是否严格要求汇编才能成为操作系统的“最低”部分？

Question

Im a mid-level(abstraction) programmer, and some months ago i started to think if i should reduce or increase abstraction(i've chosen to reduce).

Now, i think i've done most of the "research" about what i need, but still are a few questions remaining.

Right now while im "doing effectively nothing", im just reinforcing my C skills (bought "K&R C Programing Lang"), and im thinking to (after feel comfortable) start studying operating systems(like minix) just for learning purposes, but i have an idea stuck in my mind, and i don't really know if i should care.

In theory(i think, not sure), the higher level languages cannot refer to the hardware directly (like registers, memory locations, etc...) so the "perfect language" for the base would be assembly.

I already studied assembly(some time ago) just to see how it was (and i stopped in the middle of the book due to the outdated debugger that the book used(Assembly Language Step By Step, for Linux!)) but from what i have read, i din't liked the language a lot.

So the question is simple: Can an operating system(bootloader/kernel) be programmed without touching in a single line of assembly, and still be effective?

Even if it can, it will not be "cross-architecture", will it? (i386/arm/mips etc...)

Thanks for your support

Answer 1

You can do a significant amount of the work without assembly. Linux or NetBSD doesnt have to be completely re-written or patched for each of the many targets it runs on. Most of the code is portable and then there are abstraction layers and below the abstraction layer you find a target specific layer. Even within the target specific layers most of the code is not asm. I want to dispell this mistaken idea that in order to program registers or memory for a device driver for example that you need asm, you do not use asm for such things. You use asm for 1) instructions that a processor has that you cannot produce using a high level language. or 2) where high level language generated code is too slow. For example in the ARM to enable or disable interrupts there is a specific instruction for accessing the processor state registers that you must use, so asm is required. but programming the interrupt controller is all done in the high level language. An example of the second point is you often find in C libraries that memcpy and other similar heavily used library functions are hand coded asm because it is dramatically faster.

Although you certainly CAN write and do anything you want in ASM, but you typically find that a high level language is used to access the "hardware directly (like registers, memory locations, etc...)". You should continue to re-inforce your C skills not just with the K&R book but also wander through the various C standards, you might find it disturbing how many "implementation defined" items there are, like bitfields, how variable sizes are promoted, etc. Just because a program you wrote 10 years ago keeps compiling and working using a/one specific brand of compiler (msvc, gcc, etc) doesnt mean the code is clean and portable and will keep working. Unfortunately gcc has taught many very bad programming habits that shock the user when the find out they didnt know the language a decade or so down the road and have to redo how they solve problems using that language.

Answer 2

You have answered your question yourself in "the higher level languages cannot refer to the hardware directly".

Whether you want it or not, at some point you will have to deal with assembly/machine code if you want to make an OS.

Interrupt and exception handlers will have to have some assembly code in them. So will need the scheduler (if not directly, indirectly). And the system call mechanism. And the bootloader.

Answer 3

What I've learned in the past reading websites and books is that: a) many programmers dislikes assembly language because of the reasons we all know. b) the main programming language for OS's seems to be C and even C++ c) assembly language can be used to 'speed up code' after profiling your source code in C or C++ (language doesn't matter in fact)

So, the combination of a mid level language and a low level language is in some cases inevitable. For example there is no use to speed up code for waiting on user input. If it matters to build the shortest and fastest code for one specific range of computers (AMD, INTEL, ARM, DIGITAL-ALPHA, ...) then you should use assembler. My opinion...