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Some more RISC-V Emulation

I am still looking into understanding risc-v a bit more, this will be a little scattershot. First I was looking at understanding noteed/riscv-hello. This uses just a few features of the libfemto So this starts by creating a minimal UART driver. The header file just defines one function, and runs in C.

This is stdio.h

#pragma once

#ifdef __cplusplus
extern "C" {
#endif

int putchar(int);

#ifdef __cplusplus
}
#endif

Then they build the the driver by setting the address of UART in memory (in this case 0x10013000). If waits for the uart register to be available. and then writes the char to the register location.

// See LICENSE for license details.

#include <stdio.h>

enum {
    /* UART Registers */
    UART_REG_TXFIFO = 0,
};

static volatile int *uart = (int *)(void *)0x10013000;

int putchar(int ch)
{
    while (uart[UART_REG_TXFIFO] < 0);
    return uart[UART_REG_TXFIFO] = ch & 0xff;
}

Next there is a small wrapper function, which actually calls main.

void main();

void libfemto_start_main(){
    main();
}

The actual main function creates a char array and pushes it out one character at a time, and then loops forever.

#include <stdio.h>

void main(){
    const char *s = "Hello\n";
    while (*s) putchar(*s++);
    while(1);
}

There are two more files. One is the C runtime file which jumps to the main of the actual program. The default one that gcc pulls is not correct so we have to override it.

# See LICENSE for license details.

.section .text.init,"ax",@progbits
.globl _start

_start:
    j       main

Then define the memory layout of the program. 

MEMORY
{
  ram   (wxa!ri) : ORIGIN = 0x80000000, LENGTH = 128M
}

PHDRS
{
  text PT_LOAD;
  data PT_LOAD;
  bss PT_LOAD;
}

SECTIONS
{
  .text : {
    PROVIDE(_text_start = .);
    *(.text.init) *(.text .text.*)
    PROVIDE(_text_end = .);
  } >ram AT>ram :text

  .rodata : {
    PROVIDE(_rodata_start = .);
    *(.rodata .rodata.*)
    PROVIDE(_rodata_end = .);
  } >ram AT>ram :text

  .data : {
    . = ALIGN(4096);
    PROVIDE(_data_start = .);
    *(.sdata .sdata.*) *(.data .data.*)
    PROVIDE(_data_end = .);
  } >ram AT>ram :data

  .bss :{
    PROVIDE(_bss_start = .);
    *(.sbss .sbss.*) *(.bss .bss.*)
    PROVIDE(_bss_end = .);
  } >ram AT>ram :bss

  PROVIDE(_memory_start = ORIGIN(ram));
  PROVIDE(_memory_end = ORIGIN(ram) + LENGTH(ram));
}

I found a good breakdown of the linker file at Mastering the GNU linker script - AllThingsEmbedded.The MEMORY section sets the ram origin to the boot location. Then it sets the Program Headers(PHDRS), these descrive how the program should be loaded into memory. Some more info on that section here. Then it describes the sections of memory in SECTION . There are 4 sections used here.