Font OSBYTEs and Extensions
===========================
*FX20 and OSBYTE 20
-------------------
On entry: X=&00+n : set font explosion state, resetting characters &20-&7F
                    n=0 &80-&9F   n=4 &20-&3F, &80-&FF
                    n=1 &80-&BF   n=5 &20-&5F, &80-&FF
                    n=2 &80-&DF   n=6 &20-&FF
                    n=3 &80-&FF   n>6 &20-&FF
          Extensions:
          X=&07   : reserved
          X=&08+n : explode full font and set font base
                    the font base is set to X*256, eg X=&11 sets the
                    font to occupy &1100-&17FF.

On exit:  X<&80   : font permanently exploded
          X>&7F   : font explodable/implodable

Implementation:
On exit:  Master:       X=&00
          RISC OS:      X=&01 (first font bank reset)
          BB4W:         X=&20 (character under cursor)
          BBC/Electron: X=&FF (return value from Service Call &11)

All calls reset characters &20-&7F. On the Master and later the font is
always fully exploded, and OSBYTE 20 just resets characters &20-&7F.

BB4W and similar systems just return without performing any action.

*FX25 and OSBYTE 25
-------------------
On entry: X=&00+n : set font group n to ROM, resetting characters
          X=&80+n : set font group n to use RAM
                    n=0 &20-&FF   n=4 &80-&9F
                    n=1 &20-&3F   n=5 &A0-&BF
                    n=2 &40-&5F   n=6 &C0-&DF
                    n=3 &60-&7F   n=7 &E0-&FF
          Extensions:
          X=&08+n : reserved
          X=&10+n : soft teletext functions
          X=&88+n : reserved

On exit:  X<&80   : implemented
          X>&7F   : unimplemented

Implementation:
On exit:  Master:       X=&00
          BBC/Electron: X=&00 (if implemented)
          RISC OS:      X=&01 if X=&00 on entry
          RISC OS:      X=preserved if X>&01 on entry
          BB4W:         X=&20 (character under cursor)
          BBC/Electron: X=&FF (unimplemented OSBYTE)

If you wamt to ensure the font is fully exploded and reset, possibly
changing PAGE. In BASIC:
    A%=FNbyte(25,0):*FX20,6

In 6502 machine code:
    LDA #25:LDX #0:JSR OSBYTE  :\ Try 'reset all font'
    LDA #20:LDX #6:JSR OSBYTE  :\ 'explode full font'

If you want to reset the font, but leave the explosion state unchanged and
avoid overwriting memory at PAGE. In BASIC:
    A%=FNbyte(25,0):A%=FNbyte(20,FNbyte(182,&FF00))

In 6502 machine code:
    LDA #25:LDX #0:JSR OSBYTE           :\ Try 'reset all font'
    LDA #182:LDX #0:LDY #&FF:JSR OSBYTE :\ Read current explosion state
    LDA #20:JSR OSBYTE                  :\ 'explode full font'

The BASIC code uses:
    DEFFNbyte(A%,X%):LOCAL Y%:Y%=X%DIV256:=((USR&FFF4)AND&FF00)DIV256

On platforms such as BBC BASIC for Windows, use the following code:
    DIM ctrl% 31:X%=ctrl%:Y%=X%DIV256
    ...
    DEFPROCfont_Reset
    A%=FNbyte(25,0):IF FNbyte(20,FNbyte(182,&FF00))<>32:ENDPROC
    A%=10:FOR B%=32 TO 255:?X%=B%:CALL &FFF1:VDU 23,B%
    FOR C%=1 TO 8:VDU X%?C%:NEXT:NEXT
    ENDPROC

*FX163 and OSBYTE 163
---------------------
On entry: X=&80+n : set font control for font group n for n=1..7
          Y=0     : set font page soft (RAM)
          Y=1     : set font page hard (ROM)
          Y=2     : copy current page to RAM (no effect if soft)
          Y=3     : copy ROM page to RAM
          Y>3     : set font page soft and RAM page number to Y
          Y=???   : set font uses SROM bank

On exit:  X,Y     : preserved if unimplemented
          X<&80   : implemented