RE: Article PC-card
Date: 2001-03-01 10:43:34

> Correct me if I"m wrong, but the ISA bus's only NMI line, per se, is the
> parity error line.

As I said before, I develloped a card for the PC where we deliberatly used 
the NMI line. I don't see why other designers could not have done the same. 
FYI: handling the NMI interrupt is, just like with the C64, a matter of 
software. You only have to bend the vector.

>   Since the c64 wont be generating parity (anyone got a
> parity gen circuit?), 

Here you got me confused. The idea is to connect a PC-card to the C64, not 
to connect a C64 to a PC ????

> I feel it's best to leave it unhooked so the poor
> thing doesnt get an NMI on every bus access. ;)

A memory card only will generate a NMI during a read if A) something is 
wrong with the memory or B) nothing has been written to the address before 
(parity is not set). A programmer only has to disable the NMI by letting it 
point to "RTI" for the time he needs to write dummy data to the card and to 
check if it returns the data in good order. If everything is OK, the 
NMI-vector can be restored again. (this is the way a PC does it) 


For the moment it is impossible to attach DYNAMIC memorycards to the C64 
because there is one vital thing missing: memory refresh. In the PC the 
8237 (DMA) disables the CPU so it can supply a memory refresh for all 
memory including cards. This is where REFRESH/DACK0 is for. 
Attaching video-cards is no problem as they have there own refresh(= read 
by video-chip)

Groetjes, Ruud

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