# Assignment1: RISC-V Assembly and Instruction Pipeline

**LeetCode: 326.Power of Three**

> Given an integer n, return true if it is a power of three. Otherwise, return false.
>
> Example 1:
> Input: n = 27
> Output: true
>
> Example 2:
> Input: n = 0
> Output: false
>
> Example 3:
> Input: n = 9
> Output: true
>
> Example 4:
> Input: n = 45
> Output: false

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**C code:**

#include <stdio.h>
#include <stdbool.h>

bool isPowerOfThree(int n){
    if(n==0)
        return false;
    if(n==1)
        return true;
    while(n%3 == 0){
        if(n == 3)
            return true;
        n=n/3;
        
    }
    return false;

}

int main() {
    int n = 27;
    bool x = isPowerOfThree(n);
    printf("%s\n", x ? "true" : "false");
    return 0;
}

---

**Assembly code:**

.data
argument: .word   27
str1: .string "true"
str2: .string "false"

.text
main:
    lw a0, argument      # load the argument from static data
    jal isPowerOfThree   # Jump to 'isPowerOfThree' and link
    li a7, 10            # exit program
    ecall

isPowerOfThree:
    addi t0, x0, 1       # t0 = 1
    beq a0, x0, else     # if a0 == 0, go to 'else'
    beq a0, t0, then     # if a0 == 1, go to 'then'
    addi t0, x0, 3       # t0 = 3
    rem t1, a0, t0       # t1 = a0 % 3
    bne t1, x0, else     # if t1 != 0, go to 'else'
    j loop               # jump to 'loop'
    
    
loop:
   beq a0, t0, then      # if a0 == 3, go to 'then'
   div a0, a0, t0        # a0 = a0 / t0
   rem t1, a0, t0        # t1 = a0 % 3
   bne t1, x0, else      # if t1 != 0, go to 'else'
   j loop                # jump to 'loop'
    
    
then:
    la a0, str1          # load string
    addi a7, x0, 4       # print string
    ecall
    jr ra                # go back to main

    
else:
    la a0, str2          # load string
    addi a7, x0, 4       # print string
    ecall
    jr ra                # go back to main

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**Five-stage pipeline in RISC-V: IF-ID-EX-MEM-WB**

> IF = Instruction fetch, ID = Instruction decode, EX = Execute, MEM = Memory access, WB = Register write back

There are multiple processer models which can be selected in Ripes, the following image shows RISC-V 5-Stage Processor model.


Here we discuss the pipeline process of the example above in details.

# IF:

PC passes 0x00000000 to Instr. memory and then will be equal to 0x00000004 or jump address.

# ID:


Decode the instruction. Store "rd:0x0a" in IDEX and pass "opcode:AUIPC" to Imm.

Note: "auipc" is U-type.


# EX:

The output of ALU is 0x10000000.

# MEM:

Pass the value 0x10000000 to MEM/WB.

# WB:

The value 0x10000000 will be passed to a0 register.

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**Memory update**

Before the instruction discussed above is performed, the value in a0 register is 0x00000000.


After it is performed, the value in a0 register changes to 0x10000000.


Note: The image below shows the address of stored data in memory.


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