My codes run well,look:
.386P
cseg segment use16
assume cs:cseg
start:
;Enter into P-Mode
mov eax,cr0
or eax,1
mov cr0,eax ;（1）
;
mov ax,10 ;（2）
mov bx,20
;Return to R-Mode
mov eax,cr0
and eax,0fffffffeh
mov cr0,eax
mov ax,4c00h
int 21h
cseg ends
end start
After executing instruction:mov cr0,eax,it enters into P-Mode,just now the value
of segment CS is not changed,I suppose it is 2000h,and CPU fetch next instruction
according to CS,and just now the valuse(such as 2000h) is regarded as selector,
so CPU should not fetch the instruction:mov ax,10,but my codes run well,why?
The values in segment registers are always selectors. In real mode, they're 'fixed' so they work correctly, but they're still really just selectors.
When you switch to protected-mode, the segment registers still have their old values (valid selectors). So they still work, and code continues to execute from CS - because CS still has the old selector.
Their values won't change until they're explicitly reloaded with new valid protected-mode selectors. For the other segment registers this is simple, but for CS it must be done through executing a far jump (usually just to the next instruction.)
...or FAR RET :bg