Real Mode
When generating a software interrupt, the processor calls one of the 256 functions pointed to by the interrupt address table, which is located in the first 1024 bytes of memory while in real mode (See Interrupt vector). It is therefore entirely possible to use a far-call instruction to start the interrupt-function manually after pushing the flag register.
One of the most useful DOS software interrupts was interrupt 0x21. By 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. This is accomplished by entering a 32-bit value corresponding to a kernel function into the EAX register of the processor and then executing INT 0x80.
Read more about this topic: INT (x86 Instruction)
Famous quotes containing the words real and/or mode:
“The Afro-American experience is the only real culture that America has. Basically, every American tries to walk, talk, dress and behave like African Americans.”
—Hugh Masakela (b. 1939)
“Young children learn in a different manner from that of older children and adults, yet we can teach them many things if we adapt our materials and mode of instruction to their level of ability. But we miseducate young children when we assume that their learning abilities are comparable to those of older children and that they can be taught with materials and with the same instructional procedures appropriate to school-age children.”
—David Elkind (20th century)