Actually it depends on how far you want to go. You can jump to protected mode with a few lines more. But you won't have a very interesting exercise.
Basically, one would have to setup multiple registers like the GDT which sets the protection on different memory ranges (hence protected mode). Once you have your GDT setup, set a few bits in CR registers and execute a long jump to the code segment of the GDT (provided you have instructions loaded there).
Then once in protected mode, everything changes. The system allows you to set interrupts habdlndlers through the LIDT, to enable memory paging and you'll have to reload the GDT again. But no more access to the convenient BIOS functions.
You also set up the PIC of your system to get a ticking clock for scheduling your kernel operations. Once you got all that is where the fun begins and you can start implementing memory managers and allocators, writing a disk operations layer and implement (or write your own!) filesystem.
OS Dev is truly a fascinating subject because there's so much to learn. Even more with 64-bit, and multiple core initialization and usage.
I don't know how all of this works on ARM, but I might order myself an experimentation board to get my hands on.
The traditional ARM answer is that at a low level and at bootup the details are all very board specific -- what kind of BIOS equivalent you have (if any), where it starts you off, whether there's a bootloader like u-boot involved, where devices like the serial port are: all these things vary by board. For 64-bit ARM things are moving a bit more towards a standard setup with UEFI. In general, ARM assumes that the initial boot code will be customised for the specific chip, so unlike x86 which starts out looking like an 8086 until you flip it into protected mode, a 64-bit ARM CPU will start out in 64-bit and you can get at the backwards-compatibility 32-bit support by dropping down from 64-bit to 32. Back-compat is for applications, not OSes and bootloaders :-)
this is a terrible hack, but if you leave the bios areas alone, you can make a system call like machine that downshifts back into real mode, makes the call, and shifts back up into protected mode (or all the way back up into 64 bit mode)
its not _really_ worth it, but if you're just noodling around and want to send packets or write to disks you don't have to write drivers. of course you have to have dedicated buffers visible in both spaces.
the only other reason i can think of to use real mode is to actually explore the segment model rather than just treating it as trash you have to walk over to get something up from reset...but it would probably be better just to invent some kind of segmented virtual machine
Basically, one would have to setup multiple registers like the GDT which sets the protection on different memory ranges (hence protected mode). Once you have your GDT setup, set a few bits in CR registers and execute a long jump to the code segment of the GDT (provided you have instructions loaded there).
Then once in protected mode, everything changes. The system allows you to set interrupts habdlndlers through the LIDT, to enable memory paging and you'll have to reload the GDT again. But no more access to the convenient BIOS functions.
You also set up the PIC of your system to get a ticking clock for scheduling your kernel operations. Once you got all that is where the fun begins and you can start implementing memory managers and allocators, writing a disk operations layer and implement (or write your own!) filesystem.
OS Dev is truly a fascinating subject because there's so much to learn. Even more with 64-bit, and multiple core initialization and usage.
I don't know how all of this works on ARM, but I might order myself an experimentation board to get my hands on.