1st the usual disclaimer:
ttlworks once again joined and took over the majority of documentation work which I'm proud to reproduce here.
Without his tremendous collaboration this would not have been possible anytime soon!
And as in previous dissections I just copy&paste the text written by Dieter/ttlworks... which should not confuse
anyone if he speaks of himself... so here we go:
//Previous thread: MOS6530 (MCS6530-004 aka TIM) dissected
This thread is about a transistor level dissection of the 8726 REC (RAM Expansion Controller),
brought to you by Frank Wolf and ttlworks.
8726 was used in the Commodore REU (RAM Expansion Unit),
a cartridge to be plugged into the C64 and C128 expansion port.
Basically, the 8726 is a DMA\DRAM controller.
It's not possible to access the REU DRAM from the C64\C128 directly,
it's only possible to move/swap/verify blocks of data between REU DRAM and C64\C128 memory.
There were three models of the REU:
1700 (128kB of DRAM), 1750 (512kB of DRAM), 1764 (256kB of DRAM).
8726 came in DIP64 and PLCC68 package.
8726 was designed by Victor F. Andrade, who also did the 65CE02,
and later went to AMD for working on the K7.
And from what's in the chip, I have to say that he is very good at logic design.
Some more links:
I have to say, that me and Frank had greatly underestimated size/complexity of the circuitry within the 8726,
because half of the chip space is empty (I think that the amount of pads didn't allow for a smaller chip size).
The previous dissection was about the 6530, a manually routed design from 1975.
8726 is a CAD routed design from 1985, and to me it's a bit scary to see how the game of routing chip layouts
had changed within only 10 years.
Sometimes I had bumped into the size limitations when drawing schematics with the Eagle full version during this dissection,
so I started to wonder what CAD software and what workstation Victor Andrade had used there,
and how expensive CAD software plus workstation had been back in 1985.
Some of our readers are familiar with the game, when you need a PCB done fast.
You spend some time with drawing the schematics, then you throw the components from the schematics into a PCB,
and try to arrange them until the airwires between said components don't look too scary anymore.
And from there, it's autorouter plus some hand optimization.
To me, the 8726 chip layout has that particular smell.
For consistence with Frank's notation, low_active signals are named foo#, not /foo.
Orientation for all the chip pictures: RES# pad is North.