OSDev.org

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[BITS 16]
[global main]
; Main program
main:		; Label for the start of the main program

 mov ax,0x0000	; Setup the Data Segment register
		; Location of data is DS:Offset
 mov ds,ax	; This can not be loaded directly it has to be in two steps.
		; 'mov ds, 0x0000' will NOT work due to limitations on the CPU
 mov si, Intro
 call PutStr
 call PutNewLine
 mov si, HelloWorld	; Load the string into position for the procedure.
 call PutStr	; Call/start the procedure
 call PutNewLine
 call PutNewLine
 call PutNewLine
 call PutNewLine
 call PutNewLine
 mov si, Conclusion
 call PutStr

 jmp $	; Never ending loop

; Procedures
PutStr:		; Procedure label/start
 ; Set up the registers for the interrupt call
 mov ah,0x0E	; The function to display a chacter (teletype)
 mov bh,0x00	; Page number
 mov bl,0x07	; Normal text attribute

.nextchar	; Internal label (needed to loop round for the next character)
 lodsb		; I think of this as LOaD String Block 
		; (Not sure if thats the real meaning though)
		; Loads [SI] into AL and increases SI by one
 ; Check for end of string '0' 
 or al,al	; Sets the zero flag if al = 0 
		; (OR outputs 0's where there is a zero bit in the register)
 jz .return	; If the zero flag has been set go to the end of the procedure.
		; Zero flag gets set when an instruction returns 0 as the answer.
 int 0x10	; Run the BIOS video interrupt 
 jmp .nextchar	; Loop back round to the top
.return		; Label at the end to jump to when complete
 ret		; Return to main program

PutNewLine:
 mov si, newLine
 call PutStr
ret

;ExitSequence:
;inc cx
;cmp cx, 750
;je Quit
;jmp ExitSequence


; Data
Intro db ' OSTester: An Operating System Test',13,10,0
newLine db 13,10,0
HelloWorld db ' The Current Message Is As Follows: "Hello, World!"',13,10,0
Conclusion db ' Turn off the computer and remove the floppy disk to return to the normal OS',0

;Quit:
;do nothing

times 512-($-$$)-2 db 0
dw 0xAA55

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int main()
{
  return 0;
}

int k_main()
{
  int num;
  char ch;

  char *text_video = (char*)0xB8000;
  char attrib = 0x07;
  char *str="Kernel Loaded";

  while(*str!=0)
  {
    *text_video = *str;
    *text_video++;
    *text_video = attrib;
    *text_video++;
    *str++;
  }
  return;
}

void clear_screen(char clear_to, char attrib)
{
  char *text_video = (char*)0xB8000;
  char *str="Kernel Loaded";
  int pos=0;

  while(pos<(80*25*2))
  {
    *text_video = clear_to;
    *text_video++;
    *text_video = attrib;
    *str++;
    pos++;
  }
}

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@echo off
del "C:\asm\ostest.bin"
del "C:\asm\ostest.img"
del "C:\asm\kernel.o"

echo Compiling assembly
echo          nasm
C:\asm\nasm.exe -E C:\asm\errors.txt C:\asm\*.asm -f aout -o C:\asm\ostest.bin
echo          gcc
C:\djgpp\bin\gcc.exe C:\asm\*.c
echo          ld
C:\djgpp\bin\ld.exe C:\asm\link.ld -o C:\asm\kernel.o C:\asm\ostest.bin
echo Compile Attempt Finished
if exist "C:\asm\ostest.bin" (GOTO DoPartCopy)
GOTO exita

:DoPartCopy
copy /b C:\asm\ostest.bin+C:\asm\kernel.o C:\asm\ostest.img
call :InsertImgOnDisk "C:\asm\ostest.img"
GOTO exita

:InsertImgOnDisk
echo C:\asm\PartCopy.exe %1 0 %~z1 -f0
C:\asm\PartCopy.exe %1 0 %~z1 -f0
GOTO exitc

:exitb
del "C:\asm\kernel.o"
del "C:\asm\ostest.bin"
del "C:\asm\ostest.img"
echo .
echo Installation Attempt Failed
pause

:exita
del "C:\asm\kernel.o"
del "C:\asm\ostest.bin"
del "C:\asm\errors.txt"
del "C:\asm\ostest.img"
echo .
echo Installation Attempt Succeded

:exitc

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OUTPUT_FORMAT("binary")
ENTRY(main)
SECTIONS
{
  .text  0xFF800000 : {
    *(.text)
  }
  .data  : {
    *(.data)
  }
  .bss  :
  { 					
    *(.bss)
  }
}

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[extern c_main]
jmp c_main

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[extern _c_main]
jmp _c_main

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#include <stdio.h>
int main(int argc, char **argv)
{
   printf("Hello World.\n");
}

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[ORG 0x0]
set up system (cli, a20 gate, remap pic etc)
load next sector to predefined address, e.g. 0x9000
enable pmode, set up segment selectors
jump to 0x9000
times 510-($-$$) db 0
dw 0xAA55 

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global k_main

[BITS 32]
mov esp, 0x90000
call k_main
jmp $

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int puts(const char *c);

void k_main(void) {
puts("Hi from the kernel!");
}

int puts(const char *c)
{
// code to display a string
}

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nasm -f bin -o boot.bin boot.asm
nasm -f elf -o head.o head.asm
gcc -c -o main.o main.c
ld --oformat binary -Ttext 0x9000 -o kernel.bin head.o main.o
cat boot.bin kernel.bin > image