The Mill is a 6809 coprocessor board for the Apple ][.
From an eBay auction, here's a picture of a Rev B The Mill and some documentation.
>> On my Stellation Two "The Mill" card, U6 is a 74LS14 >> and the U14 socket has a daughterboard with two IC's >> and a switch plugged into it which is an "OS9 ROM >> Interface". The daughterboard replaces a 74LS367 with a PROM + 74LS367 which runs A15-A12 through a look up table in the PROM to rearrange the memory map to better suit OS/9. Without the daughterboard, the Stellation Mill card has two address maps. The first is one-one with the 6502, the other inverts A15. With the daughterboard switch enabled, the OS/9 mapping is: 6809 6502 0xxx 1xxx ... ... Axxx Bxxx Bxxx Dxxx L/C Cxxx Exxx L/C Dxxx Fxxx L/C Exxx Cxxx I/O space Fxxx 0xxx ZP+stack+text According to the instructions that come with the OS/9 upgrade, U6 is originally a 74LS14 but is replaced with a 74LS04 when you replace the 74LS367 with the daughterboard. Must be a timing adjustment. >> The ROM socket at U11 is empty on my board--does yours >> have a ROM? >> >Mine doesn't have a ROM either. >Having a quick look at the back of the circuit board where the >ROM is, you will see a large trace between pin 24 and 21. >You could fairly safely say that it has to be either a 2716 or >a 2532 which both require VCC on pin 21 to operate. The parts list shows a 2716 but A11 is fed through U3 pins 8 and 10 (exclusive NOR gate with pin 9 grounded turning it into an inverter) before reaching pin 18 (/PE on 2716 and A11 on the 2532). To enable the ROM you write to $80 to $C0m6 (setting bit 6 of the control register). $C0m7 disables the A15 address remapping when set. The manual actually says the socket takes a 2716/2732 but is wrong - 2716/2532 are needed. When enabled, the ROM occupies $F000-FFFF (2532) or $F800..FFFF (2716) and cannot be accessed from the 6502 - only the 6809 can read it. Let me know if you have any more questions - I have the OS/9 manuals and DOS assembler manuals that came with my card. -- David Wilson School of IT & CS, Uni of Wollongong, Australia
There was also a 68000 (OK, 68008) version of The Mill, called the McMill. Here's an advert for it.
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