GATE CS 2015 Set 1 Q.18 (CS Q.8): Machine Instructions and Addressing Modes

For computers based on three-address instruction formats, each address field can be used to specify which of the following:

(S1) A memory operand
(S2) A processor register
(S3) An implied accumulator register

Either S1 or S2

Either S2 or S3

Only S2 and S3

All of S1, S2 and S3

Official answer and solution

Solution

Recall the structural definition of address fields in general-register and memory-register three-address instruction architectures.

A three-address instruction format specifies three operands, typically of the form:

\\text{Opcode } A_1, A_2, A_3 \\quad (\\text{e.g., } A_1 \\leftarrow A_2 \\text{ op } A_3)

Each address field $A_i$ can specify either a memory address/operand ( $S_1$ ) or a general-purpose processor register ( $S_2$ ).
An implied accumulator ( $S_3$ ) is a fixed, implicit register used in one-address accumulator-based architectures, where no explicit address field is needed for it.

Therefore, each address field in a three-address instruction can specify either $S_1$ or $S_2$ (option A).

Final answer

Common trap

Including $S_3$ (implied accumulator), which is characteristic of one-address architectures rather than three-address architectures.

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