硬件和程序员的约定

CPU Reset 后寄存器会有确定的初始状态

  • EIP = 0x0000fff0
  • CR0 = 0x60000010
    • 处理器处于 16-bit 模式
  • EFLAGS = 0x00000002
    • interrupt disabled

Reset 后处理器从固定地址（Reset Vector）启动

  • MIPS: 0xbfc000000
    • Specification 规定
  • ARM: 0x00000000
    • Specification 规定
    • 允许配置 Reset Vector Base Address Register
  • RISC-V: Implementation defined
    • 给厂商最大程度的自由

Demo: firmware

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mbr.img: mbr.S
	gcc -ggdb -c $<
	ld mbr.o -Ttext 0x7c00
	objcopy -S -O binary -j .text a.out $@

run: mbr.img
	qemu-system-x86_64 $<

debug: mbr.img
	qemu-system-x86_64 -s -S $< &  # Run QEMU in background
	gdb -x init.gdb  # RTFM: gdb (1)

clean:
	rm -f *.img *.o a.out
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#define SECT_SIZE  512

.code16  // 16-bit assembly

// Entry of the code
.globl _start
_start:
  lea   (msg), %si   // R[si] = &msg;

again:
  movb  (%si), %al   // R[al] = *R[si]; <--+
  incw  %si          // R[si]++;           |
  orb   %al, %al     // if (!R[al])        |
  jz    done         //   goto done; --+   |
  movb  $0x0e, %ah   // R[ah] = 0x0e;  |   |
  movb  $0x00, %bh   // R[bh] = 0x00;  |   |
  int   $0x10        // bios_call();   |   |
  jmp   again        // goto again; ---+---+
                     //                |
done:                //                |
  jmp   .            // goto done; <---+

// Data: const char msg[] = "...";
msg:
  .asciz "This is a baby step towards operating systems!\r\n"

// Magic number for bootable device
.org SECT_SIZE - 2
.byte 0x55, 0xAA
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# Kill process (QEMU) on gdb exits
define hook-quit
  kill
end

# Connect to remote
target remote localhost:1234
file a.out
break *0x7c00
layout src
continue

实现最小的操作系统

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NAME := hello
SRCS := hello.c
export ARCH := x86_64-qemu

include $(AM_HOME)/Makefile
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#include <am.h>
#include <klib.h>
#include <klib-macros.h>

typedef union task {
  struct {
    const char *name;
    union task *next;
    void      (*entry)(void *);
    Context    *context;
  };
  uint8_t stack[8192];
} Task;

Task *current;

void func(void *arg) {
  while (1) {
    putch(*(char *)arg);
    for (int volatile i = 0; i < 100000; i++) ;
  }
}

Task tasks[] = {
  { .name = "a", .entry = func },
  { .name = "b", .entry = func },
};

Context *on_interrupt(Event ev, Context *ctx) {
  if (!current) {
    current = &tasks[0];
  } else {
    current->context = ctx;
  }
  return (current = current->next)->context;
}

int main() {
  cte_init(on_interrupt);  // call on_interrupt() on traps/interrupts

  for (int i = 0; i < LENGTH(tasks); i++) {
    Task *task    = &tasks[i];
    Area stack    = (Area) { &task->context + 1, task + 1 };
    task->context = kcontext(stack, task->entry, (void *)task->name);
    task->next    = &tasks[(i + 1) % LENGTH(tasks)];
  }
  iset(true);  // Enable external interrupts (timer, I/O, ...)
  yield();  // Trap
}
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#include <am.h>

#define ESC "\033["
#define RED ESC "01;31m"
#define CLR ESC "0m"

const char *MESSAGE = RED "Hello, OS World\n" CLR;

int main() {
  for (const char *s = MESSAGE; *s; s++) {
    putch(*s);  // Prints to platform-dependent debug console
  }
}