Date   : Tue, 03 May 2005 11:43:31 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: BBC & Master VDU 'Flag'

Peter Craven wrote:

> I assume there is a flag/ memory location that
> indicates whether the VDU driver is in the 'middle of
> a routine' and awaiting further parameters. Does
> anyone know what/ where this is in The Master & BBC?

Yeah, it is accessed by OSBYTE 218 (Read/write number of items in the VDU
queue), and is stored at location &26A.

It contains 0 if nothing is queued, and otherwise 256 minus the remaining
number of characters.
e.g.

>PRINT ?&26A
         0
>VDU 19:A%=?&26A:VDU1,1,0,0,0:PRINT A%
       251

The VDU queue is located such that its last address is always &323.  You
can get a pointer to the next free location with:

LDA #&24
CLC
ADC &26A
TAX
\ next free location in queue at &300,X
\ last character written to queue at &2FF,X

Cheers,
Rich


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