How does the OS detect hardware?

Question

Does the OS get this information from the BIOS or does it scan the buses on its own to detect what hardware is installed on the system. Having looked around online different sources say different things. Some saying the BIOS detects the hardware and then stores it in memory which the OS then reads, others saying the OS scans buses (e.g pci) to learn of the hardware.

I would have thought with modern OSs it would ignore the BIOS and do it itself.

Any help would be appreciated.

Thanks.

Answer 1

Generally speaking, most modern OSes (Windows and Linux) will re-scan the detected hardware as part of the boot sequence. Trusting the BIOS to detect everything and have it setup properly has proven to be unreliable.

In a typical x86 PC, there are a combination of techniques used to detect attached hardware.

PCI and PCI Express busses has a standard mechanism called Configuration Space that you can scan to get a list of attached devices. This includes devices installed in a PCI/PCIe slot, and also the controller(s) in the chipset (Video Controller, SATA, etc).

If an IDE or SATA controller is detected, the OS/BIOS must talk to the controller to get a list of attached drives.

If a USB controller is detected, the OS/BIOS loaded a USB protocol stack, and then enumerates the attached hubs and devices.

For "legacy" ISA devices, things are a little more complicated. Even if your motherboard does not have an ISA slot on it, you typically still have a number of "ISA" devices in the system (Serial Ports, Parallel Ports, etc). These devices typically lack a truly standardized auto-detection method. To detect these devices, there are 2 options:

1. Probe known addresses - Serial Ports are usually at 0x3F8, 0x2F8, 0x3E8, 0x2E8, so read from those addresses and see if there is something there that looks like a serial port UART. This is far from perfect. You may have a serial port at a non-standard address that are not scanned. You may also have a non-serial port device at one of those addresses that does not respond well to being probed. Remember how Windows 95 and 98 used to lock up a lot when detecting hardware during installation?

2. ISA Plug-n-Play - This standard was popular for a hot minute as ISA was phased out in favor of PCI. You probably will not encounter many devices that support this. I believe ISA PnP is disabled by default in Windows Vista and later, but I am struggling to find a source for that right now.

3. ACPI Enumeration - The OS can rely on the BIOS to describe these devices in ASL code. (See below.)

Additionally, there may be a number of non-PnP devices in the system at semi-fixed addresses, such as a TPM chip, HPET, or those "special" buttons on laptop keyboards. For these devices to be explained to the OS, the standard method is to use ACPI.

The BIOS ACPI tables should provide a list of on-motherboard devices to the OS. These tables are written in a language called ASL (or AML for the compiled form). At boot time, the OS reads in the ACPI tables and enumerates any described devices. Note that for this to work, the motherboard manufacturer must have written their ASL code correctly. This is not always the case.

And of course, if all of the auto-detection methods fail you, you may be forced to manually install a driver. You do this through the Add New Hardware Wizard in Windows. (The exact procedure varies depending on the Windows version you have installed.)

Answer 2

I see a lot of info about system hardware, except for memory ,one of the main important part besides the cpu, which funnily isn't really mentioned as well. This is fair because perhaps there's so many things to enumerate, you kind of lose sight of the forest through the trees.

For memory on x86/64 platforms you will want to query either BIOS or EFI for a memory map. for BIOS this is int 0x15 handle 0xe820. EFI has it's own mechanism which provides similar information.

This will show you which memory ranges are reserved by hardware etc. in order for your OS to know to leave them alone. (ok you have to built that part too of course ;D)

For other platforms, often the OS will be configured for a fixed memory size, like in embedded platforms. There is no BIOS for you, and performing a sort of bruteforce on memory is unreliable at best. (as far as i know! - not much experience outside of x86/64!!!)

For the CPU you will definitely want to look into MSRs, control registers and CPUID functions to enumerate the CPU and see what it's capable of. you can query if for example 64 bit mode is supported, and some other features which might not be present on all cpus.

For other hardware like pci etc, i would recommend like myron-semack said to look into PCI specification, pci-express, and importantly ACPI as implementing that will make you handle hardware and powermanagement a . bit more generically / according to newer standards.