6502 traces of 6501

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The 6501 was the original product from MOS Technology (who, as a team, had recently left Motorola where they worked on the 6800.) It was pin-compatible but not code-compatible, and was withdrawn in a deal which left MOS free to sell the 6502 (even though it has many resemblances to the 6800)

There are several places on the NMOS 6502 where the layout shows the history: it is a modified 6501 and not an independent design.

Contents

Pinout differences

As a quick piece of background: the 6501 was pinout compatible with 6800 - it could use the same socket. But the pins were not identical - 6501 and 6502 cannot tristate the address pins so 6501 was not compatible with a 6800 socket in a system using DMA.

Pin 2: Halt on 6800, RDY on 6501 and 6502. Compatible if not used. Pin 5: VMA on 6800, tied high internally on 6501

Data Pads

At top left of the 6502 die, you'll see two pads, the lower of which has odd metal routing next to it.

This is because these two pads are DB0 and RnW, bonded to pins 33 and 34 on 6502. On the 6501, these pads would have been routed - using the odd extra metal - to DB1 and DB0 and bonded to pins 32 and 33.

We don't know what on 6501 occupied the space where 6502 has pad DB1

Data Bus Enable

The 6502 doesn't have the DBE input found as pin 36 on 6501/6800. But the signal exists on the chip - it's tied to cclk, which is the chip-internal version of phi2. (This is why the 6502 tristates the data bus for approximately the first half of each clock cycle)

Here's the location of the via which ties the two signals together.

Here's a picture showing the two signals running in parallel, even though they are on 6502 the same signal. See how they even interleave. This would represent a waste of space and effort if it the 6502 was an original design, but it's a minimal change to the 6501.

Here's a location where the Rockwell version of the chip has an extra short, compared to the MOS 6502D (and also showing that Nintendo's 2A03 has the MOS version of the layout) Compare-r6502-balazs-2a03-dbe-short.png

Resources

  • KIM-1 Hardware Manual showing pinouts of 6800 and 6501 (on erik.vdbroeck's site)
  • Byte article November 1975 anticipating the $20 pinout-compatible 6501 (article written in August - 6501 expected in September.) (on Michael Holley's site)
  • MOS Technology history mentioning $200k settlement with Motorola
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