Implement Your Own Operating System (week 05)

7 min readAug 20, 2021

What are the Interrupts?

The interrupt is a signal emitted by hardware or software when a process or an event needs immediate attention. It alerts the processor to a high-priority process requiring interruption of the current working process. In I/O devices one of the bus control lines is dedicated for this purpose and is called the Interrupt Service Routine (ISR).

Interrupts Handlers

An interrupt is a hardware signal from a device to a CPU. An interrupt tells the CPU that the device needs attention and that the CPU should stop any current activity and respond to the device. If the CPU is not performing a task that has higher priority than the priority of the interrupt, then the CPU suspends the current thread. The CPU then invokes the interrupt handler for the device that sent the interrupt signal. The job of the interrupt handler is to service the device and stop the device from interrupting. When the interrupt handler returns, the CPU resumes the work it was doing before the interrupt occurred.

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

The task handlers use functionality specific to the Intel version of x86, so they won’t be covered here (see the Intel manual, chapter 6, for more info). The only difference between an interrupt handler and a trap handler is that the interrupt handler disables interrupts, which means you cannot get an interrupt while at the same time handling an interrupt. In this book, we will use trap handlers and disable interrupts manually when we need to.

Creating an Entry in the IDT

An entry in the IDT for an interrupt handler consists of 64 bits. The highest 32 bits are shown in the figure below:

The lowest 32 bits are presented in the following figure:

A description for each name can be found in the table below:

The offset is a pointer to code (preferably an assembly code label). For example, to create an entry for a handler whose code starts at 0xDEADBEEF and that runs in privilege level 0 (therefore using the same code segment selector as the kernel) the following two bytes would be used:

0xDEAD8E00
0x0008BEEF

If the IDT is represented as an unsigned integer idt[512] then to register the above example as a handler for interrupt 0 (divide-by-zero), the following code would be used:

idt[0] = 0xDEAD8E00
idt[1] = 0x0008BEEF

Handling an Interrupt

Interrupt handling is a key function in real-time software and comprises interrupts and their handlers. Only those physical interrupts which of high enough priority can be centered into the system interrupt table. The software assigns each interrupts to a handler in the interrupt table. An interrupt handler is just a routine containing a sequence of operations. Each of these may request input and output while running.

When an interrupt occurs the CPU will push some information about the interrupt onto the stack, then look up the appropriate interrupt handler in the IDT and jump to it. The stack at the time of the interrupt will look like the following:

[esp + 12] eflags
[esp + 8] cs
[esp + 4] eip
[esp] error code?

The reason for the question mark behind the error code is that not all interrupts create an error code. The specific CPU interrupts that put an error code on the stack are 8, 10, 11, 12, 13, 14, and 17. The error code can be used by the interrupt handler to get more information on what has happened. Also, note that the interrupt number is not pushed onto the stack. We can only determine what interrupt has occurred by knowing what code is executing — if the handler registered for interrupt 17 is executing, then interrupt 17 has occurred.

Once the interrupt handler is done, it uses the iret instruction to return. The instruction iret expects the stack to be the same as at the time of the interrupt (see the figure above). Therefore, any values pushed onto the stack by the interrupt handler must be popped. Before returning, iret restores eflags by popping the value from the stack and then finally jumps to cs:eip as specified by the values on the stack.

The interrupt handler has to be written in assembly code since all registers that the interrupt handlers used must be preserved by pushing them onto the stack. This is because the code that was interrupted doesn’t know about the interrupt and will therefore expect that its registers stay the same. Writing all the logic of the interrupt handler in assembly code will be tiresome. Creating a handler in assembly code that saves the registers, calls a C function, restores the registers, and finally executes iret is a good idea!

The C handler should get the state of the registers, the state of the stack, and the number of the interrupt as arguments. The following definitions can for example be used:

Creating a Generic Interrupt Handler

Since the CPU does not push the interrupt number on the stack it is a little tricky to write a generic interrupt handler. This section will use macros to show how it can be done. Writing one version for each interrupt is tedious — it is better to use the macro functionality of NASM. And since not all interrupts produce an error code the value 0 will be added as the “error code” for interrupts without an error code. The following code shows an example of how this can be done:

The common_interrupt_handler does the following:

Push the registers on the stack.
Call the C function interrupt_handler.
Pop the registers from the stack.
Add 8 to esp (because of the error code and the interrupt number pushed earlier).
Execute iret to return to the interrupted code.

Since the macros declare global labels the addresses of the interrupt handlers can be accessed from C or assembly code when creating the IDT.

Loading the IDT

The IDT is loaded with the lidt assembly code instruction which takes the address of the first element in the table. It is easiest to wrap this instruction and use it from C:

Programmable Interrupt Controller (PIC)

A Programmable Interrupt Controller (PIC) is an interrupt controller that manages interrupt signals received from devices by combining multiple interrupts into a single interrupt output. PIC is found on most PCs today. One of the most well-known Programmable Interrupt Controllers is Intel 8259A, which was included in the x86 PC as a part of the motherboard chipset.

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.

In the beginning, there was only one PIC (PIC 1) and eight interrupts. As more hardware was added, 8 interrupts were too few. The solution chosen was to chain on another PIC (PIC 2) on the first PIC (see interrupt 2 on PIC 1).

Every interrupts from the PIC has to be acknowledged — that is, sending a message to the PIC confirming that the interrupt has been handled. If this isn’t done the PIC won’t generate any more interrupts.

Acknowledging a PIC interrupt is done by sending the byte 0x20 to the PIC that raised the interrupt. Implementing a pic_acknowledge function can thus be done as follows:

Now you can make files for interrupt:

idt.c file

idt_asm.s file

isr.c file

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:

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.

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.

now you can make keyboard.c file:

Hope you all get a good idea about OS interrupt and input.

Reference: Helin, E., & Renberg, A. (2015). The little book about OS development