Multi-Byte NOP locks up system.

Multi-Byte NOP locks up system.


Opened up Win9x DOS under Virtual PC 2008 on a system running W7 x64 on an i7 processor.

I created the smallest program which is:

B8 00 4C ; MOV AX,4C00h
CD 21 ; INT 21h

That causes the system to lock up.

If I remove the 0F 1F 00 then it works just fine. So my question is why does it cause a hang? I confirmed CPUID reports the family as 6 (0110b).



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I found that if I actually start Win98 GUI and open a command prompt and run it, it works fine, however if I shutdown to DOS mode and run it from there, again the VM locks up.

I didn't think multi-byte nop operations needed anything special? do they?


It appears to be a bug in Virtual PC - works ok on physical machines and confired it's throwing an execption (and must return to same instruction) as I set one up like this and got looping zeros:

push ax
push bx
mov ax,0E30h
mov bx,7
int 10h
pop bx
pop ax

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