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Interrupts

Interrupts are organized into 256 vectors termed INT_00 through INT_FF. The first sixteen ones (INT_00 through INT_0F) are for CPU internal use; all others (INT_10 to INT_FF) are available to peripherals. Vectors point to memory addresses in the very first block of the addressable space taking two consecutive addresses per vector as we shall see.

Nested interrupt service is allowed according to vector priority. INT_00 has the highest priority whereas INT_FF has the lowest.

The external bus (E-BUS) provides two lines for managing interrupts: IRQT and IACK.

Devices request CPU attention by activating the IRQT line. The CPU grants the bus to the requester device by activating the IACK line. This line is connected in daisy-chain from device to device. Devices that have not made the request pass the signal down to the chain; the device who made the request disconnects itself from the chain so the acknowledge signal can not propagate further.

Once the requester device is in control of the E-BUS, it puts its interrupt vector on the Data lines and drops the IRQT signal. This vector have probably been set by DIP switches into the device at installation time.

When the CPU senses the IRQT dropped (with IACK still active), it reads the vector from the E-BUS, drops the IACK signal and calls the appropriate interrupt service routine (ISR).

Interrupt vectors points to consecutive locations in the very first block of the addressable space. Each vector takes two consecutive addresses; thus INT_00 points to 0x00000, INT_01 points to 0x00002 and so on. INT_FF points to 0x001fe.

For servicing an interrupt, the CPU will do nothing but a fetch bus cycle toward the corresponding address. What is expected to be there is a branch instruction to the actual ISR.

INT_00 is the computer's clock tick, raised at 1 ms intervals. A multitasking operating system can make use of INT_0 to define processes (or threads) time-slices.

INT_01 through INT_FF are used by the CPU to generate exceptions. Software code is allowed to raise interrupts within this range by using the INT instruction.

All interrupts are maskable. Not masked interrupts can be served in a nested fashion according to interrupt priority.

Queuing interrupt requests is the job of the Operating System. The CPU limits  itself to service interrupts in the simple way we just described.