The process of calling C++ functions from assembly code requires careful attention to calling conventions and function name mangling.
Step 1: Write Your C++ Function
First, write the C++ function you want to call from assembly. Make sure to declare it as extern "C" to prevent name mangling.
// example.cpp
extern "C" {
void myFunction();
}
void myFunction() {
// Your C++ function code here
// For demonstration, let's print "Hello from C++" to the screen
const char* message = "Hello from C++";
for (int i = 0; message[i] != '\0'; ++i) {
// Print character to screen using BIOS interrupt
asm volatile(
"mov $0x0E, %%ah \n" // AH = 0Eh (Display character)
"mov $0x07, %%bh \n" // BH = 0 (Page number)
"mov %0, %%al \n" // AL = character to print
"int $0x10 \n" // Call BIOS video interrupt
:
: "m" (message[i])
: "ah", "al", "bh"
);
}
}
Step 2: Write Your Assembly Code
Now, write your assembly code and call the C++ function.
section .text
extern myFunction ; Declare the external C function
global _start ; Entry point for the program
_start:
; Your assembly code here
; Print "Hello from Assembly" to the screen
mov ah, 0x0E ; AH = 0Eh (Display character)
mov bh, 0x00 ; BH = 0 (Page number)
mov al, 'H' ; AL = 'H'
int 0x10 ; Call BIOS video interrupt
mov al, 'e' ; AL = 'e'
int 0x10 ; Call BIOS video interrupt
mov al, 'l' ; AL = 'l'
int 0x10 ; Call BIOS video interrupt
mov al, 'l' ; AL = 'l'
int 0x10 ; Call BIOS video interrupt
mov al, 'o' ; AL = 'o'
int 0x10 ; Call BIOS video interrupt
mov al, ' ' ; AL = ' '
int 0x10 ; Call BIOS video interrupt
mov al, 'f' ; AL = 'f'
int 0x10 ; Call BIOS video interrupt
mov al, 'r' ; AL = 'r'
int 0x10 ; Call BIOS video interrupt
mov al, 'o' ; AL = 'o'
int 0x10 ; Call BIOS video interrupt
mov al, 'm' ; AL = 'm'
int 0x10 ; Call BIOS video interrupt
mov al, ' ' ; AL = ' '
int 0x10 ; Call BIOS video interrupt
mov al, 'A' ; AL = 'A'
int 0x10 ; Call BIOS video interrupt
mov al, 's' ; AL = 's'
int 0x10 ; Call BIOS video interrupt
mov al, 's' ; AL = 's'
int 0x10 ; Call BIOS video interrupt
mov al, 'e' ; AL = 'e'
int 0x10 ; Call BIOS video interrupt
mov al, 'm' ; AL = 'm'
int 0x10 ; Call BIOS video interrupt
mov al, 'b' ; AL = 'b'
int 0x10 ; Call BIOS video interrupt
mov al, 'l' ; AL = 'l'
int 0x10 ; Call BIOS video interrupt
mov al, 'y' ; AL = 'y'
int 0x10 ; Call BIOS video interrupt
; Call the C++ function
call myFunction
; Infinite loop
cli ; Clear interrupts
.endloop:
hlt ; Halt processor
jmp .endloop ; Infinite loop
section .data
; Define any data variables here
section .bss
; Define any uninitialized variables here
Step 3: Compile and Link
Compile the C++ code and the assembly code separately and then link them together.
g++ example.o bootloader.o -o bootloader.bin -m32 -nostartfiles -nostdlib -ffreestanding
nasm -f elf32 bootloader.asm -o bootloader.o
Step 4: Create a Bootable Disk Image
Create a bootable disk image containing both the bootloader and the C++ function code.
dd if=/dev/zero of=disk.img bs=512 count=2880
dd if=bootloader.bin of=disk.img conv=notrunc
Step 5: Run Your Program
Run your program using a virtual machine like QEMU.
qemu-system-i386 -fda disk.img