# Lab2: RISC-V `RV32I[MA]` emulator with ELF support
###### tags: `RISC-V`
* choose [Find Largest Element](https://hackmd.io/4-oWQOprRnCLu3ZeJy31kA?both) assembly program from [許龍君](https://github.com/jayhua267/Computer_Architecture-HW1)

.data
array: .word 9, 28, 63, 88, 52, 9, 75, 6, 26,7
range: .word 10 # range of the array
str1:  .string "The largest number in array of "
str2:  .string " is "

.text 
#s1 = Base address of current array 
#s2 = i
#s3 = Range to compare (10)
#t0 = Flag
#t1 = Current Address
main:
	la  s1, array         # s1 = base address of array
	add s2, x0, x0        # i = 0
	add t0, x0, x0        # define Flag
	lw  s3, range         # size = 10
        jal ra, forloop       # Jump-and-link to the 'forloop' label

        # Exit program
        li a7, 10
        ecall		

forloop:
	addi	s2, s2, 1		#i = i+1
	lw 	t1,0(s1)		#t1 = next value of the array
	addi	s1, s1, 4		#update new position of the next address
        #if-else
	bge	t0, t1, then 	       #if Flag >= A[i] jump to 'then'
	add	t0, x0, t1		#else Flag = A[i]
	beq 	s2, s3, printResult	#if i = 10 => stop
	j 	forloop			#back to for loop
then:
	beq 	s2, s3, printResult	#if i = 10 => stop
	j	forloop			#else continue run for loop
    
printResult:
	la a0, str1
        li a7, 4        #print string
        ecall 
		mv a0, s3
        li a7, 1        #print integer
        ecall
		la a0, str2
        li a7, 4
        ecall
        mv a0, t0
        li a7, 1
        ecall
        ret

* **Find largest element** C program by 許龍君

#include <iostream>
using namespace std;

int main()
{
    int i, flag=0, size = 10;
    int arr[size] = {9,28,63,88,52,9,75,6,26,7};


    // Loop to store largest number to arr[0]
    for(i = 0;i < size; i++)
    {
       // Change < to > if you want to find the smallest element
       if(flag < arr[i])
           flag = arr[i] ;
    }
    cout << "The largest number in array of " << size << " is "<< flag;

    return 0;
}

## Rewrite assembly to C
* **Find largest element** by myself

#include<stdio.h>
int _start()
{
    int i;
    static const int arr[10] = {9, 28, 53, 88, 52, 9, 75, 6, 26, 7};
    int maxNumber = arr[0];

    for(i = 1;i < 10; i++)
    {
       if(maxNumber < arr[i])
           maxNumber = arr[i] ;
    }

    volatile char *tx1 = (volatile char *)0x40002000;
    const char *result = "The largest element is ";
    while(*result)
    {
        *tx1 = *result;
        result++;
    }


    *tx1 = maxNumber;

    return 0;
}

objdump a assembly program based on C as above

riscv-none-embed-objdump -d test1

**compile in O3(optimize speed)**

Disassembly of section .text:

00010054 <_start>:
   10054:	000107b7          	lui	a5,0x10
   10058:	08478793          	addi	a5,a5,132 # 10084 <_start+0x30>
   1005c:	05400713          	li	a4,84
   10060:	400026b7          	lui	a3,0x40002
   10064:	00e68023          	sb	a4,0(a3) # 40002000 <__global_pointer$+0x3fff0764>
   10068:	00178793          	addi	a5,a5,1
   1006c:	0007c703          	lbu	a4,0(a5)
   10070:	fe071ae3          	bnez	a4,10064 <_start+0x10>
   10074:	05800793          	li	a5,88
   10078:	00f68023          	sb	a5,0(a3)
   1007c:	00000513          	li	a0,0
   10080:	00008067          	ret

its ***make check*** result

./emu-rv32i test1
The largest element is X
>>> Execution time: 520717 ns
>>> Instruction count: 101 (IPS=193963)
>>> Jumps: 23 (22.77%) - 0 forwards, 23 backwards
>>> Branching T=22 (95.65%) F=1 (4.35%)

**compile in Os(optimize size)**

Disassembly of section .text:

00010054 <_start>:
   10054:	000107b7          	lui	a5,0x10
   10058:	08878793          	addi	a5,a5,136 # 10088 <_start+0x34>
   1005c:	400026b7          	lui	a3,0x40002
   10060:	0007c703          	lbu	a4,0(a5)
   10064:	00071c63          	bnez	a4,1007c <_start+0x28>
   10068:	400027b7          	lui	a5,0x40002
   1006c:	05800713          	li	a4,88
   10070:	00e78023          	sb	a4,0(a5) # 40002000 <__global_pointer$+0x3fff0760>
   10074:	00000513          	li	a0,0
   10078:	00008067          	ret
   1007c:	00e68023          	sb	a4,0(a3) # 40002000 <__global_pointer$+0x3fff0760>
   10080:	00178793          	addi	a5,a5,1
   10084:	fddff06f          	j	10060 <_start+0xc>

its ***make check*** result

./emu-rv32i test1
The largest element is X
>>> Execution time: 358229 ns
>>> Instruction count: 126 (IPS=351730)
>>> Jumps: 47 (37.30%) - 23 forwards, 24 backwards
>>> Branching T=23 (95.83%) F=1 (4.17%)

### Compare between O3 and Os
Less instruction count in O3, Jump is also less used.
I don't really understand much about the whole code process, but I think it can already find out the answer in the compile process.

and ELF file

$ riscv-none-embed-readelf -h test1

Output:

ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x10054
  Start of program headers:          52 (bytes into file)
  Start of section headers:          560 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         1
  Size of section headers:           40 (bytes)
  Number of section headers:         7
  Section header string table index: 6

size

$ riscv-none-embed-size test1

Output:

   text	   data	    bss	    dec	    hex	filename
     72	      0	      0	     72	     48	test1