INT 21H 8086 PDF

INT 21H 8086 PDF


July 2, 2020

MS-DOS uses INT 21H for its main API functions which provide a low-level is the assembly code to transfer data from one file to other using INT 21H?. INT is an assembly language instruction for x86 processors that generates a software interrupt. For example, INT 21H will generate the software interrupt 0x21 (33 in decimal), causing the function pointed to by the 34th vector in the interrupt. int 21h Dos Interrupt. Int 21h is a common function. The INT 21H instruction in is a software interrupt to vector 21H. It is used for input/output operations.

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INT is an assembly language instruction for x86 processors that generates a software interrupt.

INT (x86 instruction) – Wikipedia

It takes the interrupt number formatted as a byte value. Depending on the context, compileror assemblera software interrupt number is often given as a hexadecimal value, sometimes with a prefix 0x or 80886 suffix h.


For example, INT 21H will generate the nit interrupt 0x21 33 in decimalcausing the function pointed to by the 34th vector in the interrupt table to be executed, which is typically a DOS API call.

When generating a software interrupt, the processor calls one of the functions pointed to by the interrupt address table, which is located in the first bytes of memory while in real mode See Interrupt vector.

It is therefore entirely possible to use a far-call 808 to start the interrupt-function manually after pushing the flag register.

One of the most useful DOS software interrupts was interrupt 0x 2h calling it with different parameters in the registers mostly ah and al you could access various IO operations, string output and more. Most Unix systems and derivatives do not use software interrupts, with the exception of interrupt 0x80, used to make system calls.

assembly language int 21h Dos Interrupt

This is accomplished by entering a bit value corresponding to a kernel function into the EAX register of the processor and then executing INT 0x The INT 3 instruction is defined for use by debuggers to temporarily replace an instruction in a running program in order to set a intt. Other INT instructions are encoded using two bytes. Since the dedicated 0xCC opcode has some desired special properties for debugging, which are not shared by the normal two-byte opcode for an INT 3, assemblers do not normally generate the generic 0xCD 0x03 opcode from mnemonics.


From Wikipedia, the free encyclopedia. INT X where X is the software interrupt that should be generated Instruction Set Reference Manual”.

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