Lecture 03

Class: CSE 120 Subject: computer-science computer-architecture Date: 2025-04-27 Teacher: **Prof. Nath

RISC-V Cont

Arrays in RISC-V

Example 1

C code

a = b + pow[7];

Assume pow2 base address is in x3 pow2’s element size is 4B(1 word)

RISC-V code

lw x4, 28(x3)   // x4 = Memory[pow2[7]]
add x2, x1, x4  // a = b + pow2[7]

Example 2

C code

a[i] = b + c

Assume: i in x3, b in x1, c in x2, a in x4, element size is 8B

RISC-V code

add  x5, x1, x2   // x5 = b + c  
 
slli x6, x3, 3    // x6 = 8 * i
 
add  x6, x4, x6   // x6 = a + 8*i
 
sd   x5, 0(x6)    // Memory[a[i]] = b+c
 

RISC-V Conditional Branch

beq

• essentially branch if equal

beq reg1, reg2, label1

• consider the code:

SB Instruction Format

if (a == b) goto L1;
a = a + c;
L1: // continue

• here’s how it would look like in RISC-V

RISC-V

// address    instruction
8080:         beq x1, x2, L1
8084:         add x1, x3, x4
 
...
 
8092(L1)      Continue

bne

• branch if not equal

bne reg1, reg2, label

SB Instruction Format

if (a != b) goto L1;
a = a + c;
L1: // continue

RISC-V code

bne s0, s1, L1
add s0, s0, s2
 
...
 
L1: // continue

While Loop

Example

C code

while (a[i] == k)
{
	i = i + j;
}

assume: i = s0, j = s1, k = s2, a[i] holds word size elements(4B)

RISC-V code

j Cond                // goto Cond
 
Loop:
	add s0, s0, s1    // i = i + j
 
Cond:
	slli t0, s0, 2    // t0 = 4 * j
	add t1, t0, s3    // t1 = &(a[i])
	lw t2, 0(t1)      // t2 = a[i]
	beq t2, s2, Loop  // goto Loop if (a[i] == k)
 
Exit:

For Loop

Example

C code

int result = 0;
for (i = 0; i < n; i ++)
{
	result += a[i];
}

RISC-V Pseudo Code

int result = 0;
int i;
i = 0;
 
loop:
	result += a[i];
 
test:
	if (i < n) goto lopp;

RISC-V code

// a0 = a(at start)
// a1 = n
// a0 = result(at end)
// t0 = i
 
sum_array:
	add a2, a0, x0 // save a before overwriting it
	add a0, x0, x0 // result = 0
	add t0, x0, x0 // i = 0
	j test
 
loop:
	slli t1, t0, 2  // t1 = i * 4
	add t1, a2, t1  // &(a[i])
	lw t1, 0(t1)    // a[i]
	add a0, t1, a0  // result += a[i]
	addi t0, t0, 1  // i ++
 
test:
	blt t0, a1, loop  // if (i < n) goto loop