OSDev.org

.SUFFIXES:.cpp .cpp.o .asm .asm.o

THIS_FILE      := $(abspath $(lastword $(MAKEFILE_LIST)))
FILE_PATH      = $(shell dirname $(THIS_FILE))
OUTPUT         = $(shell basename $(FILE_PATH)).elf

TARGET         = i686-elf.ld

GXX            = g++
NASM         = nasm
QEMU         = qemu-system-x86_64
ECHO         = echo
GDB            = gdb
MAKE         = make
RM            = rm

XXFLAGS      = -fno-rtti -nodefaultlibs -no-canonical-prefixes -nostdlib -ffreestanding -Wall
COMPILE      = -c
OFLAG         = -o
TGTFLAG      = -T
BITS         = -m32
BITS_MODE      = -m32
NFLAGS         = -felf32
FORCE         = -f
PRL            = -j4
MKFLAG         = -C

SRCS_CXX      = $(shell find $(FILE_PATH) -maxdepth 1 -type f -name "*.cpp")
SRCS_ASM      = $(shell find $(FILE_PATH) -maxdepth 1 -type f -name "*.asm")

CXX_OBJS      :=   $(SRCS_CXX:.cpp=.cpp.o)
ASM_OBJS      :=   $(SRCS_ASM:.asm=.asm.o)   

default:
   $(MAKE) $(PRL) $(OUTPUT)

rebuild: clean
   $(MAKE) $(PRL) $(OUTPUT)

clean:
   $(RM) $(FORCE) *.o
   $(RM) $(OUTPUT)

debug:
   $(QEMU) -kernel $(OUTPUT) -s -S &
   ($(ECHO) "target remote 127.0.0.1:1234"; \
      $(ECHO) "symbol-file $(OUTPUT)"; cat) | \
   $(GDB)

$(OUTPUT): $(ASM_OBJS) $(CXX_OBJS)
   $(GXX) $(BITS_MODE) $(ASM_OBJS) $(CXX_OBJS) $(OFLAG) $(OUTPUT) $(TGTFLAG) $(TARGET) $(XXFLAGS)

%.asm.o: %.asm
   $(NASM) $(NFLAGS) $< $(OFLAG) $@

%.cpp.o: %.cpp
   $(GXX) $(BITS_MODE) $(COMPILE) $< $(XXFLAGS) $(OFLAG) $@

/* The bootloader will look at this image and start execution at the symbol
   designated as the entry point. */
ENTRY(_start)

/* Tell where the various sections of the object files will be put in the final
   kernel image. */
SECTIONS
{
   /* Begin putting sections at 1 MiB, a conventional place for kernels to be
      loaded at by the bootloader. */
   . = 1M;

   /* First put the multiboot header, as it is required to be put very early
      early in the image or the bootloader won't recognize the file format.
      Next we'll put the .text section. */
   .text BLOCK(4K) : ALIGN(4K)
   {
      *(.multiboot)
      *(.text)
   }

   /* Read-only data. */
   .rodata BLOCK(4K) : ALIGN(4K)
   {
      *(.rodata)
   }

   /* Read-write data (initialized) */
   .data BLOCK(4K) : ALIGN(4K)
   {
      *(.data)
   }

   /* Read-write data (uninitialized) and stack */
   .bss BLOCK(4K) : ALIGN(4K)
   {
      *(COMMON)
      *(.bss)
   }

   /* The compiler may produce other sections, by default it will put them in
      a segment with the same name. Simply add stuff here as needed. */
}

; Declare constants for the multiboot header.
MBALIGN  equ  1<<0              ; align loaded modules on page boundaries
MEMINFO  equ  1<<1              ; provide memory map
FLAGS    equ  MBALIGN | MEMINFO ; this is the Multiboot 'flag' field
MAGIC    equ  0x1BADB002        ; 'magic number' lets bootloader find the header
CHECKSUM equ -(MAGIC + FLAGS)   ; checksum of above, to prove we are multiboot

; Declare a multiboot header that marks the program as a kernel. These are magic
; values that are documented in the multiboot standard. The bootloader will
; search for this signature in the first 8 KiB of the kernel file, aligned at a
; 32-bit boundary. The signature is in its own section so the header can be
; forced to be within the first 8 KiB of the kernel file.
section .multiboot
align 4
   dd MAGIC
   dd FLAGS
   dd CHECKSUM

; The multiboot standard does not define the value of the stack pointer register
; (esp) and it is up to the kernel to provide a stack. This allocates room for a
; small stack by creating a symbol at the bottom of it, then allocating 16384
; bytes for it, and finally creating a symbol at the top. The stack grows
; downwards on x86. The stack is in its own section so it can be marked nobits,
; which means the kernel file is smaller because it does not contain an
; uninitialized stack. The stack on x86 must be 16-byte aligned according to the
; System V ABI standard and de-facto extensions. The compiler will assume the
; stack is properly aligned and failure to align the stack will result in
; undefined behavior.
section .bss
align 4
stack_bottom:
resb 16384 ; 16 KiB
stack_top:

; The linker script specifies _start as the entry point to the kernel and the
; bootloader will jump to this position once the kernel has been loaded. It
; doesn't make sense to return from this function as the bootloader is gone.
; Declare _start as a function symbol with the given symbol size.
section .text
global _start
; extern kernel_main


_start:
   ; The bootloader has loaded us into 32-bit protected mode on a x86
   ; machine. Interrupts are disabled. Paging is disabled. The processor
   ; state is as defined in the multiboot standard. The kernel has full
   ; control of the CPU. The kernel can only make use of hardware features
   ; and any code it provides as part of itself. There's no printf
   ; function, unless the kernel provides its own <stdio.h> header and a
   ; printf implementation. There are no security restrictions, no
   ; safeguards, no debugging mechanisms, only what the kernel provides
   ; itself. It has absolute and complete power over the
   ; machine.

   ; To set up a stack, we set the esp register to point to the top of our
   ; stack (as it grows downwards on x86 systems). This is necessarily done
   ; in assembly as languages such as C cannot function without a stack.
   mov esp, stack_top

   ; This is a good place to initialize crucial processor state before the
   ; high-level kernel is entered. It's best to minimize the early
   ; environment where crucial features are offline. Note that the
   ; processor is not fully initialized yet: Features such as floating
   ; point instructions and instruction set extensions are not initialized
   ; yet. The GDT should be loaded here. Paging should be enabled here.
   ; C++ features such as global constructors and exceptions will require
   ; runtime support to work as well.

   ; Enter the high-level kernel. The ABI requires the stack is 16-byte
   ; aligned at the time of the call instruction (which afterwards pushes
   ; the return pointer of size 4 bytes). The stack was originally 16-byte
   ; aligned above and we've since pushed a multiple of 16 bytes to the
   ; stack since (pushed 0 bytes so far) and the alignment is thus
   ; preserved and the call is well defined.
   extern kernel_main
   call kernel_main

   ; If the system has nothing more to do, put the computer into an
   ; infinite loop. To do that:
   ; 1) Disable interrupts with cli (clear interrupt enable in eflags).
   ;    They are already disabled by the bootloader, so this is not needed.
   ;    Mind that you might later enable interrupts and return from
   ;    kernel_main (which is sort of nonsensical to do).
   ; 2) Wait for the next interrupt to arrive with hlt (halt instruction).
   ;    Since they are disabled, this will lock up the computer.
   ; 3) Jump to the hlt instruction if it ever wakes up due to a
   ;    non-maskable interrupt occurring or due to system management mode.
   cli
.hang:   hlt
   jmp .hang
.end:

class Kernel {
public:
   Kernel() {
      *((char*) 0xb8000) = 'C'; // 'C' into 1, 1
      member = 100;
      *((char*) 0xb8004) = 'M'; // 'M' into 3, 1
   }

   ~Kernel() {
      *((char*) 0xb8002) = 'D'; // 'D' into 2, 1
   }

private:
   int member;

public:
   void run() {
      char* vid = ((char*) 0xb8006);
      char* msg = "welcome!";

      while(*msg) {
         *vid = *msg;

         vid += 2;
         msg++;
      }
   }
};

char kernelObject[sizeof(Kernel)];

inline void* operator new (__SIZE_TYPE__, void* p) throw() { return p; }
inline void operator delete (void*, void*) throw() { }

extern "C" int kernel_main(void) {
   Kernel* kernel = new (kernelObject) Kernel;

   kernel->run();

   while(1);
}

CeMwelcome!rsion Ubuntu-1.8.2-1ubuntu1)
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