Posted By
JamesD
on 2016-09-21
13:53:07
|  Re: What would I do differently in the Plus/4
Placing the I/O lines on the 6510 was an ill conceived change IMHO. It was easy to do and reduced the parts count, but it ate up pins that should have been used for something else. And it meant no easy drop in replacement CPUs like the 65802.
A MOS 6509 could have been implemented using a similar technique as used by the 6803 and 65816.
Multiplex new address lines on the data buss. Just set the upper address lines when they need to change by loading then during the first part of the buss cycle. This requires an external latch and a signal from the CPU. Sadly, that means another pin and I have no idea where they would get that unless they combined pins with an encoder inside and used an external decoder to generate the proper signals. While it would work, it needs more support chips or a custom external chip. And it just seems like an ugly way to add one pin.
*edit* I'm referring to combining the 6509 with the 6510 I/O port here. Without the I/O port there are plenty of pins.
But that's not the chip I was talking about. The 6516 added features similar to the 6809, hence why it might have been referred to as the 6509... before the MOS 6509 existed.
|
