To override the association for one instruction of one of the registers BX, BP, SI or DI, we use the segment override prefix. For example “mov ax, [cs:bx]” associates BX with CS for this one instruction. For the next instruction the default association will come back to act. The processor places a special byte before the instruction called a prefix, just like prefixes and suffixes in English language. No prefix is needed or placed for default association. For example for CS the byte 2E is placed and for ES the byte 26 is placed. Opcode has not changed, but the prefix byte has modified the default association to association with the desired segment register for this one instruction.

In all our examples, we never declared a segment or used it explicitly, but everything seemed to work fine. The important thing to note is that CS, DS, SS, and ES all had the same value. The value itself is not important but the fact that all had the same value is important. All four segment windows exactly overlap. Whatever segment register we use the same physical memory will be accessed. That is why everything was working without the mention of a single segment register. This is the formation of COM files in IBM PC. A single segment contains code, data, and the stack. This format is operating system dependant, in our case defined by DOS. And our operating system defines the format of COM files such that all segments have the same value. Thus the only meaningful thing that remains is the offset.

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