Compile an asm bootloader with external c code

Question

I write a boot loader in asm and want to add some compiled C code in my project.

I created a test function here:

test.c

__as         


        

Answer 1

Compiling & Linking NASM and GCC Code

This question has a more complex answer than one might believe, although it is possible. Can the first stage of a bootloader (the original 512 bytes that get loaded at physical address 0x07c00) make a call into a C function? Yes, but it requires rethinking how you build your project.

For this to work you can no longer us -f bin with NASM. This also means you can't use the org 0x7c00 to tell the assembler what address the code expects to start from. You'll need to do this through a linker (either us LD directly or GCC for linking). Since the linker will lay things out in memory we can't rely on placing the boot sector signature 0xaa55 in our output file. We can get the linker to do that for us.

The first problem you will discover is that the default linker scripts used internally by GCC don't lay things out the way we want. We'll need to create our own. Such a linker script will have to set the origin point (Virtual Memory Address aka VMA) to 0x7c00, place the code from your assembly file before the data and place the boot signature at offset 510 in the file. I'm not going to write a tutorial on Linker scripts. The Binutils Documentation contains almost everything you need to know about linker scripts.

OUTPUT_FORMAT("elf32-i386");
/* We define an entry point to keep the linker quiet. This entry point
 * has no meaning with a bootloader in the binary image we will eventually
 * generate. Bootloader will start executing at whatever is at 0x07c00 */
ENTRY(start);
SECTIONS
{
    . = 0x7C00;
    .text : {
        /* Place the code in hw.o before all other code */
        hw.o(.text);
        *(.text);
    }

    /* Place the data after the code */
    .data : SUBALIGN(2) {
        *(.data);
        *(.rodata*);
    }

    /* Place the boot signature at LMA/VMA 0x7DFE */
    .sig 0x7DFE : {
        SHORT(0xaa55);
    }

    /* Place the uninitialised data in the area after our bootloader
     * The BIOS only reads the 512 bytes before this into memory */
    .bss : SUBALIGN(4) {
        __bss_start = .;
        *(COMMON);
        *(.bss)
        . = ALIGN(4);
        __bss_end = .;
    }
    __bss_sizeb = SIZEOF(.bss);

    /* Remove sections that won't be relevant to us */
    /DISCARD/ : {
        *(.eh_frame);
        *(.comment);
    }
}

This script should create an ELF executable that can be converted to a flat binary file with OBJCOPY. We could have output as a binary file directly but I separate the two processes out in the event I want to include debug information in the ELF version for debug purposes.

Now that we have a linker script we must remove the ORG 0x7c00 and the boot signature. For simplicity sake we'll try to get the following code (hw.asm) to work:

extern print_str
global start
bits 16

section .text
start:
xor ax, ax         ; AX = 0
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7C00

call print_str     ; call function

/* Halt the processor so we don't keep executing code beyond this point */
cli
hlt

You can include all your other code, but this sample will still demonstrate the basics of calling into a C function.

Assume the code above you can now generate the ELF object from hw.asm producing hw.o using this command:

nasm -f elf32 hw.asm -o hw.o

You compile each C file with something like:

gcc -ffreestanding -c kmain.c -o kmain.o

I placed the C code you had into a file called kmain.c . The command above will generate kmain.o. I noticed you aren't using a cross compiler so you'll want to use -fno-PIE to ensure we don't generate relocatable code. -ffreestanding tells GCC the C standard library may not exist, and main may not be the program entry point. You'd compile each C file in the same way.

To link this code to a final executable and then produce a flat binary file that can be booted we do this:

ld -melf_i386 --build-id=none -T link.ld kmain.o hw.o -o kernel.elf
objcopy -O binary kernel.elf kernel.bin

You specify all the object files to link with the LD command. The LD command above will produce a 32-bit ELF executable called kernel.elf. This file can be useful in the future for debugging purposes. Here we use OBJCOPY to convert kernel.elf to a binary file called kernel.bin. kernel.bin can be used as a bootloader image.

You should be able to run it with QEMU using this command:

qemu-system-i386 -fda kernel.bin

When run it may look like:

You'll notice the letter A appears on the last line. This is what we'd expect from the print_str code.

---

GCC Inline Assembly is Hard to Get Right

If we take your example code in the question:

__asm__ __volatile__("mov $'A' , %al\n");
__asm__ __volatile__("mov $0x0e, %ah\n");
__asm__ __volatile__("int $0x10\n");

The compiler is free to reorder these __asm__ statements if it wanted to. The int $0x10 could appear before the MOV instructions. If you want these 3 lines to be output in this exact order you can combine them into one like this:

__asm__ __volatile__("mov $'A' , %al\n\t"
                     "mov $0x0e, %ah\n\t"
                     "int $0x10");

These are basic assembly statements. It's not required to specify __volatile__on them as they are already implicitly volatile, so it has no effect. From the original poster's answer it is clear they want to eventually use variables in __asm__ blocks. This is doable with extended inline assembly (the instruction string is followed by a colon : followed by constraints.):

With extended asm you can read and write C variables from assembler and perform jumps from assembler code to C labels. Extended asm syntax uses colons (‘:’) to delimit the operand parameters after the assembler template:

asm [volatile] ( AssemblerTemplate
                : OutputOperands 
                [ : InputOperands
                [ : Clobbers ] ])

This answer isn't a tutorial on inline assembly. The general rule of thumb is that one should not use inline assembly unless you have to. Inline assembly done wrong can create hard to track bugs or have unusual side effects. Unfortunately doing 16-bit interrupts in C pretty much requires it, or you write the entire function in assembly (ie: NASM).

This is an example of a print_chr function that take a nul terminated string and prints each character out one by one using Int 10h/ah=0ah:

#include 
__asm__(".code16gcc\n");

void print_str(char *str) {
    while (*str) {
        /* AH=0x0e, AL=char to print, BH=page, BL=fg color */
        __asm__ __volatile__ ("int $0x10"
                              :
                              : "a" ((0x0e<<8) | *str++),
                                "b" (0x0000));
    }
}

hw.asm would be modified to look like this:

push welcome
call print_str ;call function

The idea when this is assembled/compiled (using the commands in the first section of this answer) and run is that it print out the welcome message. Unfortunately it will almost never work, and may even crash some emulators like QEMU.

---

code16 is Almost Useless and Should Not be Used

In the last section we learn that a simple function that takes a parameter ends up not working and may even crash an emulator like QEMU. The main problem is that the __asm__(".code16\n"); statement really doesn't work well with the code generated by GCC. The Binutils AS documentation says:

‘.code16gcc’ provides experimental support for generating 16-bit code from gcc, and differs from ‘.code16’ in that ‘call’, ‘ret’, ‘enter’, ‘leave’, ‘push’, ‘pop’, ‘pusha’, ‘popa’, ‘pushf’, and ‘popf’ instructions default to 32-bit size. This is so that the stack pointer is manipulated in the same way over function calls, allowing access to function parameters at the same stack offsets as in 32-bit mode. ‘.code16gcc’ also automatically adds address size prefixes where necessary to use the 32-bit addressing modes that gcc generates.

.code16gcc is what you really need to be using, not .code16. This force GNU assembler on the back end to emit address and operand prefixes on certain instructions so that the addresses and operands are treated as 4 bytes wide, and not 2 bytes.

The hand written code in NASM doesn't know it will be calling C instructions, nor does NASM have a directive like .code16gcc. You'll need to modify the assembly code to push 32-bit values on to the stack in real mode. You will also need to override the call instruction so that the return address needs to be treated as a 32-bit value, not 16-bit. This code:

push welcome
call print_str ;call function

Should be:

    jmp 0x0000:setcs
setcs:
    cld
    push dword welcome
    call dword print_str ;call function

GCC has a requirement that the direction flag be cleared before calling any C function. I added the CLD instruction to the top of the assembly code to make sure this is the case. GCC code also needs to have CS to 0x0000 to work properly. The FAR JMP does just that.

You can also drop the __asm__(".code16gcc\n"); on modern GCC that supports the -m16 option. -m16 automatically places a .code16gcc into the file that is being compiled.

Since GCC also uses the full 32-bit stack pointer it is a good idea to initialize ESP with 0x7c00, not just SP. Change mov sp, 0x7C00 to mov esp, 0x7C00. This ensures the full 32-bit stack pointer is 0x7c00.

The modified kmain.c code should now look like:

#include 

void print_str(char *str) {
    while (*str) {
        /* AH=0x0e, AL=char to print, BH=page, BL=fg color */
        __asm__ __volatile__ ("int $0x10"
                              :
                              : "a" ((0x0e<<8) | *str++),
                                "b" (0x0000));
    }
}

and hw.asm:

extern print_str
global start
bits 16

section .text
start:
    xor ax, ax            ; AX = 0
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov esp, 0x7C00
    jmp 0x0000:setcs      ; Set CS to 0
setcs:
    cld                   ; GCC code requires direction flag to be cleared 

    push dword welcome
    call dword print_str  ; call function
    cli
    hlt

section .data
welcome db 'Developped by Marius Van Nieuwenhuyse', 0x0D, 0x0A, 0

These commands can be build the bootloader with:

gcc -fno-PIC -ffreestanding -m16 -c kmain.c -o kmain.o 
ld -melf_i386 --build-id=none -T link.ld kmain.o hw.o -o kernel.elf
objcopy -O binary kernel.elf kernel.bin

When run with qemu-system-i386 -fda kernel.bin it should look simialr to:

---

In Most Cases GCC Produces Code that Requires 80386+

There are number of disadvantages to GCC generated code using .code16gcc:

• ES=DS=CS=SS must be 0
• Code must fit in the first 64kb
• GCC code has no understanding of 20-bit segment:offset addressing.
• For anything but the most trivial C code, GCC doesn't generate code that can run on a 286/186/8086. It runs in real mode but it uses 32-bit operands and addressing not available on processors earlier than 80386.
• If you want to access memory locations above the first 64kb then you need to be in Unreal Mode(big) before calling into C code.

If you want to produce real 16-bit code from a more modern C compiler I recommend OpenWatcom C

• The inline assembly is not as powerful as GCC
• The inline assembly syntax is different but it is easier to use and less error prone than GCC's inline assembly.
• Can generate code that will run on antiquated 8086/8088 processors.
• Understands 20-bit segment:offset real mode addressing and supports the concept of far and huge pointers.
• wlink the Watcom linker can produce basic flat binary files usable as a bootloader.

---

Zero Fill the BSS Section

The BIOS boot sequence doesn't guarantee that memory is actually zero. This causes a potential problem for the zero initialized region BSS. Before calling into C code for the first time the region should be zero filled by our assembly code. The linker script I originally wrote defines a symbol __bss_start that is the offset of the BSS memory and __bss_sizeb is the size in bytes. Using this info you can use the STOSB instruction to easily zero fill it. At the top of hw.asm you can add:

extern __bss_sizeb
extern __bss_start

And after the CLD instruction and before calling any C code you can do the zero fill this way:

; Zero fill the BSS section
mov cx, __bss_sizeb       ; Size of BSS computed in linker script
mov di, __bss_start       ; Start of BSS defined in linker script
rep stosb                 ; AL still zero, Fill memory with zero

---

Other Suggestions

To reduce the bloat of the code generated by the compiler it can be useful to use -fomit-frame-pointer. Compiling with -Os can optimize for space (rather than speed). We have limited space (512 bytes) for the initial code loaded by the BIOS so these optimizations can be beneficial. The command line for compiling could appear as:

gcc -fno-PIC -fomit-frame-pointer -ffreestanding -m16 -Os -c kmain.c -o kmain.o