Samsung KS57C2308 manual Multiple Interrupts, Two-Level Interrupt Handling

KS57C2308/P2308/C2316/P2316 INTERRUPTS

MULTIPLE INTERRUPTS

The interrupt controller can service multiple interrupts in two ways: as two-level interrupts, where either all interrupt requests or only those of highest priority are serviced, or as multi-level interrupts, when the interrupt service routine for a lower-priority request is accepted during the execution of a higher priority routine.

Two-Level Interrupt Handling

Two-level interrupt handling is the standard method for processing multiple interrupts. When the IS1 and IS0 bits of the PSW (FB0H.3 and FB0H.2, respectively) are both logic zero, program execution mode is normal and all interrupt requests are serviced (see Figure 7-3).

Whenever an interrupt request is accepted, IS1 and IS0 are incremented by one and the values are stored in the stack along with the other PSW bits. After the interrupt routine has been serviced, the modified IS1 and IS0 values are automatically restored from the stack by an IRET instruction.

IS0 and IS1 can be manipulated directly by 1-bit write instructions, regardless of the current value of the enable memory bank flag (EMB). Before you can modify an interrupt service flag, however, you must first disable interrupt processing with a DI instruction.

When IS1 = "0" and IS0 = "1", all interrupt service routines are inhibited except for the highest priority interrupt currently defined by the interrupt priority register (IPR).

	Normal Program	High or Low Level Interrupt
	Processing		Processing
	(Status 0)		(Status 1)	High Level Interrupt
INT Disable				Processing
				(Status 2)
SET IPR
INT Enable
Low or High
Level		High-Level
Interrupt		Interrupt
Generated		Generated

Figure 7-3. Two-Level Interrupt Handling

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