96 lines
5 KiB
Markdown
96 lines
5 KiB
Markdown
---
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layout: page
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title: PDP-11/70 Control Panels
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permalink: /devices/pdp11/panel/1170/
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---
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PDP-11/70 Control Panels
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------------------------
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The following Control Panel configurations are available:
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* [PDP-11/70 Front Panel](front.xml) (with [Debugger](debugger/front.xml))
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Use this [PDP-11/70](/devices/pdp11/machine/1170/panel/debugger/) to see the [PDP-11/70 Front Panel](debugger/front.xml) in action.
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Front Panel Basics
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------------------
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From the [PDP-11/70 Handbook (1979)](https://s3-us-west-2.amazonaws.com/archive.pcjs.org/pubs/dec/pdp11/1170/PDP1170_Handbook_1979.pdf), Chapter 10,
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p. 325:
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> ### CONSOLE OPERATION
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> The PDP-11/70 console allows direct control of the computer system. It contains a power switch for the CPU,
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which is also usually used as the Master Switch for the system. The console is used for starting, stopping, resetting,
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and debugging. Lights and switches provide the facilities for monitoring operation, system control, and maintenance.
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Debugging and detailed tracing of operations can be accomplished by having the computer execute single instructions or
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single cycles. Contents of all locations can be examined, and data can be entered manually from the console switches.
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The PDPjs Front Panel replicates most (but not all) the features of a real PDP-11/70 Console. The switches, which
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PDPjs displays as green to make them stand out from the rest of the traditional pink-and-magenta controls, are summarized
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below.
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### 0..21
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Switches 0 to 21 control bits 0 to 21 of the SWITCH register. The UP position represents a 1, and the DOWN position
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represents a 0. Once all the bits have been set, you can choose to load the contents of the SWITCH register into the
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ADDRESS register (as displayed by the top row of LEDs), or directly into the memory location referenced by the ADDRESS
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register (where it will become the DATA displayed by the second row of LEDs).
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The SWITCH, ADDRESS, and DATA (aka DISPLAY) registers are all internal to the Front Panel hardware, and should not be
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confused with CPU registers.
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### TEST
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This is a momentary switch that is normally in the DOWN position. When held in the UP position, it turns on all LEDs.
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This "lamp test" was more important in the days when the lights were produced by bulbs with filaments that could burn out,
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and if you didn't realize that a particular light was "burned out", you could mistake it for a zero.
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While the TEST switch is more "cute" than useful on a virtual Front Panel, we have added one "innovation" to the switch:
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pressing it also returns all the SWITCH register switches (0-21) to their DOWN position.
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### LOAD [ADRS]
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This is a momentary switch that is normally in the UP position. When pressed DOWN, it copies the SWITCH register to the
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ADDRESS register.
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### EXAM
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This is a momentary switch that is normally in the UP position. When pressed DOWN, it reads the memory location referenced
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by the ADDRESS register and displays it in the DATA register. If pressed repeatedly, each repetition also automatically
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adds 2 to the ADDRESS register, making it easier to examine a series of words. Note that if the CPU registers are being
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examined (starting at address 177700), the ADDRESS register will advance by 1 instead of 2.
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Another "innovation" unique to PDPjs: if the STEP switch is toggled from the UP to the DOWN position, repeated EXAM or DEP
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operations will decrease the ADDRESS register instead of increasing it.
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### DEP
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This is a momentary switch that is normally in the DOWN position. When pressed UP, it writes the contents of the SWITCH
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register to the memory location referenced by the ADDRESS register, and displays the result in the DATA register. Like the
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EXAM switch, if DEP is pressed repeatedly, each repetition also automatically adds 2 to the ADDRESS register, making it easier
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to deposit the same value to a series of words.
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### CONT
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This is a momentary switch that is normally in the UP position. When pressed DOWN, it will start the CPU if it's currently
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halted. If the CPU is already running, CONT has no effect. Also, if the ENABLE/HALT switch is in the DOWN (HALT) position,
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then the CPU will continue for only ONE instruction and then stop again.
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### ENABLE/HALT
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This switch is a toggle that is normally in the UP (ENABLE) position. If it pressed DOWN while the CPU is running, the CPU will
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stop. Once the CPU is stopped, this switch has no effect, other than determining what CONT will do when pressed.
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### STEP
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On a real PDP-11, this would select between cycle-stepping vs. instruction-stepping. Since PDPjs doesn't support cycle-stepping,
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this switch has no effect (except as described above when the EXAM or DEP switches are pressed repeatedly).
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### START
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The START switch is similar to CONT, but it also performs the equivalent a RESET instruction before starting the CPU, so
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unless you're starting execution at the beginning of a program, using START instead of CONT to continue execution is probably
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not a good idea. Also, if the ENABLE/HALT switch is in the DOWN (HALT) position, START will still reset the machine but not
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execute anything.
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