Talk:Motorola 6845

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There are broken links to http://www.howell1964.freeserve.co.uk/logic/vhdl_6845/ ... domain does not exist. 213.220.234.121 (talk) 12:23, 14 July 2008 (UTC)[reply]

Used in the Commodore 64 / 128 ???[edit]

The MOS Technology 6545 was used in the CBM-II, but I don't think it was ever used in the C64, which used the VIC-II (6567/8562/8564 (NTSC versions), 6569/8565/8566 (PAL)). The C128 used the VDC 8563/8568, but I don't think that this chip was based on any Motorola chip. — Preceding unsigned comment added by 192.98.122.205 (talk) 19:26, 11 February 2021 (UTC)[reply]

I think you're correct that the bit about the C= 64 is plain wrong. But the article MOS Technology 8568 does say [t]he 8568 embodied many of the features of the older 6545E suggesting a relation. Still, a relation is not an, as it says here, implementation of the chip, IMO. I'm going to be WP:BOLD and remove the references. Digital Brains (talk) 15:00, 12 February 2021 (UTC)[reply]

Thanks for making the changes. I just read Bil Herd's new book (Back Into The Storm), where he shares many details on the C128's VDC, the 8568. He makes it clear that the 8568 was NOT a superset of the 6545, that it behaved differently in many ways (double-write to registers required, for example), and that it also lacked features present in the Motorola chip (interrupts on completed execution, for example). So the MOS chip may have been originally inspired by the Motorola, but it was surely not software compatible, and thus it's not an implementation of the chip. Hope this helps to clarify things. joskirps

Apparent contradiction[edit]

"The 6845 is intended for character based displays"

However, the text then goes on to talk about using it for bitwise graphics, including several examples like CGA. It appears to be general purpose, and not specifically character-based? Maury Markowitz (talk) 18:36, 9 May 2019 (UTC)[reply]

The 6845 is character-based: it has been designed with the idea that video memory specifies which character is at which row and column, and the data from video memory will index a look-up of the pixels of that character in character memory. In this scheme, character memory is separate from the video memory and either fully constant (ROM) or semi-constant (loadable fonts). To use it for bitwise graphics, the trick is to constantly "change" what such a "character" looks like based on where you are on the display. I might be wrong in the details, but I think this is the general gist. I hope my explanation makes sense. Digital Brains (talk) 20:04, 9 May 2019 (UTC)[reply]

I understand what you are saying, I recall the Exity Sorcerer used this concept. However, that seems to contradict the bit-wise memory access? Or am I reading that incorrectly? Maury Markowitz (talk) 12:08, 13 May 2019 (UTC)[reply]

While you still program a "character height" into the 6845, what you can do with a bitmapped display is that you combine the linear character address with the character row address to produce a unique address for every group of pixels on the screen. So the first 4 pixels of the first display line could be stored in the first byte of memory and the next 4 pixels in the second byte of memory and so on. Every pixel has a unique location in memory, even though there might (or might not) be some interleaving of different display lines. Is that what you mean with bit-wise memory access? Digital Brains (talk) 17:09, 13 May 2019 (UTC)[reply]