ZyXOS

The OS for ZyX-20x4

Booting

• When the computer is turned off, BIOS is stored in the ROM of controller
• After the computer is turned on, BIOS is loaded to its RAM
• BIOS loads boot loader from the computer's ROM
• Boot loader loads and starts the OS
  • #TODO: should be more detailed here

Syscalls

The main way of process communication with the operating system is syscalls.

What happens when the system is called in ZyXOS:

• The current process tries to make a system call, which means, to access the memory, it is not allowed
• When that forbidden address is tried to be accessed, MMU raises an NMI for the processor
• The NMI handler (stored in 0066h) can recognize that address as a location of syscall handler
• The OS enters the kernel mode
• The program counter is set to that address, and the system call is performed
• The OS enters user mode
• Process continues execution

Implemented syscalls

The ZyXOS will have all 5 types of syscalls:

• Process control
• File management
• Device management
• Information maintenance
• Communication

TODO: Description of each syscall, present in the ZyXOS

Memory management

ZyXOS works with an external Memory Management Unit, but also keeps the copy of MMU mapping in RAM. Each process is associated with own pages mapping table with such properties:

• Some fixed pages are mapped 1 to 1 that are not accessible from user mode (e.g. first pages will contain syscalls handlers, BIOS functions and so on.)
• When the process starts, most of the pages (except stack and .text section) are not commited until they are needed

Filesystem

The ZyXOS will use the FAT16 file system, with memory controller made in Arduino.