![]() ![]() I386-elf-_g++ x86_64-elf-g++ -m64 kmain.cpp boot4.o -o kernel.bin -nostdlib -ffreestanding -std=c++11 -mno-red-zone -fno-exceptions -nostdlib -fno-rtti -Wall -Wextra -Werror -T linker.ld You want to compile the assembly, then link it all together. We’re ready to compile and run this thing! Here are my versions of the linker script, cpp file and bootsector assembly! Compiling and Running We reserve 16 KB for a kernel call stack. Next we’ll hack the disk reading function call to load more than one sector: mov al, 6 sectors to readįinally we set up a call stack for C++ to use and call the actual function. boot section so it’s placed first as well as define a global boot: symbol so the linker knows about the entry point. First we’re going to put everything in the. Next we need to modify our assembly so it can be linked with gcc. = 0x7c00 tells it to start outputting code at that offset, similar to the org 0x7c00 command. It will otherwise output a binary file in ELF. OUTPUT_FORMAT("binary") tells gcc to output raw assembly directly. The ENTRY(boot) means the entry point of the program is the boot symbol. We then want all of the C++ code placed after that in the file. ![]() We want the boot4.asm code positioned at offset 0x7c00 so that the 510th bytes equal 0xAA55 which makes it a valid bootsector. We need to tell gcc how to link our cpp file and asm files together. This will install i386-elf-_g++ on your path which will compile and link stuff for us! Installing takes a while - you might want to grab a cuppa whilst it compiles. Interestingly upgrading Mac OSX can mess gcc and all sorts of things up so try these commands if things fall over. ![]() ![]() Since we’re building a 32 bit operating system we also want the compiler to output 32 bit instructions - not 64 bit ones!Ĭompiling a cross compiler is an absolute pain - but luckily I’ve hacked a homebrew tap you can use: brew tap zanders3/homebrew-gcc_cross_compilers I also can’t for the life of me convince clang to use a linker script and stop adding loads of OSX specific stuff. To do this safely we need to create a cross compiler - this is safer than using your system’s C++ compiler since we can be certain about what instruction set and function call method the compiler uses. We now need to compile and link this function. The extern "C" prevents C++ from name mangling the function allowing us to call it from assembly. I’ve intentionally kept the function as simple as possible - it does almost the same thing as the assembly code. Here is the C++ function we want to compile: extern "C" void kmain() Next we’re going to load a C++ function that prints Hello world into memory and then call it from the bootloader. You can compile and run it with nasm -f bin boot3.asm -o boot.bin & qemu-system-x86_64 -fda boot.bin getting this result: It’s probably easiest to download boot3.asm directly. The last line pads our bootloader to 1024 bytes so we’re not copying uninitialised bytes from disk. Hello: db "Hello more than 512 bytes world!!",0 So we can now load 512 bytes from the second sector into memory! Let’s make use of that and move the hello world printing code from last time beyond the first 512 bytes of disk. Earlier on we stashed it into memory with mov, dl. The disk number is implicitly placed into dl by the BIOS on startup. The ah=0x2 int 0x13 command reads sectors from a drive to a target location, like this: mov ah, 0x2 read sectors To do this we’re going to use the int 0x13 interrupts which provide disk services. If we want to write programs larger than 512 bytes (maybe you don’t want to and like a challenge?) we’re going to have to load more off the disk. The BIOS only loads the first 512 bytes of the bootsector. This time we’re going to compile and load a C++ function into memory and call it! Going beyond 512 bytes In our previous article we got our CPU into 32-bit protected mode and printed the screen using the directly mapped VGA memory. ![]()
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