TomatOS

Commands

$ nasm -f bin ./boot.asm -o ./boot.bin to compile the boot.asm into binary file. $ ndisasm boot.bin prints a disassembly listing of the binary file infile. $ qemu-system-x86_64 -hda boot to execute a virtual machine x86-64 using boot as hard disk.

Protection ring

It exists mechanisms that protects data from faults and malicious behaviours, with differents level of privileges.

++ Kernel
+  Device drivers
-  Device drivers
-- Applications

Bootloader

The computer starts in reel mode, the initial state of an x86 processor. This is a restricted mode (only 16 bits) where the memory is addressed without protection. It is used at the start of the BIOS to execute the bootloader. A bootloader is the first program loaded by the BIOS at startup to initialise devices and load the operating system. It configures the registers, the stack, or the displays, loads the kernel and stores it in memory, and finally switches to protect mode (32 bits) or long mode (64 bits) and gives it the controls.

The memory is accessed by a segment and an offset,

• CS Code Segment
• SS Stack Segment
• DS Data Segment
• ES Extra Segment

Implementing Global Descriptor Table (GDT)

GDT is a datastructure used to define caracteristics of the memory segments and their use in the system. It defines how the memory segments are organised in the processor, it teaches the processor how to interpret the memory addresses It is 8 bytes long and contains:

• base: address where the segment begins
• limit: maximum address (<=> the size of the segment)
• flags: defines granularity flags and tells how limits should be interpreted.
• access byte: has its own structure and describes various elements in one code:

  _   _ _   _   _    _   _    _
| P | DPL | S | E | DC | RW | A |

• P: Present bit -> if the segment is present in memory
• DPL: Descriptor Privilege Level -> CPU privilege level of the segment (from kernel to user application)
• S: Scriptor type bit (define if its system segment or code or data segment)
• E: Executable bit Defines if its executable (code segment) or not (data segment)
• DC: Direction bit if data segment or Conforming bit if code segment
• RW: Readable bit or Writable bit
• A: Accesses bit, when the CPU accesses the segment, it is sets to one Each descriptor stores an information about a specific thing (service routine, task, data, code ...). There is three types of tables:
• Global Descriptor Table
• Local Descriptor Table
• Interrupt Descriptor Table These tables are only placed into memory once, at boot time, and then edited later when needed.

Calling c code ! (GCC cross compiler)

Using CHS which is Cylinder Head Sector.

make clean ./build.sh && make qemu