computer organization

Question

A CPU has 24 bit instructions and we have to calculate the sum of n number by using below code:

The value of X, if target address of branch is loop, when instruction is uses PC relative addressing mode is ________. (Assume memory is byte addressable)

Answer 1

X=-12 because each instruction in the memory is 3 bytes, if we the program is stored in the memory block starts from 800(say) as address then loop label is having address 806 and the instruction where PC is used as instruction's operand is having address 815,so PC contains 818,so displacement X=(818-806)=12, i.e. -12[PC]

Comments

Why not -12 ?

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As PC + X = Loop Address

Here , PC=818  X=? Loop Address = 806

818 + x=806

x=-12