This is the front panel of the PDP-8/I while it is running Focal. The light are indicating the state of various registers with the intensity showing how often that bit is set. The switches allow control of the machine. Early computers had lots of light and switches to help debug software before software debugging tools became common such as the early debugger ODT. The front panel also help in fixing the machine when it failed which was more common in the early machines. DEC had a list of recommended spares the users should have to use in fixing them. As time progressed the models slowly lost most of the lights and switches.
The printed table of octal numbers on the left is the rim loader instructions which needed to be entered with the switches to boot the machine from a Teletype. The masking tape has the instructions needed to boot from the high speed paper tape reader. The notes stuck to the left side are instructions to enter using the switches to boot the configuration they were running.
The top row of lights is the current program address and data field. The PDP-8 memory is addressed in 4k 12 bit word fields. The next row of lights is the memory address being accessed and below it the data value being read or written to memory. The next row of lights is the accumulator, the register were all calculation took place. The link light is the carry flag. The processor had a multiply option available that this machine does not have which added an additional registers the step counter and multiplier quotient register. The step counter was used in shifting and normalizing numbers. The option only did fixed point operations but the shift counter assisted in software support for floating point.
The lights on the right show what state the processor is in. The column starting with and is the current instruction that was executing, the machine has 8 instructions More detail. The next column of lights showed the processor internal state. The instruction execution cycle was a series of steps and these lights showed which were being executed. The last column shows interrupt enable state, when in pause when using slow devices and the machine running indicator.
The row of switches allowed you to control the machine operation. The left most 6 controlled the data field and instruction field. The next 12 are either the address to load or data to write. The Start momentary switch loads the program counter from the switches and start the machine running. The Load Add momentary switch loads the address register for memory access. The Dep momentary switch stores the switch value into memory and increments the memory address. The Exam momentary switch reads the memory address and displays the result in the memory buffer lights and increments the address. The Cont momentary switch continues processor operation if stopped by the Stop momentary switch. The Sing Step switch causes the Cont switch to execute one processor state each time the Cont switch is pressed. This was useful for troubleshooting the machine. The Single Inst switch makes the machine run one instruction each time the Cont switch is pressed. This was useful for debugging software or machine troubleshooting.
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