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A single dim-white blinking underline cursor in the top left cell of an otherwise black screen.

I remember that, in the old days, a serious boot error could manifest in a screen like the above.

(I have no idea if this may still happen. I haven't seen it for years, so I assume it is legacy as of UEFI so I chose to asked this question here. Please migrate it if this is not the case)

That screen is not completely blank - it contains a blinking text cursor. This seems to hint that some sort of interactivity was intended, as such cursors commonly appear if the user is expected to type something.

Was it a primitive command prompt? Some sort of low-level BIOS console?

I tried typing random characters, but it did not work. Instead, I had to resort to the power button.

  • Was a hard power-off truly the only option when such a screen was encountered?
  • If yes, then why was a blinking text cursor put there?
  • If no, then how could I interact with this screen?
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    But cursors were generated by the CRTC chip. Nothing to do with the BIOS or OS and therefore, in your blank screen, nothing to do with user input. There is nothing running with which to interact.
    – Chenmunka
    Oct 15, 2023 at 14:48
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    It's not really any different to a powered-on terminal with nothing listening at the other end of the wire.
    – dave
    Oct 15, 2023 at 18:13
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    I think swearing loudly at the screen is an option…
    – Jon Custer
    Oct 15, 2023 at 19:35
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    Just wait a bit longer.... I'm sure something is going to happen here the next few minutes....
    – tofro
    Oct 16, 2023 at 21:57
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    The question's based on a misunderstanding, but it's a very plausible misunderstanding for someone familiar with modern computers. 3 out of 4 currently-posted answers make the same point (the blinky cursor is generated asynchronously by the graphics hardware based on the initial state of the video memory), which fundamentally is the answer to this question, so I don't think it's unclear at all.
    – wizzwizz4
    Oct 17, 2023 at 11:13

4 Answers 4

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The screen you're seeing is presented asynchronously and autonomously by the graphics hardware in your PC. It doesn't mean anything in itself, other than the graphics hardware is powered, it's receiving a clock, and your monitor is connected correctly. If the registers of the graphics hardware are set up to display a text mode screen with a cursor enabled, it will blink there indefinitely regardless of whether the CPU is halted.

Without knowing more about the system you're specifically talking about, it's not possible to say what the intention of the screen is, since it's going to be different for every system.

I know what you're suggesting though, and can offer an analogous situation in the Guru Meditation screen on the Amiga. On an unrecoverable detectable error, an Amiga halts all processing and displays a blinking red rectangle with some error codes. It isn't stated as an instruction on screen, but this screen is an invitation for developers to begin a debugging session over a serial connection.

If you would please edit your question to focus on a more specific release of an IBM PC (/ compatible) then there might be a more specific answer available, but in the general case, this screen can mean either 'please wait' (perhaps it's waiting for some part of a network boot), or that the system is locked up and won't respond to anything (a bad boot sector went into an infinite loop or halted).

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    On the IBM PCjr and P/S2 systems running DOS say... 3 - 6ish, this screen would usually respond to IPL (ctrl-alt-del) if one didn't want to power cycle the system. Oct 16, 2023 at 3:03
  • That's true. It skipped my mind because I wouldn't consider it a thing the blinking screen offers, but unless the system is really messed up CAD would respond.
    – knol
    Oct 16, 2023 at 15:57
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    @ToddWilcox Is ctrl-alt-del considered keystroke input from the perspective of whatever software is controlling the screen, or is it an interrupt detected by the OS and/or hardware? Is it really the screen (or whatever is controlling it) that's involved in that case?
    – JBH
    Oct 18, 2023 at 0:13
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    @JBH In the 80s, it was an interrupt. According to Wikipedia: "By default, when the operating system is running in real mode (or in a pre-boot environment, when no operating system is started yet), this keystroke combination is intercepted by the BIOS. The BIOS reacts by performing a soft reboot (also known as a warm reboot). Examples of such operating systems include DOS, Windows 3.0 in Standard Mode as well as earlier versions of Windows." and "Perform a soft reboot without memory initialization by jumping to IPL reset vector..." Oct 18, 2023 at 1:59
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This seems to hint that some sort of interactivity was intended

Not necessarily.

The cursor in CGA/MGA/EGA/VGA text modes is a hardware feature, independent of the main CPU. There is no canonic way of disabling it. Different programs used different hacks in order to hide it, e.g. putting it on the non-existent 26-th line, setting its height to 0 - and not all video cards were compatible with all of these hacks.

Modern video systems generally emulate this behavior when forced into text mode.

Otherwise, the cursor was assumed to exist and it was generally send at the place where the next symbol was expected to appear. E.g. in the upper left corner for a clear screen.

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It depends on what the executed code is doing and why, or has it executed an illegal instruction.

Sometimes the system may be waiting for something else than user input and it may be still possible to use the keyboard for resetting the system with Ctrl-Alt-Del, if it has not completely hanged.

What you see just means that the BIOS has successfully initialized the video subsystem into text mode, and is stuck after that, due to some hardware or software issue. If you simply call the BIOS to set a text mode, you do end up with an empty screen and blinking cursor.

There is no console where you can enter anything, it does not expect any user input, there is no user interaction possible that would allow doing anything useful.

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Many PCs of the era also had a reset button, which would restart it without cycling the power. This was thought to be easier on the hardware than cycling the power.

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    s/thought to be/is/ Many types of hardware have limits to the number of times they can be power cycled. Making sure that limit is sufficient for expected use cases is a part of hardware design and is tested when doing reliability testing.
    – Makyen
    Oct 17, 2023 at 21:06
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    'Power on' is like a whack round the head to a lot of electronics. Anecdotally, when I worked on minicomputers, they'd tend to get switched off for long holiday weekends, and chances were good that at least one would need field service attention when switched back on again.
    – dave
    Oct 20, 2023 at 18:03
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    That was a serious flaw in the IBM PC, XT and (IIRC) AT. The theory was that Control+Alt+Delete would be a reliable soft way to reboot, but that depended on the nature of the problem. Oct 20, 2023 at 20:31

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