Dos interrupt handler. In general, interrupts can be emitted by hardware as well as by INT 21,0 Program terminate INT 21,1 Keyboard input with echo INT 21,2 Display output INT 21,3 Wait for auxiliary device input INT 21,4 Auxiliary output INT 21,5 Printer output INT 21,6 Direct console I/O INT 21,7 Wait for direct console input without echo INT 21,8 Wait for console input without echo INT 21,9 Print string INT 21,A Buffered Interrupt Processing in Real Mode Uses an interrupt vector table that stores pointers to the associated interrupt handlers. When the CPU calls the interrupt handlers, it changes the value in the RSP register to the value specified in the IST, and if there is none, the stack stays the same. "DOS Services" (as opposed to low-level or hardware-level services) are provided through software interrupt number 21 (hex. An assortment of microcomputers, minicomputers, terminals, programmable calculators, and arcade machines are also available, along with an archive of historical software and documentation. Since interrupt handling depends on PC's BIOS and may be changed by software, MS-DOS 7 cannot be responsible for keeping it strictly defined once and for ever. By calling INT 21h with a subfunction number in the AH processor register and other parameters in other registers, various DOS services can be invoked. This code that is run in response to the interrupt is known as a interrupt service routine (ISR) or an interrupt handler. Executing this program yields no results, the system doesn't even hang. If the request is accepted, the processor will suspend its current activities, save its state, and execute a function called an interrupt handler (or an interrupt service routine, ISR Dec 2, 2024 · For a large part of interrupts one can't be sure whether their handlers belong to BIOS or to DOS or to something else: it may depend on particular configuration settings in your PC. ∗ This table is located at base address zero. Interrupts get their name because they interrupt the normal flow of execution, stop whatever code was running on the cpu, execute a handler function, and then resume the previously running code. ); the specific service being requested is indicated by pre-setting the AH register with a code before performing the "int 21h". It provides the function codes, purpose, parameters, and return values for each interrupt. A proper and timely handling of interrupts is critical to the performance, but also to the security of a computer system. DOS interrupts are called by the assembly code instruction INT followed by the interrupt number, which causes the processor to prepare for an interrupt service request. Taxonomy of a simple interrupt In digital computers, an interrupt[a] is a request for the processor to interrupt currently executing code (when permitted), so that the event can be processed in a timely manner. Run DOS, Windows, OS/2 and other vintage PC applications in a web browser on your desktop computer, iPhone, or iPad. See the following topics for more information: INT 20 Program terminate INT 21 DOS Function Dispatcher INT 22 Program Terminate INT 23 Ctl-break exit address INT 24 Critical error handler address INT 25 Absolute disk read INT 26 Absolute disk write INT 27 Terminate but stay resident INT 28 DOS idle loop/scheduler (undocumented) INT 29 Fast Sep 12, 2018 · Function 31h is an enhanced version of DOS Interrupt 27h, Terminate and Stay Resident, that offers two extra features: It allows more than 64K of memory to remain resident, and allows the terminating process to report a return code. A semi-unexpected cross-country drive combined with a distinct lack of surviving programming documentation has made getting this article written up take far longer than expected. The MS-DOS software - particularly IO. SYS - is intended to insulate you from the real hardware, in effect providing a virtual machine, so that MS-DOS is in principle transportable An interrupt, on the other hand, is an asynchronous event that occurs at random times during execution of a pro- gram in response to a signal from hardware. Jun 1, 2019 · The Interrupt handler sets the video mode to 13h, tries to fill the entire screen with a light blue color, restores the original Interrupt 28h handler, restores all registers and flags involved, and returns to DOS. Interrupts can signal a number of events from the system, from fatal errors to a device telling us it has some data ready to be read. These are usually not the general interrupts we describe here, but software interrupts implementing what we have called supervisor calls or system calls. The help topics include BIOS interrupts, DOS interrupts, DOS functions, EMS and Mouse functions, BIOS and DOS data structures, diagnostic codes, DOS commands, 8086 assembler instructions, standard and vendor specific C functions and various hardware specifications. Jul 29, 2025 · After the CPU finds the entry for the interrupt, it jumps to the code the entry points to. Most calls to the DOS API are invoked using software interrupt 21h (INT 21h). . Entry: AL = interrupt number Return: ES:BX -> current interrupt handler SeeAlso: AH=25h,AX=2503h ( Interrupts are often discussed in literature about MS-DOS. Each entry in this table consists of a CS:IP pointer to the associated ISRs ∗ Each entry or vector requires four bytes: Two bytes for specifying CS Aug 30, 2016 · Retro-Malware: DOS TSRs, Interrupt Handlers, and Far Calls, Part 2 Before we get into this installment, I do want to apologize for the delay into getting this article up. This document summarizes various DOS interrupt functions including reading/writing characters to standard input/output, getting/setting system date and time, managing files and directories, and getting disk space information. Feb 15, 2024 · In this article I explain how to handle CPU interrupts in code, by writing an interrupt handler with Borland Pascal 7 for MS-DOS. xqa rwn qts uud wgj hew epq icx soo wjh ale oox zjd dqd xja