With Revision’s deadlines looming and the party build-up commencing today, the time budget for a possible size-coded Amiga entry doesn’t look too good anymore. Well, that’s life!

I did, however, find a little time – between my last work day before Easter and tying up my luggage – for some compression optimization. Packing before packing! :)

Specifically, I was looking to shorten the decompression header in my bootblocks. I knew I had used some magic constants before, but that was at an earlier version of Shrinkler, and maybe I haven’t used it for salvador/ZX0 at all. So let’s take a fresh look!

What exactly do the registers contain when your bootblock gets executed?

Register	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 the values for some typical configurations: Amiga 500 with 512KB of extra RAM for Kickstart 1.2 and 1.3, Amiga 500+ for 2.04, and a vanilla Amiga 1200 for 3.1

But it seems there are some specific fixed values we can rely on across different Kickstart versions!*

*) If we only need to support classic Commodore-made AmigaOS editions, that is. This will probably not work with AROS68k or AmigaOS 3.2.x.

Register	Contents
d2	Constant 00000001
d5	Constant 00000000
d6	Constant ffffffff
d7	Constant 00000000
a1	IORequest, as specified by the OS (bootblock sequence)
a6	exec.library, as specified by the OS (ibid.)
pc	Noteworthy: The 12 bytes before the entrypoint contain the bootblock header as it is stored on disk (i. e. 'DOS',0 followed by checksum and rootblock longwords

Nifty! Now let’s delete some code. With Shrinkler, we can save four bytes in the initialization code:

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

And two bytes for ZX0:

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

I love that each of the values 0, 1, and -1 is of use somewhere!

For both packers, the bootblock starts like this:

disk:   dc.b    'DOS',0
        dc.l    0               ; checksum gets inserted here
root:   dc.l    $20000          ; destination; usually 880=rootblock 
entry:  move.l  root(pc),aX     ; destination address
        lea     meat(pc),aY     ; start of compressed data
        pea     (aX)            ; rts will jump into unpacked code
        ...                     ; decompression code starts here

With this, my current bootblock stubs take up this much space:

Compression	Stub size	Room for compressed data
Shrinkler	168	856
Shrinkler, parity disabled	162	862
Savlador/ZX0	100	924

Note: It pays out to have build scripts ready for all compression schemes – with my current work-in-progress payload, the “winner” (in terms of best overall compression) fluctuates constantly!

Now, either I find some more bytes to spare during the party or I put some of that saved space to good use with an entry! :)

Then again, I already have my working gloves on…