Make it boot and start interpreting the .image with text output for tracing and debugging.
The old SqueakNOS used a custom made mutant boot loader, borned from mixing LILO, the Linux kernel boot loader, and our dirty fingers. We had quite a few problems with it, and it was really complicated to put the needed environment to compile it. So we chose to replace it for something better and more modern: GRUB.
We first thought on GRUB because it, of course, handles all initialization, switches to flat 4G mode can load huge kernles in high memory, and also has support for modules loading (module and modulenounzip GRUB commands). We wanted to detach the native part and the Squeak .image, which were glued in a single file in the old SqueakNOS, making it easier for Squeakers to touch the .image without having to set up the compiling environment.
We needed to implement the switch to graphics mode before starting jumping to the interprenter. In the old SqueakNOS we did it from real mode (that's why we had so poor video support). But this time GRUB gave us control already in protected mode, so we couldn't just call the right INT. Luckily, GRUB already has the roots for supporting graphic video mode selection, and although it is not usable in the standard GRUB distro, we could solve the problem quite easily, by copying the testvbe command into the setvbe command, and removing everything after the command switches the video mode. We later found out that the video memory address changes from box to box, so we also hacked the vbeprobe command to show the video memory address.
We also learned that GRUB supports part of the multiboot protocol and is able to load ELF binaries, this led us reduce the amount of assembly code to barely around 20 line. It's not that we are afraid of assembly, but if multiboot was supported in some other platform (Apple/PowerPC), porting SqueakNOS' kernel should be really easy. [Never tried]
At this point we had GRUB loading the kernel and loading the .image all in memory, we had some glue code to make the .image look like a file so Squeak's intepreter could start interpreting it. We coded console text output support, hacked some debugging messages in the interpreter loop, some stack traces here and there and figured out what Plugins needed to be compiled for Squeak to start up: we finally managed to make the interpreter loop run loose.