New Filesystem p2: Booting The Disk Image | 32 Bit OS Dev (x86 asm / C)
48Changing the boot sector, 2nd stage, and 3rd stage boot loaders for the new disk image (and debugging).
This continues from the last video over making the disk image: https://www.youtube.com/watch?v=OC9lD4MqxzQ
OS Dev playlist:
https://www.youtube.com/playlist?list=PLT7NbkyNWaqajsw8Xh7SP9KJwjfpP8TNX
Source code for this video:
https://git.sr.ht/~queso_fuego/amateur_os_video_sources/tree/master/item/44_filesys_disk_img_boot
Git Repos:
Sourcehut: https://git.sr.ht/~queso_fuego/quesos
Github mirror: https://github.com/queso-fuego/amateuros
NOTE: Depending on certain commits, the OS will be broken from new filesystem changes until completed. Loading/running a program like the calculator/editor, printing a directory, or other things may not work at all in the interim.
The last commit before breaking changes can be used after git clone with:
git checkout 08d3416cf5edbbab9684b8f911350e03ca9fe8af
Misc. links:
https://en.wikipedia.org/wiki/Hexspeak#Notable_magic_numbers
Join the Community Discord: https://discord.gg/yKm4T89QFn
Contact:
email: fuegoqueso@gmail.com
twitter: @Queso_Fuego
twitch: https://www.twitch.tv/queso_fuego
Notes:
- Due to laziness and not knowing how it would work, I didn't read from the inode single/double indirect blocks in fs_load_file() (or other places).
After thinking on it, that shouldn't be too hard to do, and could be accomplished with some nested loops.
FS_BLOCK_SIZE / sizeof(extent_t) = max number of extents to read from 1 single indirect block.
FS_BLOCK_SIZE / sizeof(uint32_t) = max number of single indirect block values to read from 1 double indirect block.
Given this, a rough outline to read the file data from a single indirect block would be:
Read that single indirect block to memory, and for each of the extents in the block, while current_extent.first_block != 0 (or length_blocks != 0) && current_extent less than max_extents, read the current extent to memory by loading disk sectors for length_blocks starting at first_block, appended to the memory from the end state of reading the inode's 4 direct extents.
Reading the double indirect block would add another outer loop to the single indirect block loop, where each 4 byte value in the double indirect block would be a single indirect block. So you could read the double indirect block to memory, loop over each single indirect block value while current_single_indirect_block != 0 && current_single_indirect_block less than max_indirect_blocks, and run the single indirect block loop for each value.
I might add this in the next video, even if it won't be used for a while.
Next video:
- Adding open/close/read/write/seek syscalls; open file table & inode table; inode or file path helper functions; file system saving/creating/deleting functions; kernel malloc/printf; printing dir entries; adding field width to printf; other things? However much gets done in an hour or two.
- Let me know if there's anything specific you'd like to see!
Outline:
0:00 Intro, small make_disk changes
2:10 change bootSect, load boot block/superblock/3rd stage bootloader
26:08 change makefile
32:25 change 2ndstage
41:39 make_disk bugfix
44:24 change 3rdstage
58:01 create fs_impl.h, fs_load_file()
1:11:09 print debugging, change kernel
1:23:20 it boots now, Coming up & Outro
Questions about setup/software/etc.? Check the FAQ: https://queso_fuego.srht.site/about.html
Music credits:
Downtown Glow by Ghostrifter & Devyzed
Creative Commons — Attribution-NoDerivs 3.0 Unported — CC BY-ND 3.0
Music promoted by https://www.chosic.com/free-music/all/
#osdev #filesystem #programming