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So much for getting into DOS How do we get home again The address in vector 21H took control into DOS, but how does DOS know where to go to pass execution back into EATEXE Half of the cleverness of software interrupts is knowing how to get there, and the other half just as clever is knowing how to get back To get into DOS, a program looks in a completely reliable place for the address of where it wants to go: the address stored in vector 21H This address takes execution deep into DOS, leaving the program sitting above DOS To continue execution where it left off prior to the INT 21 instruction, DOS has to look in a completely reliable place for the return address, and that completely reliable place is none other than the top of the stack I mentioned earlier (without much emphasis) that the INT 21 instruction pushes an address to the top of the stack before it launches off into the unknown This address is the address of the next instruction in line for execution: the instruction immediately following the INT 21 instruction This location is completely reliable because, just as there is only one interrupt vector table in the machine, there is only one stack in operation at any one time This means that there is only one top of the stack that is, SS:SP and DOS can always send execution back to the program that called it by popping the address
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off the top of the stack and jumping to that address The process is shown in Figure 85, which is the continuation of Figure 84 Just as the INT instruction pushes a return address onto the stack and then jumps to the address stored in a particular vector, there is a
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Figure 85: Returning home from an interrupt "combination" instruction that pops the return address off the stack and then jumps to the address The instruction is IRET (for Interrupt RETurn), and it completes this complex but reliable system of jumping toan address when you really don't know the address The trick, once again, is knowing where the address can reliably be found (There's actually a little more to what the software interrupt mechanism pushes onto and pops from the stack, but it happens transparently enough that I don't want to complicate the explanation at this point and you're unlikely to be writing your own software interrupt routines for a while) This should make it clear by now what happens when you execute an INT 21 instruction EATASM uses DOS services to save it the trouble of writing its string data to the screen a byte at a time The address into DOS is at a known location in the interrupt vector table, and the return address is at a known location on the stack Whereas I've described the software interrupt system in terms of the DOS service dispatcher interrupt 21H, the system is precisely the same for all other software interrupts and there are many In the next chapter we use a few more and explore some of the many services available through the BIOS interrupts that control your video display and printer
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Software Interrupts versus Hardware Interrupts
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Software interrupts evolved from an older mechanism that did involve some genuine interrupting: hardware interrupts A hardware interrupt is your CPU's mechanism for paying attention to the world outside itself
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There is a fairly complex electrical system built into your PC that allows circuit boards to send signals to the CPU An actual metal pin on the CPU chip is moved from one voltage level to another by a circuit board device like a disk drive controller or a serial port board Through this pin, the CPU is tapped on the shoulder by the external device The CPU recognizes this tap as a hardware interrupt Like software interrupts, hardware interrupts are numbered, and for each interrupt number there is a slot reserved in the interrupt vector table In this slot is the address of an interrupt service routine (ISR) that performs some action relevant to the device that tapped the CPU on the shoulder For example, if the interrupt signal came from a serial port board, the CPU would then allow the serial port board to transfer a character byte from itself into the CPU Most properly, any routine that lies at the end of a vector address in the interrupt vector table is an ISR, but the term is usually reserved for hardware interrupt service routines The only difference between hardware and software interrupts is in the event that triggers the trip through the interrupt vector table With a software interrupt the triggering event is part of the software; that is, an INT instruction With a hardware interrupt, the triggering event is an electrical signal applied to the CPU chip itself without any INT instruction taking a hand in the process The CPU itself pushes the return address on the stack when it recognizes the electrical pulse that triggers the interrupt; however, when the ISR is done, a RET instruction sends execution home, just as it does for a software interrupt Hardware ISRs can be (and usually are) written in assembly language It's a difficult business, because the negotiations between the hardware and software must be done just so, or the machine may lock up or go berserk This is no place for beginners, and I would advise you to develop some skill and obtain some considerable knowledge of your hardware setup before attempting to write a hardware ISR
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