Re: AppleLogic.org Question

[David Wilson <mcs6502@gmail.com>]

On Apr 4, 12:02 pm, "John B. Matthews" <nos...@nospam.invalid> wrote:
> LS367 for U12, U13, U14
> note: pin 1 & 15 of LS367's connect to VMA.
>
> Um, I'm guessing VMA isn't vanilmandelic acid or video music award. :-)

VMA is the Mill's way of avoiding the 6502's false memory accesses.

The 6809E has an AVMA (Advanced Valid Memory Access) pin and the Mill
generates VMA and /VMA signals from it. If VMA is asserted the 6809
uses the Apple bus and when it is de-asserted the 6502 gets to use it
(I think about 1 in 5 cycles on average).

P.S. I found my notes on the OS9 daughter board.

6809 -> 6502 address mapping (upper 4 bits of address modified)
---------------------------------------------------------------
0000..AFFF -> 1000..BFFF (main memory)
B000..DFFF -> D000..FFFF (language card)
E000..EFFF -> C000..CFFF (I/O cards)
F000..FFFF -> 0000..0FFF (6502 ZP, stack, text screen, 6809 interrupt
vectors)