Programmable Interrupt Controller. Features; Pinout; Block diagram; ICW1 ( Initialisation Command Word One); ICW2 (Initialisation Command Word Two). The A is a programmable interrupt controller specially designed to work with Intel microprocessor , A, , The main features of A. This tutorial puts everything we learned to the test. I will do my best to keep things simple. the A Microcontroller, Also known as the Programmable Interrupt.
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This series is intended to demonstrate and teach operating system development from the ground up. This tutorial covers a very important topic: The Programmable Interrupt Controller. We will need to microcontropler this microcontroller by mapping it to our IRQ’s. This will be needed when setting up interrupts, and handling interrupt requests. This is our first controller tutorial.
All of mmicrocontroller controller tutorials go very deep in each device, while building a workable interface to handling them. Remember that, as we are in protected mode, we have nothing to guide us.
mcrocontroller One wrong move can cause unpredictable results. As we have no helping hand, we have to communicate with each controller directly. Because of this, we have emphisized hardware programming concepts all througout this series so our readers have more experience and better understanding of hardware level programming.
This tutorial puts everything we learned to the test. I will do my best to keep things simple.
Get Ready This is our first of many microcontroller programming tutorials. We will cover nearly every asset of each microcontroller as we cover them. The main series will refrence these tutorials on an as needed bases to help cover what we need these controllers for. This tutorial is fairly complicated. We will cover the A Microcontroller from both hardware and software perspectives, and understand exactally how it connects and enteracts with the PC. We will also cover every command, register, and part of this microcontroller.
Interrupts An Interrupt is an mcrocontroller asynchronous signal requiring a need for attention by software or hardware. It allows a way of interrupting the current task so that we can execute something more important.
Interrupts provide a way to help trap problems, such as divide by zeros. Microconrtoller the processor find a problem with the currently executing code, it provides the processor microconroller code to execute to fix that problem.
8259 Programmable Interrupt Controller
Other interrupts may be used to provide a way to service software as routines. These interrupts can be called by any software from within the system. This is used alot for System API’s, which provide a way for ring 3 applications to execute ring 0 level routines.
Interrupts provide alot of use, espically as a way of recieving information from hardware that may change its state at asynchronous times. Interrupt Types There are two types of interrupts: Software Interrupts and Hardware Interrupts. Software Interrupts Software Interrupts are interrupts implimented and triggered in software.
Normally, the processor’s instruction set will provide an instruction to service software interrupts. For example, here we generate an interrupt through a software instruction: We will not cover software interrupts here. Software interrupts will be covered in another tutorial. Hardware Micrlcontroller A hardware interrupt is an interrupt triggered by a hardware device.
Normally, these are hardware devices 825 require attention. The hardware Interrupt handler will be required to service this hardware request. Spurious Interrupt This is a hardware interrupt generated by electrical interference in the interrupt line, or faulty hardware.
We do NOT want this!
Interrupt Modes There are several modes and classes of interrupts that we will need to cover. In programming the PIC, we will need to choose a mode. This section may require some knowedge of the A PIC hardware pin layout. A device sends a signal Setting this line to activeand keeps it at that state until the interrupt is serviced.
Level Triggered interrupt lines may be shared by multiple interrupts if the circuit is designed to handle it. This mode is the preferred mode because of how the lines are shared. When an IR line is active, the CPU searches through all of the devices sharing the same line until it finds what device is activating the signal. After finding the device, the CPU rechecks all of the devices again to insure there are no other devices that also need service. A problem with this approch is, if there is an interrupt with higher priority that needs to be serviced, all other interrupts will be perminately blocked until the other interrupts are serviced.
After all, only one line can be active at a given time. A device sends a signal Setting this line to active through a single pulse, and returns the line to its previous state. Edged Triggered interrupt lines may be shared by multiple interrupts if the circuit is designed to handle it. If the pulse is too short to be detected, then it will not be detected. As these are only pulses of current that signals interrupt requests, Edged triggered mode does not have the same problems that Level triggered does with shared IRQ lines.
Of course, we still run into the possibility of an interrupt being missed, as it is just a single pulse of current being sent through the IRQ line. This has caused early computer lockups of the CPU. However, through recent times, these lockups have decreased through time. Hybrid Both of these modes have their pros and cons. Alot of systems impliment a hybrid of both of them. The purpose of this is that the NMI pin is used to signal major problems with the system that can cause big problems, or entire system malfunctions, possibly hardware damage.
The Non Maskable Interrupt is just that — It cannot be disabled or masked off by any device. This insures, along with having a hybrid setup, that if the NMI pin is set, the system can die peacefully without big problems. Message Signaled These types of hardware interrupts do not use a physical interrupt line.
Instead, they rely on another medium, such as the system bus, to send messages over.
Block Diagram of Programmable Interrupt Controller | Interrupt Sequence
These types of interrupts cause the device to only send a pulse of current over the medium, similar to edge triggered interrupts. This means, it simply executes a routine that we define. Not mixrocontroller hard, huh?
This special routine determins the Interrupt Function to execute normally based off of the value in the AX register. This allows us to define multiple functions in an interrupt call.
Executing an interrupt simply executes an interrupt routine that you created. Each entry inside of the IVT is 4 bytes, in the following format: This allows us to create a simple function anywhere in memory Our IR. As long as the IVT containes the addresses of our functions, everything will work fine. Okay, Lets take a look at the IVT. The first few interrupts are reserved, and stay the same. Each of these interrupts are located at a base address within the IVT. The IDT will be explained further in another tutorial, as it is not directly related to this tutorial.
The IDT is an array of Interrupt Descriptorsthat describe the base address of the Interrupt Routine IR to execute, that contains 829 information about it’s protection level, segment information, etc.
Most of the interrupt routines will be inside of a microcontrlller descriptor, mapped by the GDT. Do not worry if you do not understand this right now. For now, just think of it as an array of function pointers, mapped exactally like that of the IVT It normally is, anyways.
Hardware interrupts are very important for PC’s. It allows other hardware devices to signal the CPU that something is about to happen. For example, a keystroke on the keyboard, or a single clock tick on the internal timer, for example. This way, we have a way to track these hardware changes.
Lets take a look at these hardware interrupts. The A Pins will be described in detail within the next section. In most cases, we will need to recreate a new interrupt table. We will cover how to do this later in this tutorial as well. Look back again at Tutorial This is very important to note.
This is a big limitation. As additional devices were created, IBM quickly realized that this limitation is very bad. Today, this is very common. They can be cascaded to support up to 64 IRQ’s.
Most computers have 2 PIC’s, nicrocontroller inside the processor, and 1 on the motherboard. Some systems may not have this. Remember that the PIC’s are only used during a hardware interrupt.
To make things more understandable, we are going to represent the controller using a simpler graphic. You can see these pins labled in the miicrocontroller on the top of this tutorial.