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Message: 5905

Author: andrewbroad

Date: 20/08/2006

Subject: Re: A technical question about J. G. Harston's patch - items problem

 

Daniel wrote:

I have tried applying J. G. Harston's Intel Hex 7-bit rooms patch
(message # 5879 and 5881), following John Elliott's instructions
given at http://www.seasip.demon.co.uk/Jsw/patches.html and using
his Hexpat utility.

The results very varied, but never quite satisfactory so far.

When I applied the patch to a .TAP file of my own gamma-
released "Willy's New Mansion", no items were visible, either.
What's more, I noted a glitch in the room "The Multi-Toilet
Bathroom" - the toilet sprite seems to have been slightly modified
(even though in JSWED it looks OK).

This is because J.G. Harston's patch relocates the "is this item
collected?" flags to a table at #A600-A6FF.

In the original JSW, #A600-A67F hold the toilet-sprite, so this gets
corrupted as the item-collected flags (Bit 6 of each byte: 0 if the
item has been collected, 1 if it's uncollected) get twiddled.

When I applied the patch to a .TAP file I am currently working on
(the bug-free version of Steve Worek's "Jet Set Emily"), the items
look good in JSWED 2.2.9, but when I load the game into an
emulator, they are visible, but all in the wrong places.

Jet Set Emily is based on the Softricks game-engine, which relocates
the item-table to #FE00-FFFF (with the item-count at #83FF), in
order to extend the guardian-class table into #A400-A7FF.

But J.G. Harston's patch assumes that the item-table is still
located at #A400-A5FF. For each item, it starts by examining the
item-collected flag in page #A6, then uses two DEC H instructions to
access page #A4 - which is now part of the guardian-class table!

So it wrongly interprets the data in #A400-A5FF as the room-numbers
and cell-coordinates of the items, when in fact - in a Softricks JSW
game - these data represent guardian-classes.

--
Dr. Andrew Broad
http://geocities.com/andrewbroad/
http://geocities.com/andrewbroad/spectrum/
http://geocities.com/andrewbroad/spectrum/willy/

 

 

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