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Tuesday, June 15, 2010

Real mode in C with gcc : writing a bootloader

   Usually the x86 boot loader is written in assembler. We will be
   exploring the possibility of writing one in C language (as much as
   possible) compiled with gcc, and runs in real mode. Note that you can
   also use the 16 bit bcc or TurboC compiler, but we will be focusing on
   gcc in this post. Most open source kernels are compiled with gcc, and
   it makes sense to write C bootloader with gcc instead of bcc as you get
   a much cleaner toolchain :)
   As of today (20100614), gcc 4.4.4 officially only emits code for
   protected/long mode and does not support the real mode natively (this
   may [5]change in future).
   Also note that we will not discuss the very fundamentals of booting.
   This article is fairly advanced and assumes that you know what it takes
   to write a simple boot-loader in assembler. It is also expected that
   you know how to write gcc inline assembly. Not everything can be done
   in C!

getting the tool-chain working

.code16gcc

   As we will be running in 16 bit real mode, this tells gas that the
   assembler was generated by gcc and is intended to be run in real mode.
   With this directive, gas automatically adds addr32 prefix wherever
   required. For each C file which contains code to be run in real mode,
   this directive should be present at the top of effectively generated
   assembler code. This can be ensured by defining in a header and
   including it before any other.
#ifndef _CODE16GCC_H_
#define _CODE16GCC_H_
__asm__(".code16gcc\n");
#endif

   This is great for bootloaders as well as parts of kernel that must run
   in real mode but are desired written in C instead of asm. In my opinion
   C code is a lot easier to debug and maintain than asm code, at expense
   of code size and performance at times.

Special linking

   As bootloader is supposed to run at physical 0x7C00, we need to tell
   that to linker. The mbr/vbr should end with the proper boot signature
   0xaa55.
   All this can be taken care of by a simple linker script.
ENTRY(main);
SECTIONS
{
    . = 0x7C00;
    .text : AT(0x7C00)
    {
        _text = .;
        *(.text);
        _text_end = .;
    }
    .data :
    {
        _data = .;
        *(.bss);
        *(.bss*);
        *(.data);
        *(.rodata*);
        *(COMMON)
        _data_end = .;
    }
    .sig : AT(0x7DFE)
    {
        SHORT(0xaa55);
    }
    /DISCARD/ :
    {
        *(.note*);
        *(.iplt*);
        *(.igot*);
        *(.rel*);
        *(.comment);
/* add any unwanted sections spewed out by your version of gcc and flags here */
    }
}

   gcc emits elf binaries with sections, whereas a bootloader is a
   monolithic plain binary with no sections. Conversion from elf to binary
   can be done as follows:
$ objcopy -O binary vbr.elf vbr.bin

The code

   With the toolchain set up, we can start writing our hello world
   bootloader!
   vbr.c (the only source file) looks something like this:
/*
 * A simple bootloader skeleton for x86, using gcc.
 *
 * Prashant Borole (boroleprashant at Google mail)
 * */

/* XXX these must be at top */
#include "code16gcc.h"
__asm__ ("jmpl  $0, $main\n");


#define __NOINLINE  __attribute__((noinline))
#define __REGPARM   __attribute__ ((regparm(3)))
#define __NORETURN  __attribute__((noreturn))

/* BIOS interrupts must be done with inline assembly */
void    __NOINLINE __REGPARM print(const char   *s){
        while(*s){
                __asm__ __volatile__ ("int  $0x10" : : "a"(0x0E00 | *s), "b"(7))
;
                s++;
        }
}
/* and for everything else you can use C! Be it traversing the filesystem, or ve
rifying the kernel image etc.*/

void __NORETURN main(){
    print("woo hoo!\r\n:)");
    while(1);
}


   compile it as
$ gcc -c -g -Os -march=i686 -ffreestanding -Wall -Werror -I. -o vbr.o vbr.c
$ ld -static -Tlinker.ld -nostdlib --nmagic -o vbr.elf vbr.o
$ objcopy -O binary vbr.elf vbr.bin

   and that should have created vbr.elf file (which you can use as a
   symbols file with gdb for source level debugging the vbr with gdbstub
   and qemu/bochs) as well as 512 byte vbr.bin. To test it, first create a
   dummy 1.44M floppy image, and overwrite it's mbr by vbr.bin with dd.
$ dd if=/dev/zero of=floppy.img bs=1024 count=1440
$ dd if=vbr.bin of=floppy.img bs=1 count=512 conv=notrunc

   and now we are ready to test it out :D
$ qemu -fda floppy.img -boot a

   and you should see the message!
   Once you get to this stage, you are pretty much set with respect to the
   tooling itself. Now you can go ahead and write code to read the
   filesystem, search for next stage or kernel and pass control to it.
   Here is a simple example of a floppy boot record with no filesystem,
   and the next stage or kernel written to the floppy immediately after
   the boot record. The next image LMA and entry are fixed in a bunch of
   macros. It simply reads the image starting one sector after boot record
   and passes control to it. There are many obvious holes, which I left
   open for sake of brevity.
/*
 * A simple bootloader skeleton for x86, using gcc.
 *
 * Prashant Borole (boroleprashant at Google mail)
 * */

/* XXX these must be at top */
#include "code16gcc.h"
__asm__ ("jmpl  $0, $main\n");


#define __NOINLINE  __attribute__((noinline))
#define __REGPARM   __attribute__ ((regparm(3)))
#define __PACKED    __attribute__((packed))
#define __NORETURN  __attribute__((noreturn))

#define IMAGE_SIZE  8192
#define BLOCK_SIZE  512
#define IMAGE_LMA   0x8000
#define IMAGE_ENTRY 0x800c

/* BIOS interrupts must be done with inline assembly */
void    __NOINLINE __REGPARM print(const char   *s){
        while(*s){
                __asm__ __volatile__ ("int  $0x10" : : "a"(0x0E00 | *s), "b"(7))
;
                s++;
        }
}

#if 0
/* use this for the HD/USB/Optical boot sector */
typedef struct __PACKED TAGaddress_packet_t{
    char                size;
    char                :8;
    unsigned short      blocks;
    unsigned short      buffer_offset;
    unsigned short      buffer_segment;
    unsigned long long  lba;
    unsigned long long  flat_buffer;
}address_packet_t ;

int __REGPARM lba_read(const void   *buffer, unsigned int   lba, unsigned short
blocks, unsigned char   bios_drive){
        int i;
        unsigned short  failed = 0;
        address_packet_t    packet = {.size = sizeof(address_packet_t), .blocks
= blocks, .buffer_offset = 0xFFFF, .buffer_segment = 0xFFFF, .lba = lba, .flat_b
uffer = (unsigned long)buffer};
        for(i = 0; i < 3; i++){
                packet.blocks = blocks;
                __asm__ __volatile__ (
                                "movw   $0, %0\n"
                                "int    $0x13\n"
                                "setcb  %0\n"
                                :"=m"(failed) : "a"(0x4200), "d"(bios_drive), "S
"(&packet) : "cc" );
                /* do something with the error_code */
                if(!failed)
                        break;
        }
        return failed;
}
#else
/* use for floppy, or as a fallback */
typedef struct {
        unsigned char   spt;
        unsigned char   numh;
}drive_params_t;

int __REGPARM __NOINLINE get_drive_params(drive_params_t    *p, unsigned char
bios_drive){
        unsigned short  failed = 0;
        unsigned short  tmp1, tmp2;
        __asm__ __volatile__
            (
             "movw  $0, %0\n"
             "int   $0x13\n"
             "setcb %0\n"
             : "=m"(failed), "=c"(tmp1), "=d"(tmp2)
             : "a"(0x0800), "d"(bios_drive), "D"(0)
             : "cc", "bx"
            );
        if(failed)
                return failed;
        p->spt = tmp1 & 0x3F;
        p->numh = tmp2 >> 8;
        return failed;
}

int __REGPARM __NOINLINE lba_read(const void    *buffer, unsigned int   lba, uns
igned char  blocks, unsigned char   bios_drive, drive_params_t  *p){
        unsigned char   c, h, s;
        c = lba / (p->numh * p->spt);
        unsigned short  t = lba % (p->numh * p->spt);
        h = t / p->spt;
        s = (t % p->spt) + 1;
        unsigned char   failed = 0;
        unsigned char   num_blocks_transferred = 0;
        __asm__ __volatile__
            (
             "movw  $0, %0\n"
             "int   $0x13\n"
             "setcb %0"
             : "=m"(failed), "=a"(num_blocks_transferred)
             : "a"(0x0200 | blocks), "c"((s << 8) | s), "d"((h << 8) | bios_driv
e), "b"(buffer)
            );
        return failed || (num_blocks_transferred != blocks);
}
#endif

/* and for everything else you can use C! Be it traversing the filesystem, or ve
rifying the kernel image etc.*/

void __NORETURN main(){
        unsigned char   bios_drive = 0;
        __asm__ __volatile__("movb  %%dl, %0" : "=r"(bios_drive));      /* the B
IOS drive number of the device we booted from is passed in dl register */

        drive_params_t  p = {};
        get_drive_params(&p, bios_drive);

        void    *buff = (void*)IMAGE_LMA;
        unsigned short  num_blocks = ((IMAGE_SIZE / BLOCK_SIZE) + (IMAGE_SIZE %
BLOCK_SIZE == 0 ? 0 : 1));
        if(lba_read(buff, 1, num_blocks, bios_drive, &p) != 0){
            print("read error :(\r\n");
            while(1);
        }
        print("Running next image...\r\n");
        void*   e = (void*)IMAGE_ENTRY;
        __asm__ __volatile__("" : : "d"(bios_drive));
        goto    *e;
}


   removing __NOINLINE may result in even smaller code in this case. I had
   it in place so that I could figure out what was happening.

Concluding remarks

   C in no way matches the code size and performance of hand tuned
   size/speed optimized assembler. Also, because of an extra byte (0x66,
   0x67) wasted (in addr32) with almost every instruction, it is highly
   unlikely that you can cram up the same amount of functionality as
   assembler.
   Global and static variables, initialized as well as uninitialized, can
   quickly fill those precious 446 bytes. Changing them to local and
   passing around instead may increase or decrease size; there is no thumb
   rule and it has to be worked out on per case basis. Same goes for
   function in-lining.
   You also need to be extremely careful with various gcc optimization
   flags. For example, if you have a loop in your code whose number of
   iterations are small and deducible at compile time, and the loop body
   is relatively small (even 20 bytes), with default -Os, gcc will unroll
   that loop. If the loop is not unrolled (-fno-tree-loop-optimize), you
   might be able to shave off big chunk of bytes there. Same holds true
   for frame setups on i386 - you may want to get rid of them whenever not
   required using -fomit-frame-pointer. Moral of the story : you need to
   be extra careful with gcc flags as well as version update. This is not
   much of an issue for other real mode modules of the kernel where size
   is not of this prime importance.
   Also, you must be very cautious with assembler warnings when compiling
   with .code16gcc. Truncation is common. It is a very good idea to use
   --save-temp and analyze the assembler code generated from your C and
   inline assembly. Always take care not to mess with the C calling
   convention in inline assembly and meticulously check and update the
   clobber list for inline assembly doing BIOS or APM calls (but you
   already knew it, right?).
   It is likely that you want to switch to protected/long mode as early as
   possible, though. Even then, I still think that maintainability wins
   over asm's size/speed in case of a bootloader as well as the real mode
   portions of the kernel.
   It would be interesting if someone could try this with
   c++/java/fortran. Please let me know if you do!
   at [6]June 15, 2010
   [7]Email This[8]BlogThis![9]Share to Twitter[10]Share to
   Facebook[11]Share to Pinterest
   Labels: [12]assembler, [13]bootloader, [14]c, [15]gas, [16]gcc,
   [17]kernel, [18]osdev

25 comments:

    1. [19]Girija[20]Tuesday, June 15, 2010 at 6:12:00 PM GMT+5:30
       Dokyaawarun 10 foot.. kiwwa jaastach.
       :-|
       Reply[21]Delete
       Replies
            Reply
    2. [22]descent[23]Tuesday, December 21, 2010 at 1:10:00 PM GMT+5:30
       Hi,
       Thank you for your sharing.
       in void __NOINLINE __REGPARM print(const char *s)
       I change the print function to access pointer,
       like this:
       videoram[0]='H';
       but I got the warning message:
       /tmp/cc5qsy9l.s:33: Warning: 00000000000b8000 shortened to
       0000000000008000
       Do I miss something?
       Reply[24]Delete
       Replies
            Reply
    3. [25]descent[26]Tuesday, December 21, 2010 at 2:05:00 PM GMT+5:30
       Hi,
       I use gcc-3.4 to compile again.
       I see no warning message, but in qemu,
       I still cannot see char H.
       videoram is static variable.
       static unsigned char *videoram = (unsigned char *) 0xb8000;
       Reply[27]Delete
       Replies
            Reply
    4. [28]descent[29]Tuesday, December 21, 2010 at 3:16:00 PM GMT+5:30
       Hi,
       I got something. In 16bit mode, the pointer is 16bit length. So
       0xb8000 shortened to 0x8000.
       I write a c file and a function,
       void put_char()
       {
       unsigned char *videoram = (unsigned char *) 0xb8000;
       videoram[0]='H';
       videoram[2]='H';
       videoram[40]='H';
       }
       no include code16gcc.h, I think the pointer is 32bits length, but I
       still can not see the H character.
       Reply[30]Delete
       Replies
            Reply
    5. [31]Prashant[32]Tuesday, December 21, 2010 at 7:16:00 PM GMT+5:30
       @descent: check the '--save-temps' preserved assembler version of
       the C function.
       This article assumes that the reader has low level programming
       experience with x86.
       To access the vidmem with b8000h, you have 2 options:
       1. write inline assembly to set es to b800h, and di to the address
       in the real mode segment. Then write byte/word to es:di.
       2. Enter unreal mode. Then you can use the full 4G memory,
       one-to-one mapped.
       I personally would not recommend any of these methods for printing
       - BIOS int 10h is pretty good. Remember - do not try and do
       anything fancy in the (m/v)br; do it in the next stage instead as
       you have pretty much unconstrained image size in later stages.
       Reply[33]Delete
       Replies
            Reply
    6. [34]descent[35]Wednesday, December 22, 2010 at 9:41:00 AM GMT+5:30
       Hi Prashant,
       Thank you for your explanation.
       Because in protected mode, I can use C,
       and direct access 0xb8000, so I am confused.
       real/protect mode, gcc/gas 16/32 bit also confuse me.
       They are very complicate.
       Reply[36]Delete
       Replies
            Reply
    7. [37]Sebastian[38]Saturday, March 12, 2011 at 6:26:00 PM GMT+5:30
       you are a genius!
       Reply[39]Delete
       Replies
            Reply
    8. [40]Unknown[41]Sunday, April 17, 2011 at 5:48:00 AM GMT+5:30
       I've got that infamous runtime error...
       bootloader.exe has encountered a problem and needs to close. We are
       sorry for the inconvenience.
       Reply[42]Delete
       Replies
            Reply
    9. [43]Unknown[44]Saturday, May 21, 2011 at 2:39:00 AM GMT+5:30
       Managed to do it in C++.
       Code is the same.
       Linker file needs to discard eh_frame.
       When building on x86-64 add -m32 to g++ and -melf_i386 on ld
       command line.
       Trying to rewrite it in a more c++-ish style.
       My e-mail is boskovits@cogito-top.hu .
       Reply[45]Delete
       Replies
            Reply
   10. [46]Prashant[47]Saturday, May 21, 2011 at 3:02:00 AM GMT+5:30
       @abraker95: are you trying to run the MZ/PE image in windows? that
       is like sinning and then spitting on the devil when in hell.
       @boskov1985: cool man! let us know how it goes :D
       Reply[48]Delete
       Replies
            Reply
   11. Anonymous[49]Friday, November 25, 2011 at 2:50:00 AM GMT+5:30
       It's easier to to this without objcopy. Modern ld versions support
       --oformat=binary , so just one line does the direct compilation
       job.
       gcc -g -Os -march=i686 -ffreestanding -Wall -Werror -I. -static
       -nostdlib -Wl,-Tlinker.ld -Wl,--nmagic -Wl,--oformat=binary -o
       loader.bin loader.c
       Reply[50]Delete
       Replies
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   12. [51]Prashant[52]Friday, November 25, 2011 at 8:01:00 AM GMT+5:30
       I can't verify right now whether it works, but thanks for letting
       us know, rpfh!
       Reply[53]Delete
       Replies
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   13. [54]descent[55]Sunday, December 4, 2011 at 9:42:00 PM GMT+5:30
       Hi,
       The c code uses function call, why need not set stack (ss:esp)?
       Reply[56]Delete
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   14. [57]Prashant[58]Tuesday, December 6, 2011 at 10:18:00 AM GMT+5:30
       good point @decent. I guess you will need to set up the stack first
       in main, probably in assembler.
       Reply[59]Delete
       Replies
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   15. [60]descent[61]Saturday, December 24, 2011 at 8:02:00 PM GMT+5:30
       I change %ss:%esp to 0x07a0:0000,
       Is any side effect?
       void __NORETURN main(){
       __asm__ ("mov %cs, %ax\n");
       __asm__ ("mov %ax, %ds\n");
       __asm__ ("mov $0x07a0, %ax\n");
       __asm__ ("mov %ax, %ss\n");
       __asm__ ("mov $0, %esp\n");
       print("woo hoo!\r\n:)");
       while(1);
       }
       Reply[62]Delete
       Replies
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   16. [63]descent[64]Monday, July 30, 2012 at 8:16:00 AM GMT+5:30
       Hi,
       I test c bootloader in real machine, in my eeepc 904, need add some
       code to setup stack.
       http://descent-incoming.blogspot.tw/2012/05/x86-bootloader-hello-wo
       rld.html
       The article is written by Chinese, but the code, picture can give
       some reference.
       cppb.cpp is cpp version (compile by g++), it can work, I test it in
       real machine(eeepc 904).
       Reply[65]Delete
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   17. [66]axiomfinity[67]Saturday, April 20, 2013 at 10:46:00 AM GMT+5:30
       linker fails whats up with it..?
       Reply[68]Delete
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   18. [69]Prashant[70]Sunday, April 21, 2013 at 9:34:00 AM GMT+5:30
       Fails how? Can you please elaborate?
       Reply[71]Delete
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   19. [72]Unknown[73]Wednesday, November 13, 2013 at 12:51:00 PM GMT+5:30
       Thank you for detaile explanation
       Linker failed nt sure why..ld: error: load segment overlap [0x7c00
       -> 0x7e50] and [0x7dfe -> 0x7e00]
       Reply[74]Delete
       Replies
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   20. [75]osdev[76]Saturday, May 31, 2014 at 1:35:00 AM GMT+5:30
       someone here? I need to test, but...
       "c"((s << 8) | s) <-- duplicate s in CH and CL?
       c = lba / (p->numh * p->spt); <-- 'c' is never used...
       maybe -> "c"((c << 8) | s)
       Reply[77]Delete
       Replies
            Reply
   21. [78]Unknown[79]Thursday, February 5, 2015 at 8:39:00 PM GMT+5:30
       Thank you for your nice post! I'm trying to run it on my x86-64
       linux box, but gcc reports errors like "bad register name rax", I'm
       a little confused by the various compiler options here, could you
       please give me suggestions on how to compile the C source file on
       x86-64 machines? Thanks
       Reply[80]Delete
       Replies
         1. [81]Jose Fernando Lopez Fernandez[82]Friday, January 20, 2017
            at 2:56:00 PM GMT+5:30
            rax is a 64 bit register. A bootloader is running in 16 bits,
            so you cannot use rax (64 bit) or eax (32 bit). You have to
            use ax.
            Also, you said your computer is an x86-64. Which one is it?
            x86 (32 bit) or 64 (64 bit)? If you have an x86, it will have
            no idea what rax is, since it has no knowledge of 64 bit
            registers.
            I'm just speculating as to your problem here, though. If
            anything here is incorrect/misguided by all means let me know,
            I'm only a beginner too
            [83]Delete
            Replies
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         2. [84]Jose Fernando Lopez Fernandez[85]Friday, January 20, 2017
            at 2:57:00 PM GMT+5:30
            @Jing Peng
            rax is a 64 bit register. A bootloader is running in 16 bits,
            so you cannot use rax (64 bit) or eax (32 bit). You have to
            use ax.
            Also, you said your computer is an x86-64. Which one is it?
            x86 (32 bit) or 64 (64 bit)? If you have an x86, it will have
            no idea what rax is, since it has no knowledge of 64 bit
            registers.
            I'm just speculating as to your problem here, though. If
            anything here is incorrect/misguided by all means let me know,
            I'm only a beginner too
            [86]Delete
            Replies
                 Reply
            Reply
   22. [87]Unknown[88]Thursday, February 5, 2015 at 8:40:00 PM GMT+5:30
       Thank you for your nice post! I'm trying to run it on my x86-64
       linux box, but gcc reports errors like "bad register name rax", I'm
       a little confused by the various compiler options here, could you
       please give me suggestions on how to compile the C source file on
       x86-64 machines? Thanks
       Reply[89]Delete
       Replies
            Reply
   23. [90]Unknown[91]Sunday, February 7, 2016 at 8:43:00 PM GMT+5:30
       hello i ma atif
       Reply[92]Delete
       Replies
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