Add sources, format MMU

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+## Problems we faced
+
+Problems we've faced (I mentioned to Metal (is it ok to use that pseudonym? I think it's ok), I'd include those and he seemed very enthusiastic about us describing them) (+ some stuff qualifiable as probles is already in HISTORY.md)
+
+* Our ramfs needs to be 4-aligned in memory, but when objcopy creates the embeddable file, it doesn't (at least by default) mark it's data section as requiring 2**2 alignment. There has to be .=ALIGN(4) in linker script before ramfs_embeddable.o, but I forgot about it, which caused the ramfs to misbehave
+* VERY NAUGHTY PROBLEM · Many sources mention /COMMON/ as the section, that contains some specific kind of uninitialized (0-initialized) data. Obviously, it has to be included in the linker script. Unfortunately, gcc names it differently, mainly - /COM/. This caused our linker script to not include it in the image. Instead, it was placed somewhere after the last section specified in the linker script. This happened to be after our NOLOAD stack section, where first free MMU section is (which happens to always get allocated to the first process, which gets it's code copied there). Do You imagine sitting for hours in front of radare2, searching for a bug in scheduler.c or PL0_test.c, that causes the userspace code to fail with either some kind of weird abort or undefined instruction, always on the second PL0 instruction!?
+* VERY NAUGHTY PROBLEM · I wanted to make our bootloader and kernel able to run no matter what address they are loaded at (see comment in kernel's stage1 linker script). To achieve that, I added -fPIC to compilation options of all arm code. With this, I decided I can, instead of embedding code in other code using objcopy, just put that code in separate linker script section with section_start and section_end symbols defined, so that I can copy it to some other address in runtime. I did it and it worker with interrupt vector and libkernel (see point below). But once I changed EVERYTHING to use linker symbols/sections instead of objcopy embedding it turned out it doesn't really work... and I had to make it back the old way :( The thing is -fPIC requires code to be loaded by some os or bootloader, that will fill the global offset table with symbols. I knew it's possible to generate bare-metal position-independent code, that shall work without got, but it turned out this is not implemented in gcc (it is in arm compiler, but only in 32-bits and who would like to use arm compiler anyway). I ended up writing stage1 of both bootloader and the kernel in careful position-independent assembly, thus achieving my goal (jut with a bit more of effort).
+* Linker behaves weird when section names don't start with .text, .data, etc.
+* Not strictly a problem, but a funny mistake of mine, that is worth mentioning... At first I didn't know about special features of SUBS pc, lr and ldm rn {pc} ^ instructions. So I would switch to user mode by first branching to code in PL0-accessible section and having it execute isb instruction. This worked, but was not good, because code executed by the kernel was in memory section writable by userspace code. So i separated that into "libkernel", that would be in a PL0-executable but non-writable section and would perform the switch... Well, it did work. Still, I was happy when I learned how to achieve the same with subs/ldm and could remove libkernel, making the project a bit simpler.
+* system mode has separate stack pointer from supervisor mode, so when going from supervisor to system we need to set it... We didn't know that and we were getting weeeird bugs (where changing something little in one place would make the bug occur or not occur somewhere completely else); also, it's not allowed (undefined behaviour) to switch from system mode directly to user mode... (at least this didn't cause such weird things to happen...)
+* both bcm arm peripherals manual and the manual to uart itself say, that writing 0s to PL011_UART_IMSC unmasks interrupts; its the opposite: writing 1s enables specific interrupts and writing 0s disables them. wiki.osdev code also got it the way it's written in those docs, but this didn't cause problems, since uart irq was not enabled in ARM_ENABLE_IRQS_2 (using #define names from our code), so, as intended, no irq was occuring
+* STILL UNFIXED · The very simple pipe_image program somehow manages to break stdin, so that even other programs run in that same (bash) shell can't read from it... (in zsh other interactively run commands work ok, but commands following pipe_image inside a shell function still have that problem)
+ 
+## Sources of Info
+
+* wiki.osdev (good for starting off, we could also (in a polite way) mention the things, that were broken there)
+    + getting uart irq masking wrong (not really their fault, see above)
+    + zeroing bss even tho it was placed in a section, that wasn't marked NOLOAD (I need to verify that yet)
+    + switch inside an enum!?!?!?
+    + There is already some more mentioned in HISTORY.md
+* ARM Architecture Reference Manual® ARMv7-A and ARMv7-R edition (man, this has 2720 pages! But I think this was the most useful document of all)
+* dwelch67
+* http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html - very good description of atags
+* BCM2835-ARM-Peripherals.pdf and https://elinux.org/BCM2835_datasheet_errata
+* https://buildmedia.readthedocs.org/media/pdf/devicetree-specification/latest/devicetree-specification.pdf - perhaps You've found another source for that... but if not, this seems to be good!
+* online ARM Compiler toolchain Assembler Reference
+* Christina Brook's rpi-open-firmware
+* http://infocenter.arm.com/help/topic/com.arm.doc.ddi0183g/DDI0183G_uart_pl011_r1p5_trm.pdf
+* GNU make documentation
+* description of linker scripts: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html#OUTPUT-SECTION-DESCRIPTION