OSBYTE &14

 OSBYTE &14 (20) - Explode/implode user defined character font RAM and reset font
 On entry:
   X=0: characters &80 to &9F only are redefineable (default without Tube)
   X=1: characters &80 to &BF are redefineable
   X=2: characters &80 to &DF are redefineable
   X=3: characters &80 to &FF are redefineable
   X=4: characters &20 to &3F, &80 to &FF are redefineable
   X=5: characters &20 to &5F, &80 to &FF are redefineable
   X=6: characters &20 to &FF are redefineable (default with Tube)
 On exit:
   X=&00:  font permanently exploded (MOS 3 and later)
   X=&FF:  font explodable/implodable (before MOS 3, ie Electron, BBC, BBC B+)
   Y=&00:  font permanently exploded (MOS 3 and later)
   Y<>&00: new value OSHWM (not returned across Tube)
   
   BBC:     X=return value from Service Call &11, &FF if call unclaimed, Y=new OSHWM
   Master:  X=&00, Y=0
   RISC OS: X=&01 (first font bank reset), Y=&03 (number of banks reset)
   BB4W:    X=&20 (character under cursor), Y=&00

In the "imploded" state (X=0) only characters &80-&9F can be redefined with VDU 23, as only 256 bytes is permanently allocated for font definition by the MOS. Other values of X "explode" the RAM and allocate 1 to 6 pages of font RAM pushing up host OSHWM, allowing more characters to be defined.

This OSBYTE must be used with care within a language, as it can reduce the language's memory allocation. Service call &11 is issued to notify ROMs of the change in OSHWM. BASIC does not respond to this, so the *BASIC command should be issued to get it to reset PAGE.

If unexploded, characters &20-&7F are taken from the ROM, and characters &A0-&FF are duplicates of &80-&9F.

When a Tube coprocessor is fitted, the font is fully exploded by default, as this does not take up language memory, but it can be imploded if desired.

OSBYTE &14 with any value of X also resets characters &20-&7F to the ROM font. Characters &80-&FF are not altered, contrary to some manuals.

MOS 3 and later ignores the input value of X. The font is always fully exploded, and the definitions do not take up user memory. The only effect of OSBYTE &14 is to reset characters &20-&7F. Characters &80-&FF are not altered, contrary to some manuals. To reset all characters, OSBYTE &19 should be used.

You should not examine the returned Y value, as OSBYTE calls <128 are defined to only return information in the X register, and only the X register is returned to the user in some environments (such as across the Tube).

See Also

• OSBYTE &19 reset font
• http://mdfs.net/Docs/Comp/BBC/Osbyte00

Jgharston (talk) 14:13, 31 August 2015 (UTC) Jgharston 21:30, 26 May 2009 (UTC)
KJBracey 15:04, 5 December 2009 (UTC)