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Message: 6908

Author: ian.rushforth

Date: 04/10/2016

Subject: Re: Advanced Trainer - Daniel's suggestions

 

Andrew wrote:

Standing on a Crumbly cell in row 0 causes row 8 to crumble!

I've noticed this amazing phenomenon in one of the rooms of "Manic
Scribbler".

It first appeared in MM:N-AT's "A Reel Challenge" [1], albeit for
purely aesthetic value. In Manic Scribbler's "Ambergris screen" [2],
it's actually cell-row 9 that crumbles, because the Crumbly cell is
in row 1.

The pixel-pattern crumbles in row 8 (or 9), but the colour-attribute

(and hence cell-type) changes from Crumbly to Air in row 0 (or 1) as
you'd expect.

Whilst it's true that the code which is supposed to change the colour-attribute of a Crumbly cell after it pixels have been cleared, in the circumstances described above, does affect the cell under Willy's feet in row 0 or 1, rather than the cell in row 8 or 9 that has had its pixels 'quirkily' removed...

...this does not manifest itself in a change to the colour-attribute of the Crumbly cell in row 0 or 1. Rather, it causes the top pixel-row of the Crumbly cell upon which Willy is standing, to be temporarily cleared (temporarily because it is only cleared whilst Willy is standing on it ; if he jumps up and down then the pixel-row disappears, and then reappears when he lands).

This occurs because, instead of incrementing HL from the range #5Cxx to #5Exx (i.e. switching from the Occupied Room Attribute Buffer, to the Empty Room Attribute Buffer, to permanently remove the Crumbly from the room) the code (at #8BD4-#8BDC) ends up pointing HL at an address in the range #60xx, which lies within the Occupied Room Display Buffer (and is therefore refreshed during every 'tick', refilling the errant pixel-row from #70xx in the Empty Room Display Buffer if Willy is elsewhere).

 

 

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