What are the segment and offset in real mode memory addressing?

Question

I am reading about memory addressing. I read about segment offset and then about descriptor offset. I know how to calculate the exact addresses in real mode. All this is OK,

Answer 1

I can see the question and answers are some years old, but there is a wrong statement that there are only 16 bit registers exist within the real mode.

Within the real mode the registers are not only 16 bit, because there are also 8 bit registers too. Every of these 8 bit register is a part of a 16 bit register which are divided into a lower and a higher part of a 16 bit register.

And starting the real mode with a 80386+ we become 32 bit registers and additional also two new instruction prefixes, one for to override/reverse the default operand-size and one for to override/reverse the default address-size of one instruction inside of a codesegment.

These instruction prefixes can be used in combination for to reverse the operand-size and the address-size together for one instruction. Within the real mode the default operand-size and address-size is 16 bit. With these both instruction prefixes we can use a 32 bit operand/register example for to calculate a 32 bit value in one 32 bit register, or for to move a 32 bit value to and from a memmory location. And we can use all 32 bit registers(maybe in combination with a base+index*scale+displacement) as an address-register, but the sum of the effective address do not have to be exceed the limit of the 64 kb segment-size.

(On the OSDEV-Wiki page we can find in the table for the "Operand-size and address-size override prefix" that the "0x66 operand prefix" and the "0x67 address prefix" is N/A(not aviable) for the real mode and the virtual 8086 mode. http://wiki.osdev.org/X86-64_Instruction_Encoding
But this is totaly wrong, because in the Intel manual we can find this statement: "These prefixes can be used in real-address mode as well as in protected mode and virtual-8086 mode".)

Starting with a Pentium MMX we become eight 64 bit MMX-Registers.
Starting with a Pentium 3 we become eight 128 bit XMM-Registers.
..

If i am not wrong, then the 256 bit YMM-Register and the 512 bit ZMM-Register and the 64 bit general-purpose Register of a x64 can not be used within the real mode.

Dirk