0

A little bg - I just started OS Dev and am developing my own OS in assembly.

So as I have learned, x86 processors and others that emulate them(AMD?) start in 16 bit real mode for boot process and then can switch to protected mode to further bootstrap the OS or secondary bootloader and what not. (leave the modern BIOSes and UEFI's switching them to 32/64 bit part and then switching them back to 16 bit real mode part as that is irrelevant)

My simple question is - Do embedded devices that have ARM processors also do this? I mean do our smartphones switch to 16 bit real mode for the boot process?

vigilante_stark
  • 159
  • 1
  • 6
  • 3
    AMD processors don't *emulate* x86, they *are* x86 CPUs. (Unless you're talking about [AMD's Opteron-A](http://www.amd.com/en-us/products/server/opteron-a-series) series of ARM server CPUs.) – Peter Cordes Jan 20 '18 at 09:12

1 Answers1

2

No. "Real mode" is unique to the x86 architecture; it doesn't exist on ARM systems. They boot directly into a 32-bit mode.

  • Thanks for the answer. I couldn't find this even after a bit of googling and thought this should be on SE. – vigilante_stark Jan 20 '18 at 04:04
  • @old_timer Which part of it? :P There's literally thousands and thousands of pages of ARM documentation, even before you get to the vendor-specific manuals… –  Jan 20 '18 at 04:14
  • there is only one small part of the appropriate manual for your core on reset and how it works. – old_timer Jan 20 '18 at 04:24
  • the comment was directed at the OP. – old_timer Jan 20 '18 at 04:26
  • 1
    @old_timer It's not that simple of a question. I've worked with ARM parts where there's whole books of the reference manual which are entirely about the boot process. And even then, they won't say "the part boots directly into 32-bit mdoe" -- that's just more or less assumed. –  Jan 20 '18 at 04:29
  • I have worked with pretty much every arm (architecture) made since the armv7tdmi. It is all very clear, no questions, vendors doc sends you to one trm, that one trm sends you to one arm arm. and that describes how that architecture works on a fraction of a page, there are no thousands of pages to worry about with respect to this question. If you are still confused why are you trying to answer the OP's question? – old_timer Jan 20 '18 at 14:38