EISA System Architecture

pending bit to be cleared. The board also ceases to provide a path to ground for the interrupt request line.

If more than one I/O device were generating requests simultaneously, the other board or boards are still driving a low onto the shared request line. The 8259 would therefore immediately sense another pending request and proceed as outlined above. This time, the interrupt pending bit for the board that was already serviced is determined to be cleared, a jump is executed to the next service routine in the chain to determine if its interrupt pending bit is set.

Since the program must go through a linked service routine list to determine which board(s) is currently generating a request, it stands to reason that the lower down in the list a device is, the more time it will take to service its request (if other devices, further up the list, are also generating requests). This latency, or delay, could cause problems ranging from slow servicing of a device right up to overflow conditions and missing characters. The problem can be solved in one of two ways:

1.Move some devices to other interrupt requests lines.

2.During the configuration process, install the devices requiring the smallest latency first and the others later in the process.

Phantom Interrupt Elimination

All IRQ inputs that are configured as level-sensitive, shareable inputs assume the high state when no requests are pending or when the IRQ line is unused. This renders these inputs relatively noise-free, substantially decreasing the possibility of phantom interrupts.

40