Why an Operating System(OS) is called as hardware dependent/platform dependent

Question

Why we are saying that the OS is purely hardware dependent (other than hardware peripherals like RAM/USB etc)? The word hardware independence means, the OS should run on any platform with out any underlying hardware abstraction layer like ARM/x86/xtensa/starcore etc etc.

Can you please give me the exact hardware dependencies in a simple/common OS? Meaning exactly in which are all points in the OS is accessing the hardware or depending on the platform?

Also is it possible to write a simple OS or a RTOS (using C language) with out any hardware or platform dependency(ie without any VM concept) so that it'll run on any platforms?

I would be expecting the answers from the OS kernel side and not from the peripheral side like RAM /keyboard/mouse

I will give you an example of exact hardware dependency in an OS "at context switching (context of the tasks/threads should be stored with help of underlying platform/CPU only)"
__Kanu

Answer 1

Pretty much everything about an OS is hardware dependent in some way, from memory management to timers (scheduling) to networking to video to keyboard to BIOS. All of this will require hardware-dependent C code and/or assembly.

That doesn't mean you can't extract out a lot of common C code which is shared between architectures. Linux is a classic example of this. It has been ported to a vast array of hardware platforms, requiring custom code for each platform. However, there's still a large body of shared C code (e.g. filesystem drivers).

And of course, even the parts that are ANSI C only run on your hardware if your compiler can target it.

Answer 2

If an OS had no hardware dependancies, how could it received input from the output world, and output back to them the results?

Answer 3

In general, the following things are hardware dependent:

• System startup/reset
• Interrupt handling
• Virtual memory management & protection
• Device I/O
• System-level protections for code access and security
• Some mutual exclusion primitives.

At some level, way way down, an OS kernel needs to sit on top of something. Most kernels are written such that they touch the hardware with as small a surface area as possible, but there must be some touch point there.

You can write most of a kernel in C (this is usually the case). But you'll need to run on top of something. If you fudge with the definition of an OS a little bit, you could have a "microkernel" that is hardware-dependent, and build many of the above as abstractions as a toy OS on top of it, but you'd suffer in performance/accuracy/sophistication.

Answer 4

Any operating system is at least depending on one piece of hardware: the CPU. There are different CPUs, each working differently and having a different "native language". Since an OS is "just a program" which needs to run on the CPU, it must be written in the CPUs native language is thus dependent on it. You cannot run a normal Windows on an ARM or PowerPC processor, for example. It only runs on Intel-compatible CPUs.

It is possible to write an OS that can be compiled for different CPUs and run on them, most UNIXes like Linux, FreeBSD, etc. are good examples. But the need to be compiled ("translated") for each CPU they want to run on.

Apart from the CPU, an OS also needs ways to process something, so it needs input and output like a hard-disk or ROM, a screen and a keyboard (but not necessarily; e.g. an elevator has no need for a real keyboard and often doesn't even need a screen). And there are various different ways to access each of these devices and the OS depends on these methods (for example, bus systems like the PCI bus, or dedicated chips like a 16550 for serial ports).

Answer 5

Every point where input and output occur is hardware dependent.

Every point where interrupts come into play is hardware dependent.

Every point where memory itself is managed is hardware dependent.

In other words, if you care about it, it's probably hardware dependent.

Man I like embedded systems.