add boot rom description

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/software_reference/boot_process/index.md

@@ -1,3 +1,65 @@
+!!! quote
+
+    _Analyzing the boot processes of systems that are functioning well
+    prepares users and developers to deal with the inevitable
+    failures._
+
+    _Alison Chaiken in "[Analyzing the Linux boot process][1]"_
+
+Learning how the **FunKey S** boots and shuts down is not required in
+order to develop for this platform, but it is certainly an interesting
+experience in order to understand the way to optimize a Linux embedded
+system.
+
+And then, when something goes wrong, it provides some useful insight
+on where to find the solution...
+
+## Startup Process
+
+The startup process for a Linux machine is like a space rocket launch,
+but in reverse order: you start small and end up big, each stage
+helping to launch the next one.
+
+The reason for this is because many resources are involved when
+booting a Linux system, that require a significant amount of set up
+before you can use them, as they are not available right from the
+beginning; the most critical ones are probably memory and system
+clocks.
+
+When the **FunKey S** is started, its CPU is merely as powerful as an
+Arduino board: what is available is a 32-bit ARM CPU running @ 24MHz,
+with no reliable memory besides its 30 internal registers and a 32KB
+boot ROM.
+
+In order to get up to full speed (1.2GHz) and with all its memory
+(64MB DRAM, access to the 8GB SDcard) requires several steps, in
+order:
+
+ - The [Boot ROM][2]
+
+ - The [SPL][3] (Secondary Program Loader)
+
+ - The [U-Boot bootloader][4]
+
+ - The [Linux kernel][5]
+
+ - The userland [System V init scripts][6]
+
+## Shutdown Process
+
+Unlike the startup process, the [shutdown process][7] is much more
+straightforward, but it must be optimized to run as quiclky as
+possible, as the time from when the shutdown is initiated when the
+user press the <i class="funkey-menu"></i> key or close the **FunKey
+S** lid to actual poweroff is extremely short (3 s).
+
+[1]: https://opensource.com/article/18/1/analyzing-linux-boot-process
+[2]: boot_rom
+[3]: spl
+[4]: bootloader
+[5]: kernel
+[6]: init_scripts
+[7]: shutdown_process
 
 --8<--
 includes/glossary.md