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I'm trying to set up some basic OS services for my OS...
This code compiles fine, but crashes at boot. Any ideas?

Well, one idea is that you return from your interrupt handler with RET instead of IRET if AH != 0.

saving/restoring registers would also be a good idea (but i'm sure you did it, but just skip it here to save keystrokes )

Ya, I be saveing/restoring the ax register...any others?

btw which address mode the statement mov word [ss:(0x20*4)],Int20_handler belong to?

80386 Real-Address Mode ;D ...

what is the value for SP at that time ? zeroing the stack segment without altering the stack pointer looks like commiting suicide in real mode: as your stack grows it could destroy its interrupt handlers or the BIOS data area ...

If you need a 0-segment, rather use GS or FS or ES if DS is busy for other things, but using SS sounds really weird to me ...

SP is at 0x1300...that may be the problem?

well, if SS was different from 0 before entering the INT 20h setup function, it will definitiely cause trouble by the time you'll issue a RET instruction, as the return address will no longer be the same ...

avoid toying with the stack segment if you don't have to switch to another stack ...