Add and link article about cross compiling binutils

_posts/2015-08-18-multiboot-kernel.md

@@ -172,6 +172,8 @@ Idx Name          Size      VMA               LMA               File off  Algn
   1 .text         0000000b  0000000000100020  0000000000100020  000000a0  2**4
                   CONTENTS, ALLOC, LOAD, READONLY, CODE
 ```
+_Note_: The `ld` and `objdump` commands are platform specific. If you're not working on x86_64 architecture, you will need to [cross compile binutils]. Then use `x86_64-elf-ld` and `x86_64-elf-objdump` instead of `ld` and `objdump`.
+[cross compile binutils]: {{ site.url }}/rust-os/cross-compile-binutils.html
 
 ## Creating the ISO
 The last step is to create a bootable ISO image with GRUB. We need to create the following directory structure and copy the `kernel.bin` to the right place:
@@ -283,6 +285,7 @@ Some comments (see the [Makefile tutorial] if you don't know `make`):
 - the `$(wildcard src/arch/$(arch)/*.asm)` chooses all assembly files in the src/arch/$(arch)` directory, so you don't have to update the Makefile when you add a file
 - the `patsubst` operation for `assembly_object_files` just translates `src/arch/$(arch)/XYZ.asm` to `build/arch/$(arch)/XYZ.o`
 - the `$<` and `$@` in the assembly target are [automatic variables]
+- if you're using [cross-compiled binutils][cross compile binutils] just replace `ld` with `x86_64-elf-ld`
 
 [automatic variables]: https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html
 

rust-os/cross-compile-binutils.md

@@ -0,0 +1,47 @@
+---
+layout: page
+title: Cross Compile Binutils
+category: "rust-os"
+---
+The [GNU Binutils] are a collection of various binary tools such as `ld`, `as`, `objdump`, or `readelf`. These tools are platform-specific, so you need to compile them again if your host system and target system are different. In our case, we need `ld` and `objdump` for the x86_64 architecture.
+[GNU Binutils]: https://www.gnu.org/software/binutils/
+
+## Building Setup
+First, you need to download a current binutils version from [here][download] \(the latest one is near the bottom). After extracting, you should have a folder named `binutils-2.X` where `X` is for example `25.1`. Now can create and switch to a new folder for building (recommended):
+[download]: ftp://sourceware.org/pub/binutils/snapshots
+
+```bash
+mkdir build-binutils
+cd build-binutils
+```
+
+## Configuration
+We execute binutils's `configure` and pass a lot of arguments to it (replace the `X` with the version number):
+
+```bash
+../binutils-2.X/configure --target=x86_64-elf --prefix="$HOME/opt/cross" \
+    --disable-nls --disable-werror \
+    --disable-gdb --disable-libdecnumber --disable-readline --disable-sim
+```
+- The `target` argument specifies the the x86_64 target architecture.
+- The `prefix` argument selects the installation directory, you can change it if you like. But be careful that you do not overwrite your system's binutils.
+- The `disable-nls` flag disables native language support (so you'll get the same english error messages). It also reduces build dependencies.
+- The `disable-werror` turns all warnings into errors.
+- The last line disables features we don't need to reduce compile time.
+
+## Building it
+Now we can build and install it to the location supplied as `prefix` (it will take a while):
+
+```bash
+make
+make install
+```
+Now you should have multiple `x86_64-elf-XXX` files in `$HOME/opt/cross/bin`.
+
+## Adding it to the PATH
+To use the tools from the command line easily, you should add the `bin` folder to your PATH:
+
+```bash
+export PATH="$HOME/opt/cross/bin:$PATH"
+```
+If you add this line to your e.g. `.bashrc`, the `x86_64-elf-XXX` commands are always available.