memory addressing(effective address or offset) in 16 bit protected or real mode

[logicman112]

We know that memory is addressed by a combination of (Base+Index*Scale+Displacement). If the code is 16 bit protected mode and addresses a memory location, can we specify 8 bit registers like AH, BH .....as base or index registers?
The same question is for a code that is executed in 16 bit real address (or 64 bit mode).

Intel 64 and IA-32 software developer's manual mentions 32 or 64 bit registers in 64 bit mode I think.

[SHIH_K_Intel]

Sounds to me like you're asking if a post 16-bit cpu could operate as iftreating thememory address space as 8-bit.
I don't think so

[logicman112]

Thank you to answer my question.

My question is how we can write effective address in 16, 32 or 64 bit modes. We know that the following is a valid effective address in 16 bit IA-32 protected:
[BX+SI]+disp8
Can we specify: [BH+SI]+disp8 as an address?

Also suppose we write a code for IA-32 protected, the following is a valid effective address:
[EAX]+disp32

But is , [AX]+disp32 a valid valid effective address? how about [AL]+disp32?

In real mode(16-bit) we have [BX]+disp8 but is "[BL]+disp8" valid?

And my last question is that how you write a valid address in 64 bit mode?

[SHIH_K_Intel]

"Can you specify [BH+SI]+disp8 as an address?"

No.
First you cannot mix register of different sizes. In addressing form, the size of register is governed by the ADDRESS_SIZE attribute, if your codelive in16-bit mode, that attribute is 16 bit by default. If you override ADDRESS_SIZE attribute with the 67H prefix, the ADDRESS_SIZE becomes 32-bit.

So, an assembler will reject the syntax if you try, and the hardware will operate in 16/32/64-bit address space, depending on the operatingmode. And ADDRESS_SIZE prefix override onlyapply in 16/32 bit modes.