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Re: What is NMI under Emulator?
- Subject: Re: What is NMI under Emulator?
- From: Linards Ticmanis <ticmanis@coli.uni-sb.de>
- Date: Mon, 04 Mar 2002 06:46:16 +0100
- Newsgroups: comp.sys.apple2
- Organization: University of Saarland, Computing Center, Germany.
- References: <Kzxg8.47236$nz4.4471935@bin4.nnrp.aus1.giganews.com>
- User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:0.9.8) Gecko/20020204
- Xref: archiver1.google.com comp.sys.apple2:17948
Bryan Parkoff wrote:
What is NMI under Emulator?
I have noticed that several emulator projects include hardware
manipulation such as IRQ and NMI. I know that IRQ is used to interrupt 6502
CPU such as keyboard and I/O controller cards. What about NMI? Please
advise.
Basically on the 6502 an NMI is almost the same as an IRQ. There are two
differences:
1.) An NMI cannot be prevented with a STI command, it gets through
anyway. NMI means "Non Maskable Interrupt".
2.) The vector to jump to is read from $FFFA/FFFB instead of $FFFE/FFFF.
An NMI can be used if you need a higher priority interrupt that can
interrupt a running IRQ handler again.
--
Linards Ticmanis
The Master said, "The business of laying on the colors follows the
preparation of the plain ground."