Date   : Tue, 07 Dec 2004 10:00:08 +0000
From   : Richard_Talbot-Watkins@...
Subject: Re: mode switching

jgh wrote on 07/12/2004 08:39:14:

> You need to program the CRTC (Cathode Ray Tube Controller) and the
VIDPROC
> (video processor). You will only be able to use the VDU to write to
screen
> in one of the MODEs. It is easier to POKE messages to a MODE 7 screen, so
> use real MODE 1 and a fake MODE 7.
>
> The following program demonstates using &3000-&7BFF to hold MODE 1
> contents and &7C00-&7FFF to hold MODE 7 contents without clearing either
> of them.
>
> [code snipola]

Yes, this is a much better solution than mine.  Setting the VDU variables
is the tricky bit, but letting the OS initialise it for Mode 1 and then
poking directly for a fake Mode 7 is infinitely preferable!

> If you are writing in machine code and know your program owns the
hardware
> and is running in the I/O processor, it is more efficient to write the
> data directly to the controllers:
>
> .ModeSetup                          :\ On entry, X=table offset
> LDY #0                              :\ Start at register 0
> .MSLoop
> LDA ModeTable,X:STY &FE00:STA &FE01 :\ Program CRTC
> INX:INY:CPY #16:BNE MSLoop          :\ Write 16 registers
> LDA ModeTable,X:STA &FE20           :\ Write VIDPROC value
> RTS

Beware of writing directly to &FE20.  If the OS IRQ handlers *ever* get a
look in, they will return ?&FE20 back to what they think it should be for
the current mode.  This is weird but seems to happen for some reason (on a
Master at least).  Safer to use OSBYTE 154, unless, as you say, your
program owns the hardware (for example, by performing all IRQ handling
yourself).



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