Member Avatar for dmanw100

Hello,
I was just wondering if the hlt instruction would produce a blinking cursor at the command line until enter is pressed or if its use is not noticeable to the user. Google wasn't too helpful for the specifics of the instruction. Thanks!

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Brief answer tied to the thread: HLT itself does not “show” or toggle a console cursor. The cursor blink and command-line updates are driven by the OS/terminal stack (timer ticks, display driver, and the console process). Whether you notice a change when HLT runs depends entirely on where that HLT executes and what else the system is doing.

On a modern multi-core OS the kernel commonly uses HLT in the CPU idle path to save power; that won’t freeze the display because other cores or the scheduler still handle timer and input events. On a single-core setup, or if HLT is reached with interrupts masked or while the kernel holds the scheduler, screen updates will stop until the next hardware event wakes the CPU — a keystroke (Enter) is only one possible wake source, not a special case.

A normal user-mode program can’t quietly execute HLT — it’s privileged, so attempting it from ring 3 will fault or be intercepted by the hypervisor. In VMs the hypervisor usually traps HLT and either emulates it or yields the vCPU, so the host/guest UI behavior is controlled by the hypervisor and guest OS, not the single instruction alone.

Practical testing tips: experiment in an emulator or a VM snapshot (safe, recoverable), try a tiny kernel or a DOS real-mode test to see raw behavior, and contrast “HLT with interrupts enabled” (idle) against “HLT reached while interrupts are disabled” (visible freeze). As suggested, consult the Intel/AMD manuals for exact semantics, and note ’s comments about privilege and where HLT actually takes effect.

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Member Avatar for gerard4143

Hello,
I was just wondering if the hlt instruction would produce a blinking cursor at the command line until enter is pressed or if its use is not noticeable to the user. Google wasn't too helpful for the specifics of the instruction. Thanks!

That would probably depend on how its initiated...

Member Avatar for gerard4143

That would probably depend on how its initiated...

You really should consult the Intel/AMD manuals..they have very good explanations of each opcode....

Member Avatar for NotNull

HLT will save CS:IP or CS:EIP of instruction after hlt
and stop instruction execution until a external interrupt
or NMI is issued.
It puts the processor in a halt state.
HLT is a priviledged instruction in virtual 8086 mode,
and will have no effect in the Windows Console.

Edited by NotNull because: n/a
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