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What do I need to put on the stack so that an IRET instruction will jump to a given location? I'm trying to do a context switch type deal by "returning" to a code entry point in an ISR.

Don't hesitate to consult reference before asking.

http://nasm.sourceforge.net/doc/html/nasmdocb.html#section-B.4.127

Ehm.. IA-32 Intel Architecture Software Developers Manual, volume 3, the Holy Bibble of PC OS development, tells you everything about interrupt/trap management you ever wanted (and didn't want) to know. Available (some where around the web) as a PDF.

btw, you may want to get a look at the FAQ which gives you hints about performing a software task switch by simply swapping stacks.