10. Consider the IBM 370 architecture and the Rx format discussed above. The eff
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10. Consider the IBM 370 architecture and the Rx format discussed above. The effective memory address (that is, the address of the operand) is computed by adding the contents of the base and index registers and then adding the displacement value. For a given setting of the base and index registers, how many distinct effective memory (byte) addresses can an instruction specify? In other words, once the base and index registers are given specific values, how many distinct bytes become addressable by varying the displacement alone? a. For a given setting of the base register, if the contents of the index register can also be varied from 0 to 255, in addition to the displacement, how many distinct effective memory (byte) addresses can an instruction specify? b.Explanation / Answer
In IBM 370 architecture, 16 general purpose 32-bit registers are there. For displacement, it uses a 12-bit number.
a) Maximum possible different displacement values, we can generate from that 12-bit number is 212.
If we fix base and index register values, maximum we can point to 212 different address bytes by varying displacement alone.
b) If we have chance to vary index register value from 0 to 255 (i.e 256 different values) along with displacement, maximum we can point to 256 x 212 (i.e 220) different address bytes by varying both.
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