Question

1.) Multiple Choice: Choose the correct letter choice. What is the correct sequence of instructions for carrying out the calculation (A * B) - (A * C) on Stack Architecture.

Assume that:

• A, B, C and D are memory locations
• R1, R2, R3 are registers.
• MUL
  • multiplies 2 operands and stores the result in the first operand in a 2 operand case
  • multiplies the second and third operands and stores the result in the first in a 3 operand case
• SUB
  • subtracts the second operand from the first and stores result in the first in a 2 operand case
  • subtracts the third operand from the second and stores in the first operand in a 3 operand case

A.) Push A Push C MUL Pop D Push B MUL Push D SUB Pop D

B.) Load R1 A MUL R2 R1 B MUL R3 R1 C Store R3 D SUB R2 R2 D Store R2

C.) Load R1 A Load R2 B MUL R2 R1 MUL R3 R1 SUB R2 R3 Store R2 D

D.) Load A MUL C Store D Load A Mul B Sub D Store D

E.) Load R1 A Load R2 B MUL R2 R1 Load R3 C MUL R3 R1 SUB R2 R3 Store R2 D

F.) Push A Push C MUL Push B MUL Push D SUB Pop D

Answer 1

Answer : OPTION D is correct . We will derive our answer as follows :

INSTRUCTION SET ARCHITECTURE :

There are three most common form of instruction set architecture :

1. STACK - The operands are implicitly on the Top of the stack .

2. ACCUMULATOR - One operator is implicitly present in the accumulator .

3. GENERAL PURPOSE REGISTER : All operands are explicitly mentioned , they are either register or memory locations .

Small Example of assembly code : C= A+B

Stack	Accumulator	General purpose register
PUSH A PUSH B ADD POP C	LOAD A ADD B STORE C	LOAD R1 , A ADD R1 , B STORE R1 , C

4. RISC (Reduced Instruction Set computer) ARCHITECTURE :

The only memory access is through explicit LOAD / STORE .

LOAD R1 , A

LOAD R2 , B

ADD R3 , R1 , R2

STORE C , R3

The correct sequence of instruction to Calculate (A*B) - (A*C) on stack architecture is :

( A*B ) - ( A*C )

Option A :

PUSH A

PUSH C

MUL ( correct instuction till this point)

POP D ( no D is present here , so what to POP ? )

thus , this instruction set fails from this point , HENCE , INCORRECT OPTION

Option B :

LOAD R1 , A R1 <-- A (loaded A In register R1)

MUL R2 R1 B R2 <-- R1 * B (R1 is multiplied with B and stired in R2) means (A * B) = R2

MUL R3 R1 C R3 <-- R1 *C means ( A * C )

Store R3 D(incorrect)    R3 <-- D (memory D is stored in R3 , BUT , the correct should be , STORE D R3 )

HENCE , INCORRECT OPTION

Option C :

LOAD R1 A R1<-- A

LOAD R2 B R2<--B

MUL R2 R1 R2 <-- R2 * R1 , means , register R2 contains (A*B)

MUL R3 R1 R3 <-- R3 * R1 , R3 is empty and R1 contains A , we want C here to be multiplied with A , but we are not getting it , so this instruction becomes incoorect at this point

HENCE , INCORRECT OPTION.

Option D :

LOAD A ACC <-- A (A is loaded in accumulator)

MUL C ACC <-- ACC*C means (A*C) (C is multiplied by the value loaded in accumulator)

STORE D D <-- ACC (A*C) (the result loaded in accumulator is stored to memory location D )

LOAD A ACC <-- A

MUL B ACC <-- A*B

SUB D ACC <-- ACC - D , means , accumulator contains (A*B) and D contains (A*C)

this instruction will result in , (A*B) - (A*C)

HENCE, CORRECT OPTION

Option E :

LOAD R1 A R1<-- A

LOAD R2 B R2<-- B

MUL R2 R1 R2 <-- R1*R2 , i.e , (A*B)

LOAD R3 C R3 <-- C

MUL R3 R1 R3 <-- R3*R1 i.e (A*C)

SUB R2 R3 R2 <-- R2 - R3 i.e . (A*B) - (A*C)

STORE R2 D R2 <-- D means , D is to be stored in R2 . BUT , here memory location D is not used so it may contain no result or some default value . Thus we well not get our desired answer .

The correct instruction should be : STORE D R2 i.e. D <-- R2

HENCE , INCORRECT OPTION

Option F :

PUSH A STACK<--A

PUSH C STACK <-- C

MUL STACK <-- A*C

PUSH B STACK <-- B

MUL STACK <-- A*C*B (this instuction becomes incorrect here )

our aim is not to calculate A*B*C , HENCE , INCORRECT OPTION