Date   : Sun, 06 Aug 2006 10:38:35
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