Path: utzoo!mnetor!uunet!husc6!rutgers!psuvax1!vu-vlsi!cbmvax!fred
From: fred@cbmvax.UUCP (Fred Bowen)
Newsgroups: comp.sys.cbm
Subject: Re: vdc
Message-ID: <3162@cbmvax.UUCP>
Date: 18 Jan 88 22:37:02 GMT
References: <2951@killer.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 58

In article <2951@killer.UUCP>, bobc@killer.UUCP (Bob Calbridge) writes:
> Those of you who have mentioned the upgrade to the vdc might be
> able to answer a question concerning that chip.  There is a 
> register that controls the starting address of the character set.
> It is the upper three bits of register 28 that does the controlling.
> The article in The Transactor did not specify just how this works.
> Bob

You had to ask.  Simply put, the starting address of the character data
can be on 8K boundarys (usually) or sometimes on 16K boundarys.  Since I
suspect that is not explanation enough for true techies, here's the poop:

During the visible part of the frame, the 8563 (8568) reads character
data from display memory.  The address of each word of character data
is determined by register 28 (bits 7-5), bit 7 of the attribute byte,
the character pointer, and the internal scan line counter.  There are
two possible ways to determine the character data address, determined
by the contents of register 9, the character vertical total.


(1) If the contents of R9 is less than 16, then four bits of the internal
scan line counter will be utilized, and the character data address will
be calculated as follows:

BIT#	15 14 13     12     11 10  9  8  7  6  5  4   3  2  1  0
        --------     --     -----------------------   ----------
SOURCE  R28(7-5)  attribute     character pointer    scan line ctr
                    bit 7

If attributes are disabled (R25 bit 6), then bit 12 of the character data
address will be 0.  Each character defined in display memory will consist
of 16 bytes (hence the 4 bit scan counter), even if the number of scan lines
actually displayed is less than 16.


(2) If the contents of R9 is greater than 15, then five bits of the internal
scan line counter are utilized, and the character data address will be
calculated as follows:

BIT#	 15 14       13     12 11 10  9  8  7  6  5   4  3  2  1  0
        --------     --     -----------------------   -------------
SOURCE  R28(7-6)  attribute     character pointer     scan line ctr
                    bit 7

If attributes are disabled (R25 bit 6), then bit 13 of the character data
address will be 0.  Each character defined in display memory will consist
of 32 bytes (hence the 5 bit scan counter), even if the number of scan lines
actually displayed is less than 32.

So begins a new year.
--

-- 
Fred Bowen			uucp:	{ihnp4|rutgers|caip}!cbmvax!fred
				arpa:	cbmvax!fred@RUTGERS.EDU
				tele:	215 431-9100

Commodore Electronics, Ltd.,  1200 Wilson Drive,  West Chester,  PA,  19380