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Status Lamps

The top panel of the Console features the following status lamps: HLT, END, CPU, WAIT, INT, BUS.


The HLT lamp (Halt) illuminates when the computer is in Halt mode, remaining off otherwise.

The END lamp monitors the microprocessor's signal Halt. This lamp illuminates when a Halt instruction is executed. As mentioned, this makes the microprocessor to stop but not the machine to enter Halt Mode. To actually halt the computer at this time you can press the HLT button in which case you will see both the HLT and the END lamps illuminated.

The CPU lamp monitors the microprocessor's signal M1. Consequently, this lamp illuminates for each time the microprocessor fetches for operational code thus indicating that the CPU is actually running code.

The WAIT lamp monitors the microprocessor's input signal Wait. This signal is used to make the CPU to hold awaiting response from a slow device which had requested such by pulling down the HLD line of the EXT-BUS.

A Wait signal is also asserted internally within the MC circuitry to coordinate DMA operations with external memory or I/O usage throughout the EXT-BUS as explained in section Master Controlller. Consequently, the lamp will also illuminate when an external device is requesting bus usage an the microprocessor has abdicated in favor of the device.

The INT lamp monitors the incoming IRQ signal before the INT switch, indicating that an external device is requesting interrupt to the Master Controller.

The BUS lamp monitors the EXT-BUS signal BSY (Bus Busy). This occurs when the MC is attempting to access the EXT-BUS, that is, trying to address shared memory or I/O.

It must be noticed that this lamp signals, not that the MC is actually using the EXT-BUS but only that it "wants" to do so. For example, if the MC tries to access external memory while a DMA transfer is taking place, you will see both the BUS and the WAIT lamps illuminated as long as the transfer takes. This is because the microprocessor have been put in hold automatically in favor of the DMA transfer (See "EXT-BUS Gating" in section Master Controller).