According to https://en.wikipedia.org/wiki/MOS_Technology_Agnus the Agnus, one of the three custom chips in the Amiga, that included the copper and blitter among other functions, contained a total of 21,000 transistors.

How many transistors did the copper and blitter contain? Or what fraction of chip area was devoted to them? Or does there exist an annotated die photo of the Agnus?

up vote 8 down vote accepted

I don't know any annotated die photos, or any other analysis of this chip, but there are some (fairly low resolution) photos out there, so in the spirit of opening a discussion, I thought I'd post this. Don't take this as gospel truth: I'm not an expert at this. I've spent a while reading about chip design, but have no practical experience, and this is a long way out of my comfort zone, but... the best die photo of Agnus I can find is a long way from being good quality, but here's my best guess at the structures I can identify there:

Agnus die photo

What I'm reasonably confident about: the section highlighted in red. There are either 12 nearly-identical or 24 mirrored units in this array. Given the unusual number, and that Agnus has 24 DMA controllers, this is almost certainly what these are.

I'm not so confident about the section highlighted blue, but this is my best guess for where Copper is. It's certainly a complex section of the chip with a high degree of feedback, and looks similar to other processor control circuits I've looked at.

I'm even less confident about the green section, but that's my best guess at the location of the blitter. It's about the size I'd expect the blitter to be, and has a degree of regularity that you'd expect from a circuit of that nature (a lot of similar functions handled with different types of shift and mask patterns, and so on). I'd be more confident, however, if it wasn't so far from the DMA channels that I'd expect it to be interacting with quite heavily, so I'd definitely bow to anybody else's reasoned opinions on where else the blitter might be.

  • Thanks! I'm surprised at the small size suggested for the blitter. That looks like it would be what, maybe 3-4,000 transistors? I was under the impression a 16-bit barrel shifter (which the blitter contains) was about that size by itself. What am I missing? – rwallace Nov 21 '17 at 1:37
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    A 16-bit barrel shifter consists essentially of 4 16-bit multiplexers. An NMOS multiplexer can be as simple as two transistors and a resistor per bit, so a 16-bit barrel shifter can be as small as 192 components, which would fit easily within the area I've highlighted. It would probably look something like the area with many diagonal connections towards the left hand side, which is one of the main reasons I thought this area a good candidate. – Jules Nov 21 '17 at 2:41
  • I would have thought it consisted of 16 16-bit multiplexers? But I misremembered, the figure I was trying to recall was 5,000 transistors for a 32-bit barrel shifter; at N^2 size, a quarter of that and four times your figure would bracket 1,000 transistors, which would be much more consistent with the highlighted area. – rwallace Nov 21 '17 at 14:18
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    For reference: circuit diagram of a barrel shifter (in this case a 32-bit one). Even this is far less than 5k transistors -- I believe there are designs that use more transistors to do more work in a single stage rather than needing log2(n) stages, and those are probably what you're expecting to see here, but this really isn't necessary for a chip operating at the speeds we're looking for here. In a 5um NMOS process, each gate in the path will add a delay of about 2-3ns (see books.google.co.uk/books?id=k803BQAAQBAJ&pg=PA377). Because the blitter needs to perform 2 DMA operations ... – Jules Nov 22 '17 at 20:40
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    ... per word it processes, each DMA operation requires 2 cycles of the 7.2 MHz system bus, and the bus is allocated a 2 cycle period between each DMA-available cycle, the blitter has over 250ns to complete each word. (Erm: missing circuit diagram from last comment is here ... patentimages.storage.googleapis.com/pages/US4583197-7.png) – Jules Nov 22 '17 at 20:47

I do not have any background in chipdesign, so I cannot add anything meaningful to Jules answer. But I backed Brian Bagnall's Kickstarter "The Amiga Years" where one stretchgoal was a poster with the fat-agnus schema. So I provide this to hope it helps.Agnus Schema

  • Welcome to Retrocomputing. This is useful information which doesn't really answer the question in itself, but I think it qualifies as a partial answer. Thanks for sharing it. – wizzwizz4 Mar 11 at 20:52
  • Hi wizzwizz, thx for the welcome.. yeah I know it does not answer.. If Jules wants to take the picture I can remove it.. – Peter Parker Mar 11 at 21:24
  • SE consensus seems to be that partial answers are allowed. I don't think you should delete it. – wizzwizz4 Mar 11 at 21:25

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