7

Did the Motorola 6847 (Video Display Generator) have any memory onboard accessible to the user?

Looking at this video display generator, which was used in the MC-10 (TRS-80 MC-10 Computer), if memory on it could have been accessed by users, to expand the meagre 4kb of the MC-10. The Wikipedia page for the Motorola 6847 does not mention any onboard memory.

1 Answer 1

13

Did the Motorola 6847 have any memory onboard accessible to the user?

No it does not. It uses its 13 address lines to address up to 8 KiB of external display memory. In the MC-10 it uses part of the 4 KiB main memory. This is why most programs only use the basic text mode or 64x64x4 and 128x64x2 graphics modes. In fact, the higher resolutions (128×192x4 and 256×192x2) are only possible with a memory expansion.

to expand the meagre 4kb of the MC-10

That's what the 16 KiB expansion (Cat.Nr.26-3013) was meant for, bringing the system up to 20 KiB.

For those with a tight budget September '83 issue of Color Computer Magazine offered a modification stacking two additional 4016 2 KiB RAM chips on top of the existing and some TTL for decoding to bring it up to 8 KiB, providing the 6 KiB the high res modes need (*1).


It's important to keep in mind that the MC-10 was intended as direct competition to the Sinclair ZX-81 and Spectrum line (*2). So cost saving was top priority. That's why it had to go with a less expensive CPU than the Color Computer and no 6883 SAM (*3).

The Coco is like the full system design as shown on page 2 of the 6847 data sheet, while the MC-10 is exactly like the minimal setup shown on page 23 (sans the external character ROM that is).

On its own the 6847 VDG is a bit of an odd beast, as it does not contain any control/configuration registers. Mode setting is done by 5 dedicated pins, set in the MC10 via a 6-bit 74LS174 latch.

All timing is derivative of a single clock and memory addressing is fixed, starting from address zero and dependent on mode. That is why the video memory always starts from the RAM base at $4000.

In the CoCo a 6883 SAM - System Address Multiplexor - will do all addressing for the 6847 (and all other system components), essentially acting as DMA. It provides registers to move video memory to other addresses.

As mentioned, the MC-10 is intended as an absolute low-end device, so saving the 6883 and replacing the expensive 6809 and PIA(s) with a 6803 with an integrated port was a great step toward that goal.


*1 - And yes, there's a modern time 128 KiB memory expansion, called the MCX-128, as this video shows.

*2 - As well the VIC-20, still going strong in 1982/83.

*3 - Of course, all that saving, plus a long development time, meant that when the MC-10 was ready, in late 1983, it was too little, too late and way too expensive despite its 120 USD price tag). A larger memory might have changed this a bit, but not much ... then again, with 64 KiB, but maybe at a 199 USD price, it would have become a true C64 killer (which cost at that time 595 USD. Quite possible for a more simple computer with a production cost considerable below 50 USD. Except that would have meant for Tandy to take a risky shot - nothing the company was known for.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.