Storing palette information in screen dumps
             ===========================================
           File: Docs.Comp.BBC.Filing.Palette - Update 0.10
                Author: J.G.Harston - Date 12-10-1989

Screens can be saved to disk with commands such as *SAVE pic FFFF3000+5000
to save a MODE 0, 1, or 2 image. However, that only saves the contents of
the screen itself, it does not save the palette. That may not be a problem
if saving a MODE 2 screen as that has 16 colours, and the default palette
can be used. However, in a 4-colour or 2-colour mode the screen dump may
not neccesarily be using the default palette.

As screen images are only *LOADed, not *RUN, the palette information can be
stored in the file's execution address. If it is known what mode the screen
dump is for, then the file can be loaded to a known address, for example
with *LOAD file FFFF3000, and so the load address can also be used to store
information.

Execution address:			Load address:
    b0-b2   = colour 0                      b0-2  = colour 4
    b3-b5   = colour 1                      b3-5  = colour 5
    b6-b8   = colour 2                      b6-8  = colour 6
    b9-b11  = colour 3                      b9-11 = colour 7
    b12-b15 = screen mode

There is no space to store colours 8 to 15 in 16-colour modes, so it is
assumed that they are set to the same as colours 0 to 7.

The palette can be set with the following code:
    tmp%=load% AND &FFFF
    FOR logical%=7 TO 4 STEP -1
    VDU 19,logical%,(tmp% DIV 512)AND 7,0,0,0:tmp%=tmp%*8
    NEXT
    tmp%=exec% AND &FFFF
    mode%=tmp% DIV &1000
    FOR logical%=0 TO 3
    VDU 19,logical%,(tmp% DIV 512)AND 7,0,0,0:tmp%=tmp%*8
    NEXT

Setting the palette information in reverse order ensures that low numbered
entries take priority in 4-colour and 2-colour modes.

The palette information can be written with the following code:
    DIM ctrl% 15:X%=ctrl%:Y%=X%DIV256

    tmp%=mode%
    FOR logical%=3 TO 0 STEP -1
    ?X%=logical:A%=11:CALL &FFF1
    tmp%=tmp%*8+(X%?1)AND7
    NEXT:exec%=tmp%
    tmp%=0
    FOR logical%=7 TO 4 STEP -1
    ?X%=logical:A%=11:CALL &FFF1
    tmp%=tmp%*8+(X%?1)AND7
    NEXT:load%=tmp%