Date   : Thu, 07 Feb 2002 17:42:06 GMT
From   : Thomas Harte <thomasharte@...>
Subject: Re[2]: Re[1]: Firetrack

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> Purely since no-one else seemed likely to, I've posted a 
> question on comp.sys.amstrad.8bit where everyone seems 
to 
> know everything, which no doubt will yield some clues. 

And to save time for everyone, here is the reply from a 
Richard Wilson :

"
On ALL CRTC types as far as I am aware, Register 5 comes 
into play after the Vertical Total match (i.e. Register 4). On A 
CRTC type 3 (ASIC), during the Vertical Total adjust period, 
the value in R5 is compared to the current raster line. 
Normally the raster line resets after matching Register 9, but 
during the Vertical Total adjust period it does not.

On A CRTC type 0, the operation is similar, but compares are 
actually performed 2 character periods after the start of a 
scan, where the ASIC does the compare at the start of each 
scan.

On A CRTC type 0 the compare is for equality (and Register 9 
compare).

On the ASIC, the scan line comparisons are all greater than or 
equal, but the character test are equality.

Regards,
Richard
"

For reference, an ASIC CRTC is, as the name suggests, an 
Amstrad functional equivalent to the 6845 resulting from a 
shrinking of the number of chips in an Amstrad CPC, and a 
type 0 CRTC is either a UM6845 (UM = UMC) or an HD6845 
(HD = Hitachi). The differences between a type 0 and a 
UM6845R or MC6845 (Motorola) appear to just be the presence 
or absense of a status register and whether the vertical sync 
period is hardcoded or read from register 3.

-Thomas
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