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CS401 Assembly Programming Language LIVE Zoom Session - VU CS401 #cs401 #VU #LIVE addressing Modes Chapter 2 Lecture no 6 and lecture 7 #CS401 #vushortlectures #vu #Assemblyprogramming
Topic of today
1. What is segmented model in 8088?
2. How effective address is calculated?
3. What is address wraparound?

The iAPX88 processor supports seven modes of memory access. Remember
that immediate is not an addressing mode but an operand type. Operands
can be immediate, register, or memory. If the operand is memory one of the
seven addressing modes will be used to access it. The memory access
mechanisms can also be written in the general form “base + index + offset”
and we can define the possible addressing modes by saying that any one,
two, or none can be skipped from the general form to form a legal memory
access.
There are a few common mistakes done in forming a valid memory access.
Part of a register cannot be used to access memory. Like BX is allowed to
hold an address but BL or BH are not. Address is 16bit and must be
contained in a 16bit register. BX-SI is not possible. The only thing that we
can do is addition of a base register with an index register. Any other
operation is disallowed. BS+BP and SI+DI are both disallowed as we cannot
have two base or two index registers in one memory access. One has to be a
base register and the other has to be an index register and that is the reason
of naming them differently.

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