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NTVDM CPU has encountered an illegal instruction

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I've been working on enhancing the debugger I've been working on, and for some error checking, I've been comparing it with the MS-DOS debugger. I've just been running through some old games I have (only 16 bit here), and ran into a problem with one of them. When the ES register is assigned a value of 0, things literally go to ****. When an XCHG is called using the ES register, NTVDM crashes every time claiming an illegal instruction. I look at where the IP register is pointing too according to the dialog box, and while it displays an invalid instruction, I have absolutely no idea how it even got there. Also, this only happens if I proceed through the debugger. If I go (as long as I don't enter an addres), everything works fine. I'd actually like to be able to proceed further on in the program, just wondering why the IP register changes to some weird value. The failing portion: 0C65:019C 26 ES: 0C65:019D 87065800 XCHG AX, [0058] ES:0058=09C4 When I proceed in the debugger, I'm at 020E:02E1 (an immediate fail)? [Edited by - Nytegard on September 16, 2006 2:10:02 AM]

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