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Why can we write to 0xB8000,Isn't the code segment is read-only?

So you mean 0xB8000 is a code segment? Hmmm.....

xtricman wrote:Isn't the code segment is read-only?

Why do you think you're actually using the code segment for a data access?

0xb8000 is the default screen buffer memory address. It contains character and color information. There is no code there, only data.

Combuster wrote:Why do you think you're actually using the code segment for a data access?

OK,but how can I know where's data where's code? At first I think when I use reference to memory like C pointer or assembly (%eax)it's using the %DS register by default, and when I use pop or push instruction it's using %SS as the selector, but when I use " movl $32 $start" and debug the kernel, I found it also worked.My code at start is overwrited .Why?

OK,but how can I know where's data where's code?

It's always where you left it. It just appears to be a mess because you aren't conciously aware of where you left it.

Memory may be used for everything, it it can only hold one thing at a time. You can't turn a plot of land into both an apartment complex and a park at the same time.

Your linker script decides where the various sections end up as segments (ELF segments, not x86 hardware segments).

Inspect your ELF kernel with tools such as objdump and readelf.