Question

The following C program fill in an array C with the larger elements from another two arrays A and B. Convert the code to its RISC-V assembly code. For reference to array elements A[i], you can use A(i) as its base+offset representation in the LW/SW instruction, e.g. to load an integer from A[i], you can use LW x5, A(i); Use register t0 and t1 for storing i and N respectively.

int i;

for (i=0; i < N; i++) {

    if (A[i] >= B[j]) C[i] = A[i];

    else C[i] = B[i];

}

Answer 1

The equivalent RISC assembly level language for the question is:

li $t0,N

li $t1,0

loop_start:

beq $t1,$t0,loop_end

lw $t2,A($t1)

lw $t3,A($t1)

beq $t2,$t3,else

lw C($t1),$t2

else:

lw C($t1),$t3

addi $t1,$t1,1

loop_end:

In RISC assembly level language the value of N is stored in t0 register and the register t1 is initialized with 0 which represents the iterator i. Then the loop begins with loop_start and the value of t0 and t1 register will be kept comparing until it reaches to the condition and it ends with loop end. In the body of the for loop two more registers are used to store the value of the arrays which are t2=A[i] and t3=B[i]. Then the value is compared with beq and if it is true C[i] loads the value of A[i] which is stored in t2 register and done by the lw command. If the above condtiion is not satisfied the control goes to the else part where the C[i] is loaded with t3 register. The value of t1 register is incremented by addi with 1 after each iteration.