# Lab1: RV32I Simulator

###### tags: `Computer Architecture`

## Two Sum [(leetcode)](https://leetcode.com/problems/two-sum/)
**Description**:

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: Because nums[0] + nums[1] == 9, we return [0, 1].

### C code

int* twoSum(int* nums, int numsSize, int target, int* returnSize){
    for (int i=0; i < numsSize; i++)
    {
        for (int j=i+1; j < numsSize; j++)
        {
            if ((nums[i]+nums[j]) == target) {
                *returnSize = 2;
                int *arr = malloc(sizeof(int) * 2);
                arr[0] = i;
                arr[1] = j;
                return arr;
            }
        }
    }
    return 0;
}

## RISC-V assembly code (R32I ISA)

This code has 5 components

1. main
* set the initial value
2. fori
* if i < numSize, then go forj
3. forj
* if j < numSize, then go if
5. if
* if (nums[i]+nums[j]) == target , then go print
7. print
* print the index of answer

.data
len:    .word 4
target: .word 26
arr:    .word 2, 7, 11, 15

.text
# t3 = i, t4 = j
main:
    la        t1, arr
    addi      t1, t1, -4
    la        t2,	arr        // t1 = arr[0]
    addi      t2, t2, 0

    la        t0, target  
    lw        s1, 0(t0)       // s1 = target
    la        t0, len
    lw        s0, 0(t0)   	 // s0 = len
    li        t3, 0	         // i = 0
    j         fori

fori:
    slt       t0, t3, s0
    beq       t0, x0, exit
    addi      t1, t1, 4
    la        t2, arr
    addi      t3, t3, 1    # i++
    mv        t4, t3
    j         forj
    
forj:
    slt       t0, t4, s0
    beq       t0, x0, fori
    addi      t2, t2, 4
    addi      t4, t4, 1    # j++
    j         if
    
if:		
    lw        s2, 0(t1)
    lw        s3, 0(t2)
    add       t5, s2, s3
    beq       t5, s1, print
    j         forj
        
print:
    mv        a0, t3
    li        a7, 1
    ecall
    mv        a0, t4
    li        a7, 1
    ecall
    j		exit

exit:

## Ripes simulator

### Instruction fetch
The Program Counter, is a register that holds the address of instruction.
it will output the instruction that read out of instruction memory.
`0x10000317` from IFID to Decode
`0x00830313` from Instruction memory to IFID


### Instruction decode

It will know the input instruction, and decide what it should output
### Execute
The computation stage. this stage consists of an ALU
The ALU is responsible for performing boolean operations(and, or, not, nand, nor, xor, xnor) and integer addition and subtraction.
Then stroed output in EX/MEM

### Memory access
if data memory needs to be accessed, it will done in this stage.

### Write back

The output will write the result in the register file
`0x10000004` will write in t0 at next cycle