Apple II Computer Info
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>It is hard to imagine any significant project being undertaken which would
>really benefit from this. It is almost impossible to get hold of devices (eg
>EPROM) that would fit into the memory space (and those you do get, you
>probably wouldn't want to use). Using larger devices would be a waste
>without making available the rest of the memory which means using some form
>of bank switching.
I had already discarded 2716/2732 and reached 2764/6264 sized chips. Bank
selection is included (1/4 is used for the slot space, the remainder is
banked 3 ways into the C800-CFFF space).
>How generic is the PAL/GAL to be (since you're also adding a TTL device as
>well)?
Just a standard 20V10 or similar.
>PLDs are becoming inexpensive and readily available. It would be easy enough
>to produce a trivial PLD design which would decode everything (get rid of
>the NAND gate), provide /CS lines and addressing for modern memory devices
>(eg 128Kx8 --> 512Kx8) making the entire space available through the 2K
>C800:CFFF window using an arbitrary location (eg C0[n+8]0) for the bank
>switching. Decoding the slot ROM (Cnxx) could force a certain bank (such as
>bank zero which would presumably always be there). The PLD could also
>convert the R/W line to /OE and /WE lines for memory.
Very similar to my GAL design. Rather than using up the limited C0[8+n]X
space I am using CFFC-E to select banks 0..2 while bank 3 is forced when
IOSEL is asserted.
The reason for a GAL is that it is relatively small and I have access to
a GAL burner.
>To make all of this happen, you need /IOSEL, /IOSTRB & /DEVSEL for the 3
>memory spaces
Got them.
> you need the 8 data bus lines to latch a bank number
> (if you wanna make it simple make this read only)
No need as I use A0-A2 instead.
> A7:0 can go straight through to the memory chips...
> A10:8 need to be latched (and driven) if you want to
> force Cnxx accesses to a certain bank
I feed A10:8 straight to the EPROM - this uses 8x256 bytes for the slot
ROM but does allow slot dependant code rather than having to write PIC.
> you need to produce the high 8 address lines
> (latched from the data bus/forced by /IOSEL)
I am producing A11 & 12.
> you need to produce at least one /CS line
Yep.
> you need to convert R/W into /OE and /WE
Currently /WE = R/W* and the GAL inverts R/W* to form /OE.
>If you were to produce some generic 'application-specific' logic parts you
>might as well do it properly...
Very interesting ideas. Thanks for the input.
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Apple II Computer Technical Information : Apple II Family Hardware Info
ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 176 of 572
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