MOS Technology MCS6501 Processor

MOS MCS6501 – November 1975

One of the classic stories of the 1970’s microprocessor boom times was that of MOS Technologies at WESCON (Western Electronics Show and Convention) on September 16th 1975 in San Francisco.  MOS Technology was a newcomer to microprocessors.  They had with them two brand new processor design, the MCS6501 and the MCS6502 which they hoped to sell on the floor at Wescon, for $20 and $25 each.  However Wescon forbid sales on the convention floor, so quick thinking by MOS Technologies Chuck Peddle directed people to a hotel room, where “the beer was free and chips were $25.”  In the room were jars of 6501 and 6502 processors, to give them impression that these were in full production.  In reality the bottoms of the jars were filled with defective parts.  It was no matter, the 6500 series was a huge hit, led largely by its availability, low price and marketing to everyone (not just ‘big corporate users’).  The 6500, and specifically the 6501 have an interesting story leading up to that fateful day at WESCON.

Motorola XC6800B – July 1975 – Pre-production part, not something MOS bothered with.

It begins at Motorola, where Chuck Peddle, Bill Mensch and several others were employed in the early 1970’s design the MC6800 processor and its peripherals.  The 6800 was not a bad design, it was however, very expensive, a development board for it costing over $300.  Chuck worked largely as the 6800 system architect, ensuring all the ICs worked well together and were what was needed to meet customers needs.  He attended many calls to potential clients and noted that many were turned off by one thing, price.  With that in mind he sought out to build a lower cost version of the 6800 using some of the newer processes available (specifically depletion mode NMOS vs the enhancement mode of the 6800).  Motorola management wouldn’t hear it, they wanted nothing to do with a lower cost processor available to the masses.  And with that, Chuck, Bill and over half the 6800 team left.

They ended up at MOS Technologies, which at the time was owned in large part by Allen/Bradley.  It was there, at MOS under the direction of Chuck Peddle that the 6501/2 was borne.  The 6501 was to be pin for pin compatible with the MC6800, though with a completely different instruction set.  One could replace a 6800 in a system with a 6501 and only really need to change the programming. It was made on a 8 micron depletion mode NMOS process where as the 6800 was made on a 6 micron enhancement mode.  Even at the large process node, depletion mode results in smaller features, allowing more die per wafer.  The 6502 was very similar, it actually used nearly the same mask set, the only difference being, the 6502 had the on chip oscillator enabled, and a slight difference in pin out.  The 6501 required an external 2 phase clock generator (just like the 6800)

The 6501 was never really meant to be a mass market chip, Chuck often said it was “an in your face to Motorola.”  Motorola took notice, and sued, resulting in Allen/Bradley pulling out of MOS, the 6501 being removed from sale, a $200,000 fine, and ultimately a greatly lowered price for the 6800 and many other processors.  It is commonly assumed that Motorola forced the redesign of the 6501 into the 6502 but that was not the case, they, and the 6503, 04 and 05 were all made concurrently and available at the same time.  Motorola forced the end of selling the 6501 but it was never meant to sell en masse, the 6502 was simpler and easier to design in.  The 6502 lived on, with MOS being purchased by Commodore.  The 6502 was used in such famous machines as the Apple 1, the Commodore 64, and many many more.  The 6502 design (in CMOS) continues to be made today by several companies, nearly 40 years after its debut with free beer.