Switch and display register
A PDP-11 console carries a switch register and a display register at
one address, 777570. A program reads the switches the operator has set and
writes a word to the display lights. XXDP diagnostics take their options from
the switches, and many programs show progress or an error code on the display.
A Q-bus processor has neither, so QUniLator supplies both on the bus. The
device is listed as dispreg, of type dispreg_c, and ships disabled.
What the machine sees
Section titled “What the machine sees”| Register | 777570. A read returns switches, a write sets display |
| Interrupt | None |
| Bus INIT | Leaves both standing, so a diagnostic’s last code stays readable after it stops |
| Power down | Clears the display |
Enabling it probes 777570 first and refuses when something already answers there. An emulated PDP-11 processor on a UNIBUS board carries its own switch register at 777570 and answers it without a bus cycle, so with one of those enabled the device refuses as well.
Using it
Section titled “Using it”Enable dispreg and add its card to the dashboard. The card shows the display
as sixteen lamps grouped by octal digit, with the octal value beside them, and
the switches as sixteen toggles with their value in a field: click a toggle, or
type an octal word and press Enter.
The lamps follow the program at its own pace, a fraction of a second behind the machine: every change the program makes is replayed in turn, so a pattern that moves, such as RSX’s idle display, moves on the card as it does on a console. A lamp that goes out fades briefly, as a bulb does. A program changing the display more often than a thousand times a second shows as a blur of the lamps it lights, which is what a real console shows too.
Parameters
Section titled “Parameters”Settings
Section titled “Settings”What a configuration stores and an operator sets.
| Parameter | Short | Value | Default | What it is |
|---|---|---|---|---|
switches | sw | octal | 0 | Console switch register, read at 777570. |
Status
Section titled “Status”What the device reports as it runs. A configuration does not store these.
| Parameter | Short | Value | What it is |
|---|---|---|---|
display | dsp | octal | Console display register, written at 777570. |
It also carries the parameters under parameters every card on the bus has.
In detail
Section titled “In detail”switches
Section titled “switches”What a program reads at 777570: the console switches, set by the operator from the dashboard or the API. A change reaches a running program at its next read.
display
Section titled “display”The word a program last wrote to 777570, as the display lights show it. Bus INIT leaves it standing, so a diagnostic’s last code stays readable after the program has stopped; powering the machine down clears it.