7. Relation of Base Registers to Memory Segments

• In indirect memory addressing the base register identifies which segment register will be used to calculate the actual memory location.

• Therefore, we need to understand the rules that define which register is the base register in indirect memory addressing mode.

• The default segment register is SS if the base register is EBP or ESP.

• However, if EBP is scaled, the processor treats it as an index register with a value relative to DS, not SS.

• All other base registers are relative to DS.

• If two registers are used, only one can have a scaling factor.

• The register with the scaling factor is defined as the index register.

• The other register is defined as the base register.

• If scaling is not used, the first register is the base.

• If only one register is used, it is considered the base for deciding the default segment, unless it is scaled.

• The following examples illustrate how to determine the base register:

          mov   eax, [edx][ebp*4] ; EDX base (not scaled - seg DS)
          mov   eax, [edx*1][ebp] ; EBP base (not scaled - seg SS)
          mov   eax, [edx][ebp]   ; EDX base (first - seg DS)
          mov   eax, [ebp][edx]   ; EBP base (first - seg SS)
          mov   eax, [ebp]        ; EBP base (only - seg SS)
          mov   eax, [ebp*2]      ; EBP*2 index (seg DS)