Day 15 was about File System.

The book that I am following has come to an end with File System as the final chapter where it teaches to implement tar file as a filesystem.

tar is an archive format (like zip) that can contain multiple files. It contains file contents, filenames, creation dates, and other information necessary for a filesystem. Compared to common filesystem formats like FAT or ext2, tar has a much simpler data structure.

First a simple directory with two files is made as an example which is then archived/compressed into the tar format using the command line tar tool. Which, also is added to the buildscript before launching the qemu VM.

(cd disk && tar cf ../disk.tar --format=ustar *.txt)

# The following flag is also added to qemu.
-drive id=drive0,file=disk.tar,format=raw,if=none \

To read the filesystem, first, the data structures related to the tar filesystem is defined in kernel.h. This implementation has all files read from the disk at boot.

To write to the disk, the contents of the files variable is written back to the disk in the tar file format. The files variable contains the copy of the disk variable declared as a static variable (It is a temporary buffer and, since the stack has a limited size it is preferred to use static variable instead of local stack variable.) which contains the disk image.

File read/write system calls are then designed in the user.c and user.h files and then the system calls are implemented on the kernel files and then the system calls are implemented in the kernel. We also add the read and write commands to the shell.

But, we still have to address the user pointers.

In RISC-V, the behaviour of S-Mode (kernel) can be configured through sstatus CSR, including SUM (permit Supervisor User Memory access) bit. When SUM is not set, S-Mode programs (i.e. kernel) cannot access U-Mode (user) pages.

All we need to do is to set the SUM bit when entering user space and voila! Our OS with a filesystem implementation is now ready.