Do You want to create your own OS (mission 5)
Hello friends, are you building your operating system, or are you interested in the building process in operating system. This is my fifth part about building my own operating system.
Are you already read my previous parts of this article series? If not I recommended you to read that parts too. In this part, I want to share with you about Itterupts and inputs.
Interrupts Handlers
In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, software interrupt instructions, or software exceptions, and are used for implementing device drivers or transitions between protected modes of operation, such as system calls.
Interrupts are handled via the Interrupt Descriptor Table (IDT). The IDT describes a handler for each interrupt. The interrupts are numbered (0–255) and the handler for interrupt i is defined at the ith position in the table. There are three different kinds of handlers for interrupts:
- Task handler
- Interrupt handler
- Trap handler
If you want to find more information about interrupts click here.
Programmable Interrupt Controller (PIC)
To start using hardware interrupts you must first configure the Programmable Interrupt Controller (PIC). The PIC makes it possible to map signals from the hardware to interrupts. The reasons for configuring the PIC are:
- Remap the interrupts. The PIC uses interrupts 0–15 for hardware interrupts by default, which conflicts with the CPU interrupts. Therefore the PIC interrupts must be remapped to another interval.
- Select which interrupts to receive. You probably don’t want to receive interrupts from all devices since you don’t have code that handles these interrupts anyway.
- Set up the correct mode for the PIC.
Input
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals or data received by the system and outputs are the signals or data sent from it. The term can also be used as part of an action; to “perform I/O” is to perform an input or output operation.
I/O devices are the pieces of hardware used by a human (or other system) to communicate with a computer. For instance, a keyboard or computer mouse is an input device for a computer, while monitors and printers are output devices. Devices for communication between computers, such as modems and network cards, typically perform both input and output operations.
Reading Input from the Keyboard
The keyboard does not generate ASCII characters, it generates scan codes. A scan code represents a button — both presses and releases. The scan code representing the just pressed button can be read from the keyboard’s data I/O port which has address 0x60. How this can be done is shown in the following example:
#include "io.h" #define KBD_DATA_PORT 0x60 /** read_scan_code:
* Reads a scan code from the keyboard
*
* @return The scan code (NOT an ASCII character!)
*/
unsigned char read_scan_code(void)
{
return inb(KBD_DATA_PORT);
}
The next step is to write a function that translates a scan code to the corresponding ASCII character. If you want to map the scan codes to ASCII characters as is done on an American keyboard then Andries Brouwer has a great tutorial [36].
Remember, since the keyboard interrupt is raised by the PIC, you must call pic_acknowledge at the end of the keyboard interrupt handler. Also, the keyboard will not send you any more interrupts until you read the scan code from the keyboard.