Date   : Wed, 09 Aug 2006 21:55:31 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Cursor sizes

>Message-ID: <009d01c6b8f0$a740c720$800101df@...>
 
"Richard Wilson" <richard@...> wrote:
> I'm stuck with cursor sizes. I've found some stuff on the Wikki
> which states the three cursor bits in the ULA are as follows:
> 
> Bit 7 - Segment 0 (1 byte wide)
> Bit 6 - Segment 1 (1 byte wide)
> Bit 5 - Segment 2 (2 bytes wide)
 
That's correct.
 
> gives no cursor, however, it doesn't make sense in mode 7. The
> problem is that the cursor is already delayed by the 6845 for two
> character periods, so not showing the first byte of the cursor
> would delay it by yet another character period. I know Teletext
 
No, the cursor isn't delayed after any previous delay. The cursor is
delayed from the same place the output is delayed from:
 
Bitmap modes:
                    +--+  +--+  +--+  +--+  +--+
                  --+  +--+  +--+  +--+  +--+  +
                    Fetch
                    Display
cell 1 cursor       -------
cell 1+2 cursor     --------------
cell 1+2+3+4 cursor ----------------------------
 
Teletext mode:
                    +--+  +--+  +--+  +--+  +--+
                  --+  +--+  +--+  +--+  +--+  +
                    Fetch
                          Display
cell 1 cursor       ------
cell 2 cursor             -------
cell 1+2 cursor     -------------
cell 1+2+3+4 cursor ---------------------------
 
> Is there anyone with a real BBC who can test the various ULA
> cursor values in Mode 7, and perhaps Mode 1 also to compare the
 
MODE 7:
ULA b765:  000   100   010   110   001   101   011   111
 
          ..#.. ..#.. ..#.. ..#.. ..#.. ..#.. ..#.. ..#..
                 ~      ~    ~~      ~~  ~ ~~    ~~  ~~~~
MODE 1:
ULA b765:  000    100    010    110    001    101    011    111
 
          ..##.. ..##.. ..##.. ..##.. ..##.. ..##.. ..##.. ..##..
                   ~       ~     ~~       ~~   ~ ~~     ~~   ~~~~
 
# or ##: displayed character
~: displayed cursor
 
> Another question, if anyone can answer it. Is the cursor size
> determined by the CRTC clock rate, or by the pixel clock rate
> (Bits 2 and 3 of the ULA)?
 
The CRTC clock rate as set by b4. To see this, observe that the ULA is set
to:
        7 6 5 4 3 2 1 0
MODE 1: 1 1 0 1 1 0 0 F
MODE 4: 1 0 0 0 1 0 0 F
 
The pixel clock rate is the same (40 chars per line), but the MODE 1 CRTC
rate is twice the MODE 4 CRTC clock rate and consequently, the MODE 1
cursor width is twice the MODE 4 cursor width.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
BBC BASIC for Windows and Internationalisation
  See http://mdfs.net/Software/BBCBasic/Windows/ProgTips