"Outline specification for the BBC MICROCOMPUTER system"

[1024MAK]

Possibly means using a 6845 instead of the 6847 CRTC? Plus the same character ROM, or something very similar.

But if they replaced the 6847 with a 6845 and support circuitry, I thought they /Chris Curry would have mentioned the higher graphics resolutions/ flexibility possible?

Keep in mind that the 6845 was originally intended for character displays. Either the pixel data coming from a ROM with the font stored in it (which is effectively what MODE 7 is in a Beeb) or from RAM. Either separate to the character code data, or in the same RAM. This system for a character text display uses far less RAM compared to a graphics video system. The BBC Micro does not use these modes to generate a character mapped display, instead using the 6845 and the VideoProc to produce graphic pixel displays (including the so called text modes).

Mark

[B3_B3_B3]

Possibly means using a 6845 instead of the 6847 CRTC? Plus the same character ROM, or something very similar.

But if they replaced the 6847 with a 6845 and support circuitry, I thought they /Chris Curry would have mentioned the higher graphics resolutions/ flexibility possible?

Keep in mind that the 6845 was originally intended for character displays. Either the pixel data coming from a ROM with the font stored in it (which is effectively what MODE 7 is in a Beeb) or from RAM. Either separate to the character code data, or in the same RAM. This system for a character text display uses far less RAM compared to a graphics video system. The BBC Micro does not use these modes to generate a character mapped display, instead using the 6845 and the VideoProc to produce graphic pixel displays (including the so called text modes).

Mark

Yes, but the bbc's specification required hi res graphics, so if the 6847 was ditched as BigEd suggested then the 6845 would have to supply them as well as the >=40column text, like the bbc micro does as you described.

[1024MAK]

Yes, but the bbc's spec required hi res graphics, so if the 6847 was ditched as BigEd suggested then the 6845 would have to supply them like the bbc micro.

The point is, that at the time (what became) the BBC Micro was being designed, RAM was expensive. So, I expect various different video display systems would have been looked at. Initially, that may have included looking at how the conventional use of the 6845 worked. We now know what Acorn came up with. But as Commodore, Sinclair, Texas Instruments and others showed, other ideas existed.

It’s not too difficult to come up with an impressive video system if you ignore the price of the design. But trying to come up with a design that is cost effective, and which does not use up lots of expensive RAM is far more of a challenge.

I keep mentioning the cost of RAM, because the BBC Model A only had 16K bytes of RAM. When the computer was being designed, which model do you think Acorn thought would sell the most?

Mark

[B3_B3_B3]

Yes, but the bbc's spec required hi res graphics, so if the 6847 was ditched as BigEd suggested then the 6845 would have to supply them like the bbc micro.

The point is, that at the time (what became) the BBC Micro was being designed, RAM was expensive. So, I expect various different video display systems would have been looked at. Initially, that may have included looking at how the conventional use of the 6845 worked. We now know what Acorn came up with. But as Commodore, Sinclair, Texas Instruments and others showed, other ideas existed.

It’s not too difficult to come up with an impressive video system if you ignore the price of the design. But trying to come up with a design that is cost effective, and which does not use up lots of expensive RAM is far more of a challenge.

I keep mentioning the cost of RAM, because the BBC Model A only had 16K bytes of RAM. When the computer was being designed, which model do you think Acorn thought would sell the most?

Mark

I didnt mention the price of the design, I just wondered what hardware Chris Curry was referring to in his original Superatom letter...

But the bbcs outline specification required high-ish resolution graphics so a solution supplying both text mode and bitmapped graphics seems required:--- in Chris Currys original Super-atom proposal he mentioned that if the 6847 32column (Capital only?) text was not satisfactory , 40 columns could be provided by other hardware..... if the 6847 was kept then the teletext board (that he had already mentioned in that letters teletext section) would have given 40col text, but he didnt mention it again in the earlier graphics/text section 1.4 of letter , so was presumably thinking of something else , it seems reasonable it might be something existing or being developed for the system series, so I wondered if anyone knew what he might have been referring to.....

It would seem nobody knows...

[algenon_iii]

Possibly means using a 6845 instead of the 6847 CRTC? Plus the same character ROM, or something very similar.

But if they replaced the 6847 with a 6845 and support circuitry, I thought they /Chris Curry would have mentioned the higher graphics resolutions/ flexibility possible?

There's actually another issue, additional chips would have made the beeb even more expensive. It's why they went with the mega ULA for the electron, to get chip count down.

Compare the BBC motherboard to other 8-bits and it has more in common with the expensive 1970s designed Apple II or Commodore PET than the cheap early 80s VIC-20, C64 or Spectrum.

[B3_B3_B3]

1) The Apple ii already existed, with large software base
when the BBC were looking for a micro: if a uk-ified* apple ii was manufactured in the Uk** would that have satisfied the BBCs Britishness test? The early apple ii variant's pal compatibility/circuitry of 1981 era was presumably not good enough for the BBC so BBC engineering might have had to be involved? Lower case and 80 columns also needed....?


There was a z80 card for the Apple and a teletext graohics mode could be provided on an optional card for those who needed it.
Comal or Basic could have been commissioned from a software company or apple?

2) Following on from above line, rather than try and persuade acompany to implement a new superbasic, a perhaps simpler? alternative would have been to require an ms compatible basic as well as as a Comal implentation be fitted, given that Pascal seemed the preferred acadamic 2st language at some point so the Pascal-like Comal seems a logical choice for a micro....?


*but with graphic resolution/modes compatible with existing software unlike the https://en.m.wikipedia.org/wiki/ITT_2020 which seems to implement artifact colour on Pal? but incompatibly in a weird way..

**or a minimum amount given some actual bbcs were made abroad?

[scruss]

One of the requirements from the BBC was broadcast quality graphics. The Apple II had, um, smudges on the screen might be the most charitable way of putting it. Its whole colour system was based on fudged timings of an incompatible video system.

I don't know much about ITT's 2020 (apart from having one for shipment, briefly) but it was on the market for such a short while that it may not have been a viable contender. It does have lots of local content (so many UK-made ITT memory chips ...) but the licensing from Apple might have limited the BBC's interest.

(I am an Apple II owner/user, btw. They're endearingly weird machines.)

[B3_B3_B3]

One of the requirements from the BBC was broadcast quality graphics. The Apple II had, um, smudges on the screen might be the most charitable way of putting it. Its whole colour system was based on fudged timings of an incompatible video system.
...

(I am an Apple II owner/user, btw. They're endearingly weird machines.)

Well, I did say I thought the video circuitry would need redone (as in completely, just keeping resolution and memory layout) to be Pal compatible to required BBC srandard whilst keeping software compatibility..

[B3_B3_B3]

On a related note given that the apple ii was 1Mhz until the gs, given the choice, would be beeb model B owners/users pick

A) 32k but 2Mhz 6502

B) 48k linear ram (ie all os and basic's current functionality was repaged to fit in upper 16k, sideways roms to be 8k in that space... I accept the graphics limitations that 80columns would have to come from a text mode(an encouragement to push the teletext chip a la Mr Beesley?) Or that a bitmap based 80x25 column mode would involve cpu contention(less of a problem if only writing text?) And mode 2 and 1 would need some sort of 'cheat' to support more colours on screen such as palette windows. The video Ula could perhaps be viewed as something that coukd be upgraded later ...

I'd still pick B.

[BigEd]

You're picking B for Apple, but I'd pick A for beeb! To cook up a reason, a system designed to run at 2MHz has a chance of later expanding the RAM, but a system with 48K RAM running at 1MHz is more difficult to upgrade to 2MHz. Plus, with sideways RAM, one has something rather like 48k linear, if an application is written that way. Plus also, George's 64k in-socket upgrade gets even more linear RAM, at modest cost, and it looks like Solidisk offered something very like his upgrade, even back in the day. Plus even more, the benefit of a Tube-connected second processor is huge, at which point the amount of linear RAM in the host is much less critical.

[B3_B3_B3]

I'd still prefer simple 48k linear for basic, and gfoot's? supershadow is a bit new unless a time machine is available and I never could work out what solidisk ads were claiming and how compatible it was, coupled with a lack of reviews of their weirder sounding boards made me cautious..

A 6502/65816 2nd processor would make even more sense to speed up a 1mhz beeb...? most 80s 6502 home computers were 1Mhz....

[BigEd]

Yeah, Solidisk had something similar, and George's design uses only parts available BITD, so the SuperShadow is certainly part of the possible thought experiments.

It's a bit hard for me to see how the BBC would have chosen a 48k machine, because you can always cost-reduce such a machine to 32k, and so there would have been great pressure to do that. (As we know, the Beeb model A had only 16k and was expected to be the most popular model. The Beeb needed an adequate amount of RAM, rather than a maximal amount.)

So, if what we're doing here is asking 'how might the BBC's choice have come out differently', I don't think a 48k RAM machine is very likely. (There are probably other threads where we are asking 'how might the Beeb have been different' - like for example Bit of fun - One change only)

To answer your much earlier question, I don't think a UK-manufactured machine from a US company would have been acceptable. That's from my understanding of the decision process.

[B3_B3_B3]

1) I should have said expandable to (from B's 32k) (I typed that somewhere... ), just like the apple ii: but according to wiki, by 1981 all apple ii plus machines came with 48k https://en.m.wikipedia.org/wiki/Apple_II_series.

Video shadow ram does't help if programming in mode 7.

2) Ok, thanks, no apple-ly beebs then (but now https://github.com/bobbimanners/Applecorn )

[BigEd]

applecorn - I like it!

[B3_B3_B3]

You're picking B for Apple, but I'd pick A for beeb! To cook up a reason, a system designed to run at 2MHz has a chance of later expanding the RAM, but a system with 48K RAM running at 1MHz is more difficult to upgrade to 2MHz. ...

But the beebs interleaved video access meant 2mhz mpu needed 4mhz ram bandwidth to cover graphics leading to a long wilderness period where its 1mhz equivalent competitors had more ram more affordably and the beeb b had no software backed way to catch up..

Perhaps a compromise could be 2mhz mpu Rom at 2mhz, zero page and stack at 2mhz by using a small 512byte-ish static ram, other ram at 1Mhz that might have encouraged a max 48k linear ram even if not populated at first.

Adding 16k of sideways ram to a 32k bbc b is ok for games but doesn't help high level programmers or applications (unless you fits lots and officially take the Bas128 approach of language/app in main ram using multiple 16 sideways ram pages for data(ie basic code)) , but that would have needed encouragement from Acorn by supplying such language/application versions. It might have allowed of official Acorn opcode originating address controlled shadow ram to be skipped, thus avoiding a damaging court case hiatus/disagreement/other better description with Aries over the patent/patent application.

Otherwise A linear 48k max beeb b would just have seemed to have a longer useful life without needing the complex patent-defendable shadow ram imo EDIT I would include Rich Talbot-Watkins idea of keeping the bbcs 4 16k chunks architecture but allowing the maxing out of linear ram to 48k to move the vdu memory beside the basic/sideways rom area where it would paged by usual software method by MOS vdu drivers , adding some 16k vdu modes, and just leaving any extra space for machine code or workspace for language.. Actually, I think Rich Talbot-Watkins idea is a much better basis for the B+ and should avoid the 20k shadow ram court case.

EDIT Interestingly? The originally 48k linear max ram Apple II, despite its (charming?) idiosyncrasy,   its successor II models seemed to facilitate the increasing need/desire for more memory better than the ( more seemingly elegantly designed)  beeb  at least from  an application user/high level language programmer pov ...?

EDIT as paulb as mentioned elsewhere, the original Bbc computer could have satisfied the Bbc studio need for proper broadcast quality teletext output with an external addon teletext box, so if the Bbc could have been persuaded to drop the mode 7 requirement, the money saved on the mallard 5050 and support circuitry could have been used to make 32k dram go further:

..............a serious computer needs an 80 column mode, so instead of teletext mode, instead supply a conventional ROM fed text only 80x25 character mode (and 40x25), needing only 2k(and 1k), so 26k free for serious apps. (Or would 80column teletext be cheaper
if possible on the beeb) An E00 dfs would mean shadow ram provided little extra for serious 80 col text apps. An 80 column text mode would also mean perhaps bitmapped modes 0 and 3 could be dropped reducing the required video memory bandwidth if desired ;*. Perhaps the character Rom could hold the appropriate teletext chars, allowing a monochrome mode 7 simulation.
More memory for machine code games could be provided by sideways ram, so if Bbc b+ had been shadow free just with 2 sideways ram banks, a plain bbc model b could be easily upgraded to match it compatibly, and no Shadow ram court case trouble...


*(in that case presumablyreducing colour bitmapped modes bandwidth would also be desired: perhaps by OS support for redefining pallette in different screen areas, a 160x128 resolution mode2 etc)

[BigEd]

Just to add a link to this 8 page pamphlet which includes nice diagram:
BBC Microcomputer System - Information Sheet G2

In fact, there's a slightly nicer scan here on Chris's Acorns) but also more linked documents in the parent page.

Related post from upthread:

Maybe not worthy of a new thread, but a friend found a copy of the 1982 “BBC Microcomputer System Technical Description”, which is now up on archive.org. Seems mostly be be a BBC BASIC language spec plus some physical/electronic details.

There's another scan of this document which has two extra pages: the keycodes for the keyboard (RTR mentioned), and a note about things still under development:

Code: Select all

The present description does not include details of various
parts of the BBC Microcomputer System which are still being
developed. These include:
   The teletext adaptor
   The Prestel adaptor
   The single-drive 100 Kbyte disk store
   The dual-drive 800 Kbyte disk store
   The 6502 second processor expansion
   The Z80 second processor expansion
   The CP/M - compatible disk system.

Outline specifications of these items will become available
separately in due course. If you would like to be sent a
copy, please send a large s.a.e. (324 x 229 mm and 20p stamp)
to:
   TECHNICAL SPECIFICATION (2)
   BBC MICROCOMPUTER SYSTEM, P.O. BOX 7
   LONDON W3 6XJ
   
           September 1982