Re: BYTE Article of Mega II

["Henry" <apl2research(.a.t.)comcast.net>]

Hey Bryan.

> I stated earlier on the newsgroups.  I know Mega II chip is ASIC type. It 
> has eight 2 KB ROMs or one 16 KB ROM holding eight languages of character 
> fonts.  It is already had the answers how big ROM size can hold in the 
> Mega II chip.  I doubt to say there are other ROMs like RGB matrix.  It is 
> to be 3,000 gates for MMU, IOU, TMG, and GLU.
>    I can't imagine how big 3,000 gates are.  I wonder how did BYTE authors 
> know?
>  What do you think?

Good question.  I too had often wondered about this fact.  The Mega II being 
one of the more interesting ICs in the IIgs has always caught my eye. 
If/when I get a few extra bucks I was going to purchase a good microscope, 
have the IC decapped and take a look around to see what I can see.  Maybe I 
can learn a little of how the chip is laid out.

3k gates really isn't that big from what I've been finding out.  I have a 
feeling either that number is wrong or Apple, who I assumed released that 
number in a technical briefing Byte used, may have misstated the number to 
obfuscate and deter reverse engineering.  Although I guess without really 
knowing what is actually going on in the chip no one can really say for sure 
that 3k isn't enough, but it sounds low to me.  I'm pretty sure I remember 
reading that 10k+ was possible back then from VLSI in that 68 pin PLCC form.

I now have the code for the GLU unit.  When I get the chance I'll have to 
study it and see what's actually going on and what logic it emulates.  The 
TMG and GLU were pretty basic as far as custom logic goes.  The TMG only 
emulates something like 6 chips functions and GLU is probably something 
similar to that I have a feeling.  I assume the IIe which doesn't have a GLU 
uses standard logic.  Since the IIc needed to be as small as possible then 
the GLU was created.  Probably also saved Apple some money too like the 
HAL/TMG did in the IIe.

The IOU and MMU are quite a better example of what custom logic can 
accomplish.  Just compare a II+ board to a IIe board and you quickly realize 
how much space and power savings there was.  The HAL/TMG unit in the IIe has 
something around 200 gates according to the data sheet schematic and even 
more when viewed under a microscope.  And that's a 'simple' chip!  Now the 
MMU or IOU must have at least 1k each.  So 3k is possible, but I have a 
feeling there's much more in that chip then we think.  Of course I'm just 
speculating at this point and have no real data to back up my suspicions - 
yet. ;-)


-- 
Henry S. Courbis
www.GSE-Reactive.com
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