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Message: 6386

Author: andrewbroad

Date: 05/03/2008

Subject: Re: Item Oddities

 

dm_boozefreek wrote:

I've been working on the sequel to the man who sold the world, and
something rather strange has happened. I don't know if anyone else
has had this happen before?

The file I used for it was a .Z80 image I saved ages ago for this
game because it had Andrews 24 hour clock patched into it. I
converted it back to a .tap and started editing again, all seemed
fine I deleted all the items but the one in the bathroom, and
started making my game. I'm about 4 screens in and I decided to
playtest the screens I'd done. All the items have vanished from
where I put them, and all the items in the unedited rooms have
appeared back in their usual spots even though JSWED displays them
and counts them as being removed.

So I thought lets put the remaining 255 in the start room and chip
them off as I need them, low and behold when I playtested again the
startroom total gives you 109 items and the ones from the rooms
I've been working on are still not there, and the ones from the
unedited rooms are back. Even though JSWED says that the items I've
put down are in place and the ones I removed from the original
screens aren't there.

Could anybody shine a light on this rather odd situation?

This is indeed odd! My first suspicion is a dislocated item-table, so
I'll start with the following questions:

1. Which JSW-engine are you using?
(a) If JSW48, are you using either the Softricks engine or J.G.
Harston's extension?
(b) If JSW64, which variant?

2. What are the bytes at #8801 and #93D2? (they should be the same,
as they are the high byte of the item-table's base-address)

3. What are the bytes at #8803 and #8804? (low byte and high byte of
the address of the item-count)

4.(a) If using JSW48/128, what are the bytes at #93D4 and #93D5?
(should be the same as the two bytes in #8803 and #8804)

4.(b) If using JSW64, what are the bytes at #8590 and #8591? (should
be the same as the two bytes in #8803 and #8804)

Daniel wrote:

So what happens in JSW64:MM when you keep playing in a loop? I know
that you keep collecting the items, but what happens when you reach
256?

I haven't actually tested this, but the on-screen item-count is
stored as an ASCII string in #857C-857E (initially "000").

Whenever you collect an item, the code at #93F3-9408 starts at the
right of this string (#857E), and increments the ASCII code for the
units until it goes above #39 ('9'), whereupon it replaces it with
#30 ('0'), moves one character to the left in this string (#857D) and
processes the tens likewise, then moves one character left again
(#857C) and processes the hundreds. So that's fine up to 999 items.

But what happens when we collect the 1000th item? The on-screen count
will reset to "000", but the algorithm doesn't check for moving past
the left end of the string, so it will start overwriting memory-
locations it shouldn't, starting with #857B (until that goes above
#39), and working backwards through the "GameOver" string!

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

 

 

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