Ok, so this is what I've found up to now.
The 'table' that starts at $9C00 in the source above ($1c00 in the ROM) is a 'description' of movement. It's kind of fun figuring out how things were done. This is kind of an 'outside the box' (at least to me) approach to movement:
The table seems to be made up of 8 bytes for each type of movement. I'll take the third entry as an example, being that it's the set I was working with:
This table entry makes up the sequence for the initial 'left loop' that you see for the first set of aliens (i.e. when they are together in 4 pairs of 2 before you shoot them or they reach the bottom of the screen):
Byte Value Description
---- ------ -------------
$00 $01 The number of frames between movements
$01 $01 What position in the 'Circle' the flight pattern starts (there seems to be a 360 degree circle of movement defined somewhere and changing this value starts them at different degrees of the circle)
$02 $1E Length (in *movement* frames) - How long to do this movement
$03 $01 Direction ($01 = normal direction, $FF = opposite direction). i.e. The 'right loop' for this set of aliens has the same values except for $FF in this position
$04 $01 Y Magnification Radius - the higher the number, the larger the sweeps in the y direction the aliens make
$05 $02 X Magnification Radius - same, but in the X direction.
$06 $00 This is the only one I'm not sure of yet. If I change this to $22 like in another entry, they fly off straight to the upper left corner of the screen.
$07 $00 This is always zero for every entry.
Once I have these values down, I need to figure out:
1 - where they are called from and how to tell which wave of aliens use which of these entries
2 - when they end, there's no indicator (that I can see) of a repeat or a stop. (i.e. the circling of the first set of aliens)
Ugh. I searched the 'net almost completely. Nobody has done this yet with this game. (Or maybe they've done it and not posted it)
I was hoping to find something along the lines of the commented arcade Pac disassembly or the arcade Frenzy disassembly I had found.
I'm guessing nobody who knows Z80 is up to the task of looking through the above disassembly.
Anyway, that's what I have so far.
Bob
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