Execunit

Because we don't actually have an order for this project yet, I am going to work on it on a part-time basis.  My intent is to pick pieces of the design, and extract the netlist.  This will provide knowledge about how the chip is put together, and hopefully make it easier for me to delegate work when we do get an order.

A very large block in the 68020 is called the execunit, which I presume is the execution unit.  It has 26 sections, and one of them has nodes labeled PC0 through PC31.  I thought that might be the program counter.  

It consisted of 32 pieces, with bits 0 and 1 different from the next 30 bits.  However, I did not find any signals going from bit to bit.  In a real program counter, I would expect a carry between the bits to allow it to be incremented.

I analyzed the bits, and just found two independent latches and some read logic.  This was somewhat disappointing, as I was expecting a counter.  Maybe it goes somewhere else to be incremented.  And maybe it isn't a program counter after all.

Date

2011-08-03