# Lab1: RV32I Simulator
###### tags: `CA2020`
## IsPrime C-Code

#include <stdio.h>

int main ()
{ 
    int n;
    int prime = 1; 
    int i;    
    printf("Please enter a positive integer\n");
    scanf("%d", &n);
    for(i = 2; i*i <= n; i++)
    {
        if(n % i == 0)
        {
            prime = 0;
            break;
        }
    }
    if(prime) printf("%d is a prime number\n", n);
    else printf("%d is not a prime number\n", n);
    return 0;
}

## IsPrime Assembly-Code
1. prologue
2. for & i++ : for (i = 2 ; i * i <= n ; i++)
3. if : if (n%i == 0)...
4. epilogue
5. print1 : n is prime .

.data
argument: .word 19
str1: .string " is a prime number."
str2: .string " is not a prime number."
.text
prologue:
       addi	sp,sp,-12    #prologue
	sw	s0,8(sp)     #prologue
	addi	s0,sp,12     #prologue
	lw	a5,argument  #n
	sw	a5,-4(s0)    #store n in -4(s0)
	li	a5,1         #prime = 1
	sw	a5,-8(s0)    #store prime in -8(s0)
	li	a5,2         #i = 2
	sw	a5,0(s0)     #store i in -0(s0)
        lw	a4,-4(s0)    #a4 = n
	j	for

if:	
	lw	a5,0(s0)     #a5 = i
	rem	a5,a4,a5     #n%i
	bne	a5,zero,i++  #whether n is divided by i
	sw	zero,-8(s0)  #if n can be divided by i, prime = 0
	j	epilogue
i++:
	lw	a5,0(s0)     #a5 = i
	addi	a5,a5,1      
	sw	a5,0(s0)
for:
	lw	a5,0(s0)     #a5 = i
	mul	a5,a5,a5     #a5 = i*i	
	bge	a4,a5,if     #for loop condition
epilogue:
        lw     a0,argument    #a5 = n (for print)        
        lw     a4,-8(s0)      #a4 = prime
        lw     s0,8(sp)       #epilogue
        addi   sp,sp,12       #epilogue
        bne    a4,zero,print1 #if(prime)      
        li     a7, 1          #print "n"
        ecall
        la     a0,str2        #print " is not a prime number."
        li     a7, 4
        ecall    
        li     a7, 10         #exit
        ecall        
        
print1:
        li     a7, 1 
        ecall
        la     a0,str1
        li     a7, 4          #print " is a prime number."
        ecall        
        li     a7, 10         
        ecall
        

* Result


## Branch
The pipeline decide whether to branch in the execution stage.
If jump,then insert 2 nop to flush because you don't have to fetch the following 2 instructions without branch.

## Load-use
The rem instruction wants to use a5 at the execution stage,but the correct a5 hasn't write back.Thus,we need to insert nop.Because of forwarding,we only need to insert 1 nop.

lw	a5,argument  #n
sw	a5,-4(s0)    #store n in -4(s0)


## Print
1. a7 = 1 , print int a0
2. a7 = 4 , print string a0
3. a7 = 10 , exit

## Memory and Register
In prologue,stackpointer -12 and create a space for stack.
This is because there are 3 local variables (n,i,prime).
sp : 0x7ffffff0 -> 0x7fffffe4