Date   : Sun, 06 Aug 2006 10:38:35 +1000
From   : "Richard Wilson" <richard@...>
Subject: Cursor sizes

Hi All,

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 something like I expected, especially since a 000 value 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 output is delayed, but the 6845 only has a delay
of
one character period (1 microsecond). Perhaps the ULA also delays the
DISPEN
from the 6845 in teletex mode to compensate for this.

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 results? I'd like to
know
where the cursor appears (pixel accurate if possible)?

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)?

Cheers,
Richard