Yehos

An operating system for the people

We are building an operating system from scratch under the guidance of @saulpw and his work on frotzos. This is our story...

Components

• Bootloader
• Keyboard IO
• Video output
• Lazy loading of applications from ISO disk
• Virtual memory and demand paging
• Multiple concurrent processes

Yehos supports two applications:

• A video player playing a text-encoded version of the film Star Wars (1977).
• A REPL for the Forth programming language, implemented in x86 assembly.

Requirements

• The qemu emulator (depending on installation, you may have to also need qemu-arch-extra to support the i386 architecture.

• nasm and ndisasm (come together)

• mkisofs command line tool

• The Star Wars vga file for the movie.

Building

Linux users can simply run make.

It is suggested that Macs first build a cross-compiler. This will allow them to generate 32-bit ELF binary files.

Mac users can either:

run sudo yehos/tools/build/cross_compiler.sh (the cross compiler will be installed to /opt/cross/)

or

execute the commands in the file manually.

Running

make run will run Yehos normally.

To debug Yehos with GDB, run

qemu-system-i386 -s -S -cdrom yehos-patched.iso &
gdb

The contents of the .gdbinit file point gdb to target qemu (localhost:1234).

It is possible to get a console for interacting with qemu by following the instructions here. Running info tlb in the console is especially helpful for debugging virtual memory mappings.

Memory Layout

Physical Memory

• ? - 0x6000: bootstack
• 0x7c00 - 0x7dff: boot sector
• 0x8000 kernel
• 0x80000 starting page directory
• 0x81000 first page table
• 0x100000 page pool

Virtual Memory

• 0 - 0x100000: identity mapped to physical memory
• 0x100000 - ?: disk iso
• 0x1000000: entry point to applications
• 0xffbfe000 - 0xffbfefff: reserved for copying to physical memory
• 0xffbff000 - 0xffbfffff: application stack
• 0xffc00000 - end: page table