...

I love that each of the constant values 0, 1, and -1 is useful! :)

For both packers, the bootblock on disk then starts like this:

        dc.b    'DOS',0
        dc.l    0               ; checksum will be inserted here
root:   dc.l    $20000          ; destinaton; normally 880 = rootblock
entry:  move.l  root(pc),aX     ; load destination address
        lea     packed(pc),aY   ; load packed data
        pea     (aX)            ; rts will jump into unpacked code
        ...                     ; decompress code starts here