Introduction To Dos And Bios Interrupts In 8086 Pdf

introduction to dos and bios interrupts in 8086 pdf

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Traditionally, BIOS calls are mainly used by DOS programs and some other software such as boot loaders including, mostly historically, relatively simple application software that boots directly and runs without an operating system—especially game software. For this reason, modern operating systems that use the CPU in Protected mode or Long mode generally do not use the BIOS interrupt calls to support system functions, although they use the BIOS interrupt calls to probe and initialize hardware during booting.

An interrupt is a condition that halts the microprocessor temporarily to work on a different task and then return to its previous task. Interrupt is an event or signal that request to attention of CPU. This halt allows peripheral devices to access the microprocessor. Whenever an interrupt occurs the processor completes the execution of the current instruction and starts the execution of an Interrupt Service Routine ISR or Interrupt Handler.

Protected mode

Traditionally, BIOS calls are mainly used by DOS programs and some other software such as boot loaders including, mostly historically, relatively simple application software that boots directly and runs without an operating system—especially game software. For this reason, modern operating systems that use the CPU in Protected mode or Long mode generally do not use the BIOS interrupt calls to support system functions, although they use the BIOS interrupt calls to probe and initialize hardware during booting.

In all computers, software instructions control the physical hardware screen, disk, keyboard, etc. In a PC, the BIOS, pre-loaded in ROM on the motherboard, takes control immediately after the CPU is reset, including during power-up, when a hardware reset button is pressed, or when a critical software failure a triple fault causes the mainboard circuitry to automatically trigger a hardware reset.

The BIOS tests the hardware and initializes its state; finds, loads, and runs the boot program usually, an OS boot loader, and historical ROM BASIC ; and provides basic hardware control to the software running on the machine, which is usually an operating system with application programs but may be a directly booting single software application. From the beginning, programmers had the choice of using BIOS or not, on a per-hardware-peripheral basis.

IBM did strongly encourage the authorship of "well-behaved" programs that accessed hardware only through BIOS INT calls and DOS service calls , to support compatibility of software with current and future PC models having dissimilar peripheral hardware, but IBM understood that for some software developers and hardware customers, a capability for user software to directly control the hardware was a requirement.

In part, this was because a significant subset of all the hardware features and functions was not exposed by the BIOS services. This, combined with a similar degree of hardware compatibility, means that most programs written for an IBM AT can still run correctly on a new PC today, assuming that the faster speed of execution is acceptable which it typically is for all but games that use CPU-based timing. Despite the considerable limitations of the services accessed through the BIOS interrupts, they have proven extremely useful and durable to technological change.

A key element of the purpose of BIOS calls is abstraction - the BIOS calls perform generally defined functions, and the specific details of how those functions are executed on the particular hardware of the system are encapsulated in the BIOS and hidden from the program. The program only needs to identify the BIOS-defined number of the drive it wishes to access and the address of the sector it needs to read or write, and the BIOS will take care of translating this general request into the specific sequence of elementary operations required to complete the task through the particular disk controller hardware that is connected to that drive.

The program is freed from needing to know how to control at a low level every type of hard disk or display adapter, or port interface, or real-time clock peripheral that it may need to access. This both makes programming operating systems and applications easier and makes the programs smaller, reducing the duplication of program code, as the functionality that is included in the BIOS does not need to be included in every program that needs it; relatively short calls to the BIOS are included in the programs instead.

In operating systems where the BIOS is not used, service calls provided by the operating system itself generally fulfill the same function and purpose. The BIOS also frees computer hardware designers to the extent that programs are written to use the BIOS exclusively from being constrained to maintain exact hardware compatibility with old systems when designing new systems, in order to maintain compatibility with existing software.

The hardware emulation is not exact, so not all programs that try to use the keyboard hardware directly will work correctly on the PCjr, but all programs that use only the BIOS keyboard services will. For example, the BIOS maintains separate cursor positions for up to eight text display pages and provides for TTY -like output with automatic line wrap and interpretation of basic control characters such as carriage return and line feed, whereas the CGA-compatible text display hardware has only one global display cursor and cannot automatically advance the cursor, use the cursor position to address the display memory so as to determine which character cell will be changed or examined , or interpret control characters.

In contrast, the ordinary capabilities of the standard PC and PC-AT keyboard hardware are limited to reporting to the system each primitive event of an individual key being pressed or released i. Operating systems and other software communicate with the BIOS software, in order to control the installed hardware, via software interrupts. A software interrupt is a specific variety of the general concept of an interrupt.

An interrupt is a mechanism by which the CPU can be directed to stop executing the main-line program and immediately execute a special program, called an Interrupt Service Routine ISR , instead.

An interrupt is invoked by its type number, from 0 to , and the type number is used as an index into the Interrupt Vector Table, and at that index in the table is found the address of the ISR that will be run in response to the interrupt.

A software interrupt is simply an interrupt that is triggered by a software command; therefore, software interrupts function like subroutines, with the main difference that the program that makes a software interrupt call does not need to know the address of the ISR, only its interrupt number. This has advantages for modularity, compatibility, and flexibility in system configuration. The messages are broken into categories, each with its own interrupt number, and most categories contain sub-categories, called "functions" and identified by "function numbers".

A BIOS client passes most information to BIOS in CPU registers, and receives most information back the same way, but data too large to fit in registers, such as tables of control parameters or disk sector data for disk transfers, is passed by allocating a buffer i.

Sometimes multiple addresses of data items in memory may be passed in a data structure in memory, with the address of that structure passed to BIOS in registers. The interrupt number is specified as the parameter of the software interrupt instruction in Intel assembly language, an "INT" instruction , and the function number is specified in the AH register; that is, the caller sets the AH register to the number of the desired function.

In general, the BIOS services corresponding to each interrupt number operate independently of each other, but the functions within one interrupt service are handled by the same BIOS program and are not independent.

This last point is relevant to reentrancy. The data itself can be as small as one bit or as large as 65, bytes of whole raw disk sectors the maximum that will fit into one real-mode memory segment. BIOS has been expanded and enhanced over the years many times by many different corporate entities, and unfortunately the result of this evolution is that not all the BIOS functions that can be called use consistent conventions for formatting and communicating data or for reporting results.

Some BIOS functions report detailed status information, while others may not even report success or failure but just return silently, leaving the caller to assume success or to test the outcome some other way. Sometimes it can also be difficult to determine whether or not a certain BIOS function call is supported by the BIOS on a certain computer, or what the limits of a call's parameters are on that computer. For some invalid function numbers, or valid function numbers with invalid values of key parameters—particularly with an early IBM BIOS version—the BIOS may do nothing and return with no error code; then it is the [inconvenient but inevitable] responsibility of the caller either to avoid this case by not making such calls, or to positively test for an expected effect of the call rather than assuming that the call was effective.

Because BIOS interrupt calls use CPU register-based parameter passing, the calls are oriented to being made from assembly language and cannot be directly made from most high-level languages HLLs. However, a high level language may provide a library of wrapper routines which translate parameters from the form usually stack-based used by the high-level language to the register-based form required by BIOS, then back to the HLL calling convention after the BIOS returns.

Support for inline assembly language is not part of the ANSI C standard but is a language extension; therefore, C modules that use inline assembly language are less portable than pure ANSI standard C modules. Invoking an interrupt can be done using the INT x86 assembly language instruction. For example, to print a character to the screen using BIOS interrupt 0x10, the following x86 assembly language instructions could be executed:.

A list of common BIOS interrupt classes can be found below. Note that some BIOSes particularly old ones do not implement all of these interrupt classes. The BIOS also uses some interrupts to relay hardware event interrupts to programs which choose to receive them or to route messages for its own use.

The table below includes only those BIOS interrupts which are intended to be called by programs using the "INT" assembly-language software interrupt instruction to request services or information.

This call would typically be invoked if the BIOS was unable to identify any bootable disk volumes on startup. Other programs also reused this vector for their own purposes. Many modern operating systems such as Linux and Windows NT bypass the BIOS interrupt calls after startup, the OS kernel will switch the CPU into protected mode or long mode at startup, preferring to use their own programs such as kernel drivers to control the attached hardware directly.

The reason for this was primarily that these operating systems run the processor in protected mode or long mode, whereas calling BIOS interrupt call requires switching to real mode or unreal mode , or using Virtual mode.

The real mode, the unreal mode and the virtual mode is slow. However, there are also serious security reasons not to switch to real mode, and the real mode BIOS code has limitations both in functionality and speed that motivate operating system designers to find a replacement for it. In fact, the speed limitations of the BIOS made it common even in the DOS era for programs to circumvent it in order to avoid its performance limitations, especially for video graphics display and fast serial communication.

The problems with BIOS functionality include limitations in the range of functions defined, inconsistency in the subsets of those functions supported on different computers, and variations in the quality of BIOSes i. By taking matters into their own hands and avoiding reliance on BIOS, operating system developers can eliminate some of the risks and complications they face in writing and supporting system software.

On the other hand, by doing so those developers become responsible for providing "bare-metal" driver software for every different system or peripheral device they intend for their operating system to work with or for inducing the hardware producers to provide those drivers.

Thus it should be apparent that compact operating systems developed on small budgets would tend to use BIOS heavily, while large operating systems built by huge groups of software engineers with large budgets would more often opt to write their own drivers instead of using BIOS—that is, even without considering the compatibility problems of BIOS and protected mode.

From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Main article: Interrupt vector table. Retrieved Hidden categories: Articles needing additional references from April All articles needing additional references. Namespaces Article Talk. Views Read Edit View history. Help Learn to edit Community portal Recent changes Upload file.

Download as PDF Printable version. This is the real time clock interrupt. It fires The BIOS increments the time-of-day counter during this interrupt. This is the Keyboard interrrupt. This is generally triggered when a key on a keyboard is pressed. Video Services AH Description 00h.

Return conventional memory size. Serial port services AH Description 00h. Get Extended Memory Size. Get Extended Memory Size Newer function, since Gives results for memory size above 64 Mb. Query System Address Map.

Keyboard services AH Description 00h. Printer services AH Description 00h. Modern machines would attempt to boot from a network through this interrupt. On modern machines this interrupt will be treated by the BIOS as a signal from the bootloader that it failed to complete its task. The BIOS can then take appropriate next steps. A program can call this interrupt to reboot the computer but must ensure that hardware interrupts or DMA operations will not cause the system to hang or crash during either the reinitialization of the system by BIOS or the boot process.

Timer tick handler - called by INT Not to be called; simply a pointer to the DPT Diskette Parameter Table , containing a variety of information concerning the diskette drives.

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In computing, protected mode , also called protected virtual address mode , [1] is an operational mode of x86 -compatible central processing units CPUs. It allows system software to use features such as virtual memory , paging and safe multi-tasking designed to increase an operating system's control over application software. When a processor that supports x86 protected mode is powered on, it begins executing instructions in real mode , in order to maintain backward compatibility with earlier x86 processors. Protected mode was first added to the x86 architecture in , [6] with the release of Intel 's processor, and later extended with the release of the in The Intel , the predecessor to the , was originally designed with a bit address bus for its memory.

BIOS and Dos Interrupts

Interrupt is the method of creating a temporary halt during program execution and allows peripheral devices to access the microprocessor. The microprocessor responds to that interrupt with an ISR Interrupt Service Routine , which is a short program to instruct the microprocessor on how to handle the interrupt. Hardware interrupt is caused by any peripheral device by sending a signal through a specified pin to the microprocessor. The has two hardware interrupt pins, i. One more interrupt pin associated is INTA called interrupt acknowledge.

View larger. Download instructor resources. Alternative formats. Praised by experts for its clarity and topical breadth, this visually appealing, comprehensive source on PCs uses an easy-to-understand, step-by-step approach to teaching the fundamentals of 80x86 assembly language programming and PC architecture.

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x86 PC: Assembly Language, Design, and Interfacing, The: International Edition, 5th Edition

Special functions of General purpose registers. Addressing modes of Instruction set of Assembler directives, simple programs, procedures, and macros. Timing diagram.

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Functional layers of a General Computers Computers can be broadly divided into four layers Application Program layer, is. Which is framed generally using High level languages. All these functions are predefined by computer. This category are used to control text and graphics on the PCs display screen. INT 10h System Services,. This category are used to obtain the encoded list of the standard hardware equipments connected to computer. Invokes services to receive characters and status of keyboard.

Join our Community. In this chapter we will explore the concept of interrupts and interrupt programming. Upon receiving an interrupt signal, the microprocessor holds whatever it is doing and serves the corresponding device. The program associated with the interrupt is called the interrupt service routine ISR or interrupt handler. In POLLING method, the microprocessor continuously monitors the status of a given device; when the status condition is met, it performs the service. After that, it moves on to the next device until each one is serviced. Although polling can monitor the status of several devices and serve each of them if certain conditions are met.

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BIOS interrupt call

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Microprocessor - 8086 Interrupts

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