# Lab2: RISC-V RV32I[MA] emulator with ELF support : pow function
###### tags: `RISC-V`

## Rewrite POW function in C

int _start()
{
        int base = 5;
        int exponent = 3;

        int result = 1;
        for(exponent; exponent>0;exponent--)
        {
                result = result * base;
        }
        int Value = result;

        volatile char* tx = (volatile char*) 0x40002000;
        int out = Value;
        *tx = out;
        
        return 0;
}

I got an experience on the other Lab2, therefore, I write in iterative method, which is not functional.

---

using command below to makefile
cf with gcc: gcc produce elf file for c, the other produce elf file for riscv

riscv-none-embed-gcc -march=rv32i -mabi=ilp32 -O3 -nostdlib pow.c -o pow

### run it

dc@dc-VirtualBox:~/rv32emu$ ./emu-rv32i pow
}
>>> Execution time: 3097 ns
>>> Instruction count: 6 (IPS=1937358)
>>> Jumps: 1 (16.67%) - 0 forwards, 1 backwards
>>> Branching T=0 (-nan%) F=0 (-nan%)

I was shocked by the NaN message. Then I know, it's because they set it as your count is 0. We don't want anything divided by 0.

---
### objdump

dc@dc-VirtualBox:~/rv32emu$ riscv-none-embed-objdump -d pow

pow:     file format elf32-littleriscv


Disassembly of section .text:

00010054 <_start>:
   10054:	400027b7          	lui	a5,0x40002
   10058:	07d00713          	li	a4,125
   1005c:	00e78023          	sb	a4,0(a5) # 40002000 <__global_pointer$+0x3fff0798>
   10060:	00000513          	li	a0,0
   10064:	00008067          	ret

I was quite surprising that it's only 6 instructions.
Then I figure out it was the optimization by compiler.

---
### readelf

dc@dc-VirtualBox:~/rv32emu$ riscv-none-embed-readelf -h pow
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:          484 (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:         6
  Section header string table index: 5

---
### size

dc@dc-VirtualBox:~/rv32emu$ riscv-none-embed-size pow
   text	   data	    bss	    dec	    hex	filename
     20	      0	      0	     20	     14	pow

---

### [Assembly code credit to 李佶龍](https://hackmd.io/JJtYOroGRL2UWdjE4LDopw?view)

.text
main:
	# Load arguments from static data
	lw a0, argument1 
	lw a1, argument2
	
	# Jump-and-link to the 'pow' label
	jal ra, pow
	
	# Print the result to console
	add a2, a0, zero
	lw a0, argument1
	lw a1, argument2
	jal ra,	printResult

	#Exit program
	li a0, 10
	ecall

## Note:
### sp = stack pointer
The default of riscv emulator does not allow we change sp(cauz we would ket it lower than 0). In this case, I got a wrong simulation in the oter code.
This problem I do not fix until I check that file and asking others. I think reading that file is quite time consuming.

### Report from this Lab2