More BeebEm MODE7 strangeness

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flaxcottage
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More BeebEm MODE7 strangeness

Post by flaxcottage » Tue Mar 19, 2019 12:05 pm

I archived the Acornsoft title "JARS" today. Whilst archiving I use BeebEm v4.14 to produce screen shots of the working programs. So far everything has worked perfectly, except for this title. :?

JARS uses MODE7 graphics to display the filling and emptying of jars of liquid. BeebEm does not display these processes accurately. It is as though the liquid graphics are shifted one place to the left. On a real BBC computer, however, the display is accurate. #-o

I investigated this further. I have two versions of JARS - version 1 and version 2. Both show this anomalous behaviour in BeebEm. It also does not matter which variant of BBC is emulated, the apparent error is the same.

Using real machines the program works correctly on BBC model Bs and BBC Masters using any filing system (Tape, DFS, RAMFS, ADFS, NET). (Technical note: The program JARS will only run on a BBC model B computer. Line 65 causes the problem. The real program JARS1 runs OK on any BBC micro.)

JARS1 will also run correctly on any of my RISCOS machines either natively or under !65Host emulation. It has been tested on A3000 to A5000 machines and on a Raspberry Pi running RISCOS.

JARS1 runs correctly under BBCB4W on all my machines that can use it - PC, Android, Linux.

If you want to check this out, JARS can be downloaded here - http://www.flaxcottage.com/Educational/ ... wnload.zip
- John

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hoglet
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Re: More BeebEm MODE7 strangeness

Post by hoglet » Tue Mar 19, 2019 2:48 pm

FYI, it works correctly in B-Em.

chrisn
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Re: More BeebEm MODE7 strangeness

Post by chrisn » Tue Mar 19, 2019 2:48 pm

The good news is that BeebEm 4.15 beta1 fixes this. I really must get this release finalised...

https://github.com/stardot/beebem-windo ... tag/4.15b1

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flaxcottage
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Re: More BeebEm MODE7 strangeness

Post by flaxcottage » Tue Mar 19, 2019 7:29 pm

@ Hoglet - I'm not surprised but I don't want to complicate things by using a different emulator as I've only just got used to this. :lol:

@chrisn - that's good news. I'll wait with baited breath, thanks.
- John

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jgharston
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Re: More BeebEm MODE7 strangeness

Post by jgharston » Tue Mar 19, 2019 9:27 pm

The first character on the line is 9E which is Hold Graphics. The second character on the line is 97 which is GraphicsWhite. As HoldGraphics is active that 97 should be displayed as the most recent held graphic, which at that point will be a solid blank block. Instead the 'Holding' is starting after the control code. This is a common gotcha in implementing a teletext display. There's a thread elsewhere investigating this in BeebEm. You can see the flaws in the graphics lines in the top left quadrant here:
Image

where it should look like:
Image

(PS: sorry for the huge pic, I can't get image size syntax working.)

Code: Select all

$ bbcbasic
PDP11 BBC BASIC IV Version 0.25
(C) Copyright J.G.Harston 1989,2005-2015
>_

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