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An indexed instruction like "load indexed xxxx AC" needs to make use of index register IX. But for simple microprocessor with limited registers, we can implement "load indexed" instruction if we treat the accumulator AC as the index register. The ONLY difference here is that the AC is also the index register.
更新1:
Question: Although it is possible to have "load indexed" with AC being the index register, it is not meaningful to implement the corresponding "store indexed" instruction. Explain why...