Protected mode

[spiner900]

Hey, I have only one question. When we try to set the first bit of CR0, many web site do
MOV EAX, CR0. But EAX is a 32 bit register and in rMode we works with 16 bit register. Why this is a good instruction ?

[AJ]

Hi,

When you 'MOV EAX, CR0' in real mode, your assembler automatically adds a 32 bit prefix to extend the operation size. There are many circumstances where it's useful to work with the full 32 bit registers even in real mode.

Cheers,
Adam

[skyking]

Because in real mode you can work with 32-bits registers as well as you in protected mode can work with 16-bits registers. The operand size prefix is used to use the size that is not default.

[spiner900]

Ok, but rMode is also in 32 bit ??

[AJ]

Hi,

In real mode, the default operand size (registers and stack operations) and addressing mode is 16 bit. You are stuck with the 16 bit addressing mode in pure Real Mode, but the default operand size can be overridden.

Cheers,
Adam

[Combuster]

You are stuck with the 16 bit addressing mode in pure Real Mode, but the default operand size can be overridden.

Not entirely true, you can use 32-bit addressing in pure real mode - you just have to be aware of the real mode access limit preventing you from using an effective address over 64k. As with 32 bit code, the assembler will add the necessary prefixes if you use [EAX+4*EBX] in real mode to make it work (just make sure that eax+4*ebx < 65536 when that gets executed).