I saw an advert on bey (China seller) for a Rockwell 6503 IC. The 6503 is a variant of the 6502 with limited number of pins to 28, the same 6502 core with less addressing space.
But something in the advert made me wonder, a 40 pin 6503?

I ordered one and indeed a 40 pin IC appeared. So this is not a 6503!
Then I searched the internet for the 338-6503 printed on the IC and found references to it being a R65C02.
And indeed, the 6502 testers recognized it as working 65C02. Weird!
The ‘6503’ was found in Apple IIe and Apple IIc computers.

I acquired a R6500/1EC IC. A beautiful white version. Week 40 1980, NMOS.The R6500/1EC is a development IC for the R6500/1 microcomputer family.

Rockwell R6500/1 family

The R6500/1 is a 6502 , ROM (2K) 64 bytes RAM ,4×8 bit I/O, a 16 bit timer with I/O pin.
The ROM contents are factory mask-programmed, not programmable by the user.

This makes it a standalone microcomputer, no extra RAM/ROM or decode logic required.

There are no external address lines or data lines, the 6502 is all internal except the interrupt lines and clock.

Rockwell R6500/1EC

To develop applications with the R6500/1 Rockwell developed the R6500/1EC. This IC has the same pins as the R6500/1 but adds address and data lines and relies on an external ROM.

Downloads

	R6500/1 One-Chip Microcomputer
	R6500/1E Microprocessor Emulator Device
	R6500/1 Microprocessor Programming Reference Card

The 6500/1 has been used for example in the AMIGA 1520 Plotter. I found this image on ebay:

Ian day published the commented source of the UPS065 monitor.