Can anyone tell me what the memory mode is at the time linux kernel is loaded (by bootloader)? As this link says, the memory mode is real mode then the linux kernel will ask user to choose video mode and then switch to protected mode, but I do not seen it ask me to choose any thing, so is it right?
From what I understand of the x86 architecture and the Linux boot process, when the computer is turned on, it is in real mode; the kernel then enters protected mode after setting the needed descriptor tables. This is done by the code in the files
The diagram on this page explains better than I could.
As for the "choose video mode", there was a
vga= parameter, which isn't much used anymore and is deprecated; those instructions you link to are quite old.