Restoring ROM character data after VDU 23

[SparkyNZ]

Just been playing with VDU 23 command on page 171 of the User Guide..

Once I've modified character data for 'A' (CHR$(65)) for example, how do I programmatically restore the ROM character data?

[james]

It looks as though "FX20,0 might be the answer. Within certain limits and assumptions.

On a BBC B, space is initially allocated for 32 User Defined Characters. *FX20,0 will reset all of these.

If you have 'exploded' the font (ie, reserved more memory for additional character definitions), then *FX20,0 will shink the allocated memory back down to the default amount and clear it see: https://beebwiki.mdfs.net/OSBYTE_%2614

If you have a Master, then "FX24,n can apparently be used, see https://beebwiki.mdfs.net/OSBYTE_%2619

[james]

Another trick, on a BBC B, is to copy the font definition back out of ROM.

Characters start in the OS ROM at &C000 and occupy 8 bytes each https://tobylobster.github.io/mos/mos/S-s4.html

All that snippet does is VDU23,65,[copy original 8 values from OS ROM]

Though note, the computer will still 'think' it's displaying a user-defined character. It's just one that looks like the original it's replacing

[SparkyNZ]

It looks as though "FX20,0 might be the answer. Within certain limits and assumptions.

On a BBC B, space is initially allocated for 32 User Defined Characters. *FX20,0 will reset all of these.

This is quite interesting. the *FX20,0 does indeed restore the characters' pixel data but the previous image remains onscreen.. So I guess the BBC must render characters to a sort of framebuffer (as it does with lines etc) which is different from the CBM machines with their character mapped text mode screens.



It's pretty cool though - here I have defined 'A' to be 3 different images and they can all be put onto the screen.

Update: Now why do my Image links never work when I use the Picture button?? It just displays "Image" in text above.

https://imgur.com/k9xgvPt

[SparkyNZ]

Another trick, on a BBC B, is to copy the font definition back out of ROM.

bbc font copy.png

Characters start in the OS ROM at &C000 and occupy 8 bytes each https://tobylobster.github.io/mos/mos/S-s4.html

That's good to know also - using the actual bytes.

The BBC never did have a concept of multicolour characters did it? (other than background and foreground colour rendering)

[SparkyNZ]

Another trick, on a BBC B, is to copy the font definition back out of ROM.

That trick didn't work for me on the b2 emulator. Maybe I'm not emulating the BBC B by default - I got spaces for the 'A' so maybe the character data is stored in a different location. Yeah, looks like I'm emulating the Master 128 at the moment.

[tom_seddon]

It's more complicated on the Master. The font data is at a documented location ($b900 in ROM 15 - see Master Reference Manual part 1, page E.4-5), but this is not very convenient to access from BASIC. But it's easy enough to use OSBYTE 25: https://beebwiki.mdfs.net/OSBYTE_%2619

(Note that the BeebWiki page previously incorrectly called this OSBYTE 24, which I've fixed. OSBYTE 24 does nothing on the Master.)

--Tom

[BeebMaster]

The BBC never did have a concept of multicolour characters did it? (other than background and foreground colour rendering)

In text mode (VDU4), characters will be printed on screen in the current text foreground colour.

In graphics mode (VDU5), characters will be printed on screen in the current graphics foreground colour. Using the GCOL extensions in the Graphics Extension ROM, or Master 128, the graphics colour can be a pattern rather than a solid colour, so this could be used to an extent to plot multi-coloured characters:

Any pixel on the screen can be any colour the current mode supports, so by plotting the individual pixels of a text character, a multi-coloured character could be displayed.

[wiggy]

I guess the BBC must render characters to a sort of framebuffer (as it does with lines etc) ...

Correct - at least for Modes 0 through 6: these do indeed draw everything to a framebuffer.

(Mode 7 is character mapped, though, using the '5050 to generate the characters).

[1024MAK]

Now why do my Image links never work when I use the Picture button?? It just displays "Image" in text above.

Because that's a link to a web page and not to an image format that the forum software recognises.

It is possible to attach an image in-line. Use the Attachments tab below the text edit box.

Mark

[SparkyNZ]

Because that's a link to a web page and not to an image format that the forum software recognises.

I could have sworn I'd used this method of attaching screenshots in the past on this website. Not to worry. I'll try pasting the actual image next time or using an attachment.