Segment Association
All the addressing mechanisms in iAPX88 return a number called effective address. For example in base + offset addressing, neither the base nor the offset alone tells the desired cell in memory to be accessed. It is only after the addition is done that the processor knows which cell to be accessed. This number which came as the result of addition is called the effective address. But the effective address is just an offset and is meaningless without a segment. Only after the segment is known, we can form the physical address that is needed to access a memory cell. We discussed the segmented memory model of iAPX88 in reasonable detail at the end of previous chapter. However during the discussion of addressing modes we have not seen the effect of segments. Segmentation is there and it’s all happening relative to a segment base. We saw DS, CS, SS, and ES inside the debugger. Everything is relative to its segment base, even though we have not explicitly explained its functionality. An offset alone is not complete without a segment. As previously discussed there is a default segment associated to every register which accesses memory. For example CS is associated to IP by default; rather it is tied with it. It cannot access memory in any other segment. In case of data, there is a bit relaxation and nothing is tied. Rather there is a default association which can be overridden. In the case of register indirect memory access, if the register used is one of SI, DI, or BX the default segment is DS. If however the register used in BP the default segment used is SS. The stack segment has a very critical and fine use and there is a reason why BP is attached to SS by default. However these will be discussed in detail in the chapter on stack. IP is tied to CS while SP is tied to SS. The association of these registers cannot be changed; they are locked with no option. Others are not locked and can be changed.
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