VPOS

VPOS is an operating system designed (primarily for fun) with the end goal of creating an OS from scratch. We hope to eventually implement advanced features such as native shared library linking support, a GUI interface, etc. with minimal support from existing tutorials, so we can arrive at our own solutions and, hopefully, learn a lot about the methodology and issues which arise through this development.

The original version of this repo can be found at BestVG/VPOS. For the moment, BestVG is working on other projects, so to ensure his repo stays true to his vision for the project, I will be making changes here for the forseeable future.

Memory Layout

We are working on migrating the built live-image file to be a functional FAT-32 partition. At the moment, is arranged as follows:

1st sector (loaded to 0x7C00): boot_section (Contains some FAT-32 metadata; ends with word 0xaa55)
2nd sector: FS_INFO sector (More FAT-32 metadata)
(dynamic) (loaded to 0x1000): kernel code (Note: due to the dynamic nature of kernel code, the following sections may not fall exactly on sector boundaries)
Next 2 sectors (1024 bytes): Reserved for the stack
Next 10 sectors (5120 bytes): Reserved for malloc (heap)
After this, any remaining space in the sector is padded with 0x00
Memory Map (Will be loaded from BIOS into 0x500)
Page Tables (Will be generated at 0x100000)

Boot Process

At the moment, the process for booting the OS is as follows:

1. Begin Program
2. Jump over FAT_32 metadata
3. Setup a small stack (ss = 0x07e0 sp = 0x1200)
4. Load the kernel from disk (to 0x1000)
5. Enable A20 line
6. Jump to kernel
7. Make sure CPU supports all required features (See Feature Checks)
8. Load the memory map from BIOS (into 0x8000) (See int 0x15, eax=0xE820)
9. Load GDT
10. Enable protected mode in cr0
11. Set all segment registers (besides CS) to the data segment
12. Update the 32-bit stack pointer to use the stack defined in the kernel
13. Enable Physical Address Extensions (PAE) in cr4
14. Generate page tables (starting at 0x100000)
15. Store page table location in cr3
16. Enable long mode in EFER_MSR
17. Enable paging in cr0
18. Jump to 64-bit kernel code
19. Reset 64-bit stack pointer to the kernel stack
20. Jump to kernel_main

Feature Checks

1. Check that the CPU supports cpuid by changing the CPUID bit in EFLAGS
2. Check that the CPU's vendor id matches "AuthenticAMD" or "GenuineIntel" (eax = 0x00 should return valid values in ebx edx and ecx)
3. Check that the cpuid instruction supports all of the leaves required to check the remaining features (eax = 0x00 should return eax >= 0x01, and eax=0x80000000 should return eax >= 0x80000001)
4. Check that long mode is supported (eax = 0x80000001 should return edx & 0x20000000 = 0x20000000)
5. Check that model specific registers are supported (eax = 0x1 should return edx & 0x00000020 = 0x00000020)
6. Check that all required paging features are supported (PSE, PAE, PAT, and PSE36) (eax = 0x1 should return edx & 0x00030048 = 0x00030048)
7. Check that APIC is supported (eax = 0x1 should return edx & 0x0000200 = 0x0000200)

If any of the above checks fail, the user will be notified and given the option to ignore the error (at risk).

Building/Running

The Makefile contains the following targets

build: Builds the bin/live-image binary which contains the compiled OS
clean: Deletes the bin directory, effectively removing the binaries from all previous builds
rebuild: Runs clean followed by a build
run: Attempts to build and run the live-image using QEMU
debug: Attempts to build and run the live-image using QEMU with guest_errors logging enabled to log.txt
disk: Builds the live-image and packages it in bin/disk.vdi
run-vbox: Builds bin/disk.vdi and attempts to start a VPOS vm in VirtualBox. (This assumes that a vm named VPOS is already configured in VirtualBox)
*.mem: Connects to a running VirtualBox machine and dumps the memory to the requested file