Creation of an operating system for learning purposes following the 539kernel book.

1. Prerequisite
2. Glossary
3. Assembly Information
4. Resources

Install QEMU.

Install Bochs

Linux:

1. sudo apt update
2. sudo apt install bochs bochs-x bochsbios vgabios

1. cd 539kernel
2. make

Bootloader: Code that is responsible for loading the main kernel from the disk to the main memory so the kernel can be executed.

Register: Small memory inside the processor’s chip. We can store and read data form it, the registers are too small and too fast.

General purpose registers: Can store any kind of data. The size of each one is 32 bits.

Special purpose registers: Provided by the architecture for some specific purposes.

Fat binary (FatELF): Binary format where the software machine code of multiple processor architectures are gathered in one binary file and the suitable machine code will be loaded on run based on the type of the system’s processor.

Flat binary: File which doesn't use any specification, instead, the output is stored as is with no additional information or organization, only the output machine language of our code.

Object file: Machine code of a source file, generated by the compiler. It’s not an executable file, and it can be linked with other object files to generate the final executable file.

Program Counter: Stores the memory address for the instruction that will be executed in the next instruction cycle of the processor.

Caller: Function which call another function.

Callee: Function which is called by the caller.

byte (b): 8 bits.

word (w): 16 bits (2 bytes).

doubleword (d): 32 bits (4 bytes).

Operating mode: Specifies the overall picture of the processor.

• e.g. Maximum size of available registers, the available advanced features for the running operating system, the restrictions and so on.

Real mode: 16-bits operating mode.

Protected mode: 32-bits operating mode.

• Ability to deal with 4GB of main memory.

Long mode: 64-bits operating mode.

• Provide more capacity for its users (e.g. Can deal with 16 exabytes of memory).

Process: Program that is currently running.

Kernel-mode: Privilege to do anything.

• e.g. access any memory location, access any resource of the system and perform any operation.

User-mode: Restricted environment where the code that runs on it doesn’t have the privilege to perform sensitive actions.

Current Privilege Level (CPL): Privilege level of the currently running code.

Nibble: Half a byte.

Flat memory model: Viewing the memory as an array of contiguous cells.

Segment: Store a bunch of related data.

Code segment: Store the code of the program under execution.

Data segment: Store the data of the program under execution.

Stack segment: Store the data of program’s stack.

Offset: Reference to a byte.

Near call (jump): Any call (or jump) to a code inside the same code segment of the caller.

Far call (jump): Any call (or jump) to a code outside the same code segment of the caller.

Global descriptor table (GDT): A data structure which is used by Intel x86-family processors starting with the 80286 for the purpose of defining the characteristics of the various memory areas (segments) which are used during program execution, including the size, the base address, and access privileges like write and executable.

• Source: https://www.geeksforgeeks.org/what-is-global-descriptor-table/

Local descriptor table (LDT): Same functionality and structure of GDT. But multiple LDT can be defined in the system, and each one of them can be private to a specific process that is currently running on the system, also, multiple running processes can share a single LDT that is considered private for them by the kernel and no other processes can reach this given LDT.

Logical memory address: Generated memory address.

Physical memory address: Real memory address.

Segment limit field: Size of a given segment.

Descriptor privilege level (DLP): Privilege level of a given segment.

Run-time heap: Region of process’ memory that provides an area for dynamically allocated objects (e.g. variables).

Run-time stack: Region of process’ memory that is used to store the values of local variables and function parameters.

• It’s a way to implement function’s invocation.

Event-driven programming: Paradigm where the program is driven by events. It keeps idle and waiting for any event to occur and once an event occurs, the program starts to work by handling this event.

Handler: Function dedicated to an event.

Non-maskable interrupts (NMI): Type of interrupts that will interrupt the execution of the current code even if the interruption is disabled.

Control register: Contain values that determine the behavior of the processor.

Programmable Interrupt Controller (PIC): Send the interrupts of other devices (e.g. hard disk) to the processor.

• Mediator between the machine’s I/O devices and the processor.
• Only 8 devices can be attached to one PIC device.

PIC remapping: Change the default mapping between IRQs and interrupt number of the processor.

Slave PIC: PIC attached to the master.

Master PIC: The first PIC.

Interrupt request (IRQ): Interrupting send to the PIC.

System Timer: Device that sends an interrupt in each unit of time (extremely useful for multitasking environment).

Memory-mapped I/O: The main memory is used to perform the communication with external devices by the processor.

Port-mapped I/O: Communication with external devices by the processor. In this method, each device has a ports, each port has its own unique number and job.

Demand paging: Memory management technique used in virtual memory systems where pages enter main memory only when requested or needed by the CPU.

• Source: https://www.geeksforgeeks.org/what-is-demand-paging-in-operating-system/

Swapping operation: When the main memory is full and a page is needed to be loaded, a page frames is chosen to be removed from the main memory (victim frame), its content is written into the disk (swapped out), and its place in the main memory is used for the new page that should be loaded.

Least recently used (LRU): Algorithm that selects the oldest element based on a time metric.

Translation lookaside buffer (TLB): Cache used for page directory and page tables.

• ESI, EDI, EBP and ESP are considered as general purpose registers in x86 architecture (according to Intel’s manual), they store some important data in some cases, and it’s better to use them carefully if we are forced to.
• call (Assembly instruction): Used to call a code that resides in a given memory address.
  • The instruction pushes the return address into the stack.
• ret (Assembly instruction): Used to resume the execution of the caller.
  • The instruction gets the return address from the stack.
• jump (Assembly instruction): Jumps to the specified memory address, but doesn’t store the return address in the stack.
• cmp (Assembly instruction): Used to compare two values.
• je (Assembly instruction): Jump if equal.
• jne (Assembly instruction): Jump if not equal.
• db, dw, dd, dq, dt, ddq and do (NASM pseudoinstructions) help us to initialize a memory location with some data.
  • db 'abc', 0: Store each value one after another.
  • our_variable db 'abc', 0: We can use the label our variable to refer to the initialized data.
• times (NASM pseudoinstruction): Repeat some source line multiple times.
  • times 5 call print_character_S_with_BIOS: Print 'S' five times.
  • times 100 db 0: Reserves 100 bytes of the memory and fills them with 0.
• $ (NASM's special expression): Points to the beginning of the assembly position of the current source line.
• $$ (NASM's special expression): Points to the beginning of the current section of assembly code.
• cli (clear interrupt flag): Disable interruption (manipulate the interrupt flag).
• sti (set interrupt flag): Enable the x86 instruction cli (manipulate the interrupt flag).

• A Journey in Creating an Operating System Kernel (The 539kernel Book)