TwistedOS

x86 32 bit OS from scratch

Design

2 stage boot loader

• stage 1 loads the FAT 12 table then loads in stage 2 of the bootloader
• stage 2 loads the entire kernel from the FAT 12 file system at above 1mb of memory
  • Initializes GDT
  • Switches to 32 bit mode
  • Jumps execution to kernel code
• 2 stages are used due to the 512 byte limit of the inital bootloader and the 1mb limit on 20bit addressing (16 bit + A gate)

Kernel

• installs TSS to allow for interupt handling in user mode
• Set up IDT for interrupt handling
• Initalizes IDT with ISRs
• remaps the PIC to allow for interrupts
• Sets up keyboard driver
• enables interrupts
• Initalizes physical memory manager
  • bit map that manges the workable memory range using blocks of 4096 (size of pages)
• Initalizes virtual memory
  • sets up page table dir and page tables
  • identiy maps first 16mb of memory
  • allocates space for pages using physical memory manager
• Initializes network card driver for rtl8139
  • card sends interrupt any time a packet is recv or sent
• ethernet layer
  • checks if packet is an ARP request or IPv4
• Console support
  • console printing support for output and debugging
  • user is able to enter commands
  • user can enumerate PCI devices
  • user can clear screen

Resources

https://www.ctyme.com/intr/rb-0106.htm https://www.ctyme.com/intr/rb-0607.htm // interrupt codes https://www.eit.lth.se/fileadmin/eit/courses/eitn50/Literature/fat12_description.pdf // FAT12 https://wiki.osdev.org/Rolling_Your_Own_Bootloader https://wiki.osdev.org/Creating_an_Operating_System https://www.sqlpassion.at/archive/2022/03/03/reading-files-from-a-fat12-partition/ https://www.calculator.net/hex-calculator.html?number1=7E00&c2op=%2B&number2=2800&calctype=op&x=Calculate https://wiki.osdev.org/Memory_Map_(x86) http://www.brokenthorn.com/Resources/OSDev18.html https://www.rapidtables.com/convert/number/hex-to-binary.html?x=D https://wiki.osdev.org/PCI https://wiki.osdev.org/RTL8139 http://realtek.info/pdf/rtl8139cp.pdf https://ics.uci.edu/~harris/ics216/pci/PCI_22.pdf https://github.com/dreamportdev/Osdev-Notes/tree/master/06_Userspace https://en.wikipedia.org/wiki/Ethernet_frame https://en.wikipedia.org/wiki/Address_Resolution_Protocol