ntoskrnl.exe (Short for Windows NT operating system kernel,) also known as kernel image, provides the kernel and executive layers of the Windows NT kernel space, and is responsible for various system services such as hardware virtualization, process and memory management, thus making it a fundamental part of the system. It contains the cache manager, the executive, the kernel, the security reference monitor, the memory manager, and the scheduler.
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Overview
This system binary is not a native application (in that it is not linked against ntdll.dll), instead containing a standard 'start' entry point, a function that calls the architecture-independent kernel initialization function. While ntoskrnl.exe is not linked against ntdll.dll, it is linked against bootvid.dll, hal.dll and kdcom.dll. Because it requires a static copy of C Runtime objects it depends on, the executable is usually about 2MB in size.
Overall, there are four kernel image files. Multiprocessor or uniprocessor files are selected at install time, and PAE or non-PAE files are selected by boot.ini or BCD option, according to the processor's features.
Windows kernel coding typically uses Hungarian notation for type names, but not for variable names.
Routines in ntoskrnl use prefixes on their names to indicate in which component of ntoskrnl they are defined. The following table lists some of them.
Initialization
When the kernel receives control, it gets a pointer to a structure as parameter. This structure is passed by the bootloader and contains information about the hardware, the path to the registry file, kernel parameters containing boot preferences or options that change the behavior of the kernel, path of the files loaded by the bootloader (SYSTEM Registry hive, nls for character encoding conversion and vga font). The definition of this structure can be retrieved by using the kernel debugger or downloading it from the Microsoft symbol database.
In the x86 architecture, the kernel receives the system already in protected mode, with the GDT, IDT and TSS ready. But since it does not know the address of each one, it has to load them one by one to fill the PCR structure.
The main entry point of ntoskrnl.exe performs some system dependent initialization then calls a system independent initialization then enters an idle loop.
Interrupt Handling
Modern operating systems use interrupts instead of I/O port polling to wait for information from devices.
In the x86 architecture, interrupts are handled through the Interrupt Dispatch Table (IDT). When a device triggers an interrupt, the interrupt flag (IF) in the flags register is set and the processor's hardware looks for an interrupt handler in the table entry corresponding to the interrupt number, or IRQ. Interrupt handlers usually save the state of all or some registers before handling it and restore the registers when done.
The interrupt table contains handlers for hardware interrupts, software interrupts, and exceptions. One example of a software interrupt handler (there are many others) is in table entry 0x2e. It points to the KiSystemService.
Memory manager
Microsoft Windows divides virtual address space into two regions. The lower part, starting at zero, is instantiated separately for each process and is accessible from both user and kernel mode. Application programs run in processes and supply code that runs in user mode. The upper part is accessible only from kernel mode, and with some exceptions, is instantiated just once, system-wide. Ntoskrnl.exe is mapped into this region, as are several other kernel mode components. This region also contains data used by kernel mode code, such as the kernel mode heaps and the file system cache.
The entire physical memory (RAM) address range is broken into many small (usually 4 KB) blocks. A few of the properties of each block are stored in structures called page table entries, which are managed by the OS and accessed by the processor's hardware. Page tables are organized into a tree structure, and the physical page number of the top-level table is stored in control register 3 (CR3).
Registry
Windows Registry is a repository for configuration and settings information for the operating system and for other software, such as applications. It can be thought of as a filesystem optimized for small files. However, it is not accessed through file system-like semantics, but rather through a specialized set of APIs, implemented in kernel mode and exposed to user mode.
The registry is stored on disk as several different files called "hives." One, the System hive, is loaded early in the boot sequence and provides configuration information required at that time. Additional registry hives, providing software-specific and user-specific data, are loaded during later phases of system initialization and during user login, respectively.
Drivers
The list of drivers to be loaded from the disk are retrieved from the Services key in the SYSTEM registry hive. That key stores device drivers, kernel processes and user processes. They are all collectively called "services" and are all stored mixed on the same place.
During initialization or upon driver load request, the kernel transverses that tree looking for services tagged as kernel services.