I started programming Spider-OS, more than a year ago before this website, and several elements are now realized.
The system runs on Raspberry Pi 2 and Pi 3 (the Neon component is needed).
Here is the list of modules made in assembly language ;-) :
the kernel with interrupt management
UART PL011 serial unit, it allows me to debug
the display via the framebuffer and the mailbox, and Videocore (difficult to program the Videocore, because the available documentation is limited)
a memory allocation module
USB support : support keyboards, and some keys : a lot of work to operate the USB correctly, but hey it is so essential
SD port : read SD cards, with FAT32 format supported
the different timers (Generic, local, system): necessary for multitask in particular
the user interface with window management
One of the many documentations I used for this work : BCM2835-ARM-Peripherals.pdf
This is the documentation on the main component of Raspberry Pi.
Here I am now designing the keypad command interpreter.