this post was submitted on 23 May 2025
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Hah! This might be the best use of this meme template I've seen. And this template has a lot of good ones.
I feel like I should understand this, but I don't.
Think of it like breaking news for your processor
"WE INTERRUPT THE CURRENTLY RUNNING APPLICATION TO BRING YOU THIS MESSAGE: e"
This is x86 assembler. (Actually, looking at the register names, it's probably x86_64. On old school x86, they were named something like al, ah (8 bit), ax (16 bit), or eax (32 bit).) Back in the old days, when you pressed a key on the keyboard, the keyboard controller would generate a hardware interrupt, which, unless masked, would immediately make the CPU jump to a registered interrupt handler, interrupting whatever else it was doing at the point. That interrupt handler would then usually save all registers on the stack, communicate with the keyboard controller to figure out what exactly happened, react to that, restore the old registers again and then jump back to where the CPU was before.
In modern times, USB keyboards are periodically actively polled instead.
Right, PS/2 cable. I remember having to reboot, if you accidentally unplugged it.
Oh, you're right! I completely forgot that was a thing. But I do remember the "keyboard not found, press f1" message.
The virgin USB: hey, uh, when you get a chance, uh, if it's not too much trouble, could you, uh, put an 'e' there? Whenever you get the chance is fine
The chad PS/2: THE USER SAID E.
Thanks for the explanation. Ironically this was the bit I didn't know:
In modern times, USB keyboards are periodically actively polled instead.
I was thinking the implication was that some computer had faulty interrupt handling that would smash the status register or something.
Honestly I think I'm just too old to understand memes.
does that mean though that if I connect a PS/2 keyboard or mouse to my relatively modern computer (a "gamer" motherboard made ~6 years ago) 's PS/2 port, that it'll still trigger such an interrupt?
The other commenter is on the right track but the chip controls both USB and PS/2 as well as others;
In the 90s and 2000s, for x86 machines, slower I/O was handled by a chip called the Southbridge which worked in conjunction with a chip called the Northbridge that handled faster I/O like IDE and PCI. Later these were integrated into a single chip and, as of recent processor generations, into the processor itself.
AFAIK ghosting and key rollover are issues when using PS/2 but it can offer some milliseconds off latency when used in high cpu games.
AFAIK ghosting and key rollover are issues when using PS/2
I think it's more of an issue for USB keyboards than PS/2 keyboards.
They are wholly independent from the protocol or interface. Ghosting is an electrical issue that is a result of keyboards being a bunch of switches arranged in a matrix. It makes the keyboard's controller register an extra keypress in certain conditions. Nothing to do with how the thing communicates with the host computer.
Key rollover issues can be related to ghosting. The limit for it is once again the keyboard's design at the circuit level, not its communication protocol.
Really they're both related to how cheaply built the keyboard is. That's the only thing.
Ghosting entirely depends on the wiring of the keyboard pcb. Key rollover can depend on the wiring of the keyboard pcb, but usually is limited by the usb HID protocol.
Generally speaking, usb can carry up to 6 keys of information in a single packet (I donβt remember off the top of my head if modifiers are included). It is possible to use extended packets and encode more info (and thus allow for more than 6 keys rollover) but it requires negotiation with the os so most vendors donβt bother as generally you donβt need to be able to press more than 6 keys at the same time for most applications.
I think there's a USB device inside the mobo to handle dumb peripherals. So it would still trigger an interrupt, but it wouldn't make it to the CPU. The USB keyboard controller would handle it and cache the strokes locally until polled by the CPU.
I would expect that any motherboard that went to the trouble of including a PS/2 port would handle it with a real hardware interrupt, because the whole point of still having those things is to avoid the latency overhead of USB.
there are loads of things in the gamer space that are just branding BS⦠i could definitely see dodgy (or honestly even reputable) motherboard manufacturers adding PS/2 ports that are just basically a USB dongle built into the board and adding $40 for the dubious privilege
Largely an urban legend. The internal electronics of the keyboard/mouse matter more than the protocol for end to end latency.
There are USB keyboards that beat a PS/2 one, at just 125 Hz polling. 1000 Hz polling pulls ahead even more.
But honestly, 1000 Hz polling is just for bigger numbers game. Even 300 ms are barely noticeable by humans, not to mention 10 ms of 100 Hz.
You're right, but rax is amd64.
I think there were a few early amd64 systems with genuine ps2, and I think you can still get one, but it wasn't common, and honestly it's probably usb->ps/2.
To be a pedantic asshole: mov eax, ecx? Unless you're commenting on the insanity of interrupt driven i/O in the modern age of high performance, deep-pipelined superscalar OOO cores.
I had to write a mini os and it handled keyboard interrupts. Certainly made it make a lot more since after writing it for my uni course
The small bird is a CPU executing its instructions. The big bird is a keyboard sending an interrupt for the CPU to process immediately.
And their interrupt routine has an error that leads to changed memory and you donβt know why your Programm calculates β2+6=Eβ and that only sometimes on every other run.
REISUB
Raising Elephants Is So Utterly Boring
!!! I just learned about this recently because my PC has an Nvidia GPU and it sometimes wakes up from sleeping to a blank screen with just the mouse cursor showing.
I try that REISUB first but if that doesn't do anything, I have to go to Ctrl-Alt-F3 and do "sudo reboot".
Linux is fun except for when it's not.
You probably have to enable it in sysctl
Iβm confused. Why would you REISUB/O if you can use the reboot command?
REISUB is a last resort before hitting the physical power button
Haha, because I'm lazy and going with the fastest, safe way to reboot when I get stuck.
reisub is not "safe". it is the "safest" way when your system has reached an otherwise broken state, but you're still interrupting things that may be saving state or changing configurations. if your system is working behind the scenes you may very well break it more.
You should really use the reboot command first because you donβt have to control the order and timing. REISU(B/O) requires pauses between letters for the specific action to run. It also requires those letters to be in that particular order - you canβt sync the file system if you put it into read only. If REISUB is sometimes not working but you can go elsewhere and reboot, youβre likely not doing it right and you should err on the side of caution and use the reboot command
Gotcha, thanks;
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