Intel 386 manual Interrupts, ADS# Synchronizer

LOCAL BUS INTERFACE

			74F109
		11
	14	PR	10
ADS		J		ADSO
			QI--'---I
	12	CK	9
	13	K
			Q
		15

ClK 10NS

231732i3-17

Figure 3-17. ADS# Synchronizer

An alternative method of generating CLK2 is to use a TTL oscillator coupled to a 74ACT244 buffer. Altliough a typical 74ACT244 datasheet does not guarantee an output compatible with the Intel386 DX microprocessor CLK2 input, some manufacturers devices have been observed to have sufficiently fast rise/fall times (4 ns at CL = 80 pf), as well as the necessary swing, to meet the Inte1386 DX microprocessor CLK2 require- ments. This approach is recommended if the Inte1386 DX microprocessor must run synchronously with an external clock (i.e., EFI). Similarly, if a CMOS level swing is required on CLK, a 74AC109 flip-flop may be used instead of the 74F109.

3.4INTERRUPTS

Both hardware-generated and software-generated interrupts can alter the programmed execution of the Inte1386 DX microprocessor. A hardware-generated interrupt occurs in response to an active input on one of two Intel386 DXmicroprocessor interrupt request inputs (NMI or INTR). A software-generated interrupt occurs in response to an INT instruction or an exception (a software condition that requires servicing). For complete information on software-generated interrupts, see the 386"1 DX Microprocessor Program-

mer's Reference Manual.

In response to an interrupt request, the Inte1386 DXmicroprocessor processes the inter- rupt (saves the processor state on the stack, plus task information if a task switch is required) and services the interrupt (transfers program execution to one of 256 possible interrupt service routines). Entry-point descriptors to service routines or interrupt tasks are stored in a table (Interrupt Descriptor Table orIDT) in memory. To access a par- ticular service routine, the Inte1386 DX microprocessor must obtain a vector, or index, to the table location that contains the corresponding descriptor. The source of this vector

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