Manual · Section 3 of 8
The DMD 5620
The bitmapped terminal, its two screen sizes, and the phosphor.
A computer that happens to have a screen
The Blit — sold by Teletype as the DMD 5620 — was designed by Bart Locanthi at Bell Labs and is the reason Eighth Edition looks like anything at all. It has its own 32-bit processor, a megabyte of memory, and an 800×1024 one-bit display, and it connects to the host over a single serial line.
The important part is what that made possible: programs could be downloaded into the terminal and run there. The host sends a binary down the wire, the terminal executes it, and from then on the two are cooperating processes on two different computers. That is whatmux is.
The two screens
ipnx offers two sizes, in Settings → Screen:
| Preset | Pixels | Text |
|---|---|---|
| Original | 800 × 1024 | 88 columns |
| Wide | 1152 × 1024 | 127 columns |
Original is the machine as it shipped. Wide is not — it is a real modification to the terminal's firmware data, made at runtime, and it is included because a portrait CRT on a modern display leaves a great deal of desk unused.
The window is locked to the shape of the screen rather than the other way round. You can scale it, but you cannot reshape it, because the terminal has a fixed number of pixels and pretending otherwise would just stretch them.
One consequence worth knowing: on the Wide screen, mux needs a differently-built window system, and the login profile arranges that for you. If you ever run the window system by an explicit path and get a blank screen on Wide, that is why.
Phosphor
A one-bit display has no colours, only lit and unlit, so the colour of "lit" is yours to choose — green, amber or white, in Settings. It is a tint applied to the emulated raster, not a filter over the app, so it affects nothing else.
Scaling, and the moiré
Eighth Edition's window system draws with stipple patterns — alternating pixels — for grey. At a fractional zoom those patterns beat against the display grid and crawl unpleasantly.Crisp scaling fixes it by rounding to a whole-number multiple, so one terminal pixel is exactly two or three screen pixels. Fill uses the whole window and accepts the shimmer.
Crisp cannot always be honoured — below 1:1 there is no whole number to round to — so in a very small window it falls back to filling.
Why it uses CPU when nothing is happening
Because a real 5620 did. The VAX idles properly: its kernel executes a wait instruction and the simulator sleeps, so a machine sitting at a login prompt costs almost nothing. The terminal has no such instruction — its processor genuinely spins, polling its keyboard and its serial port — and emulating that faithfully means emulating the spin.
Closing the 5620 window stops it. The thread ends and the core comes back. If you are working in the glass terminals and want the fans quiet, close the terminal you are not using.
Its memory survives
The 5620 had non-volatile memory for its settings, and so does this one — baud rate and the rest persist between launches. So does the screen: what was last drawn is painted back when you resume, so a restored session looks like the one you left rather than a blank tube.