Is there a detailed description of the BBB boot sequence anywhere?

I can find lots of descriptions of what is *supposed* to happen when
powering up a BBB but I don't seem to be able to find a description of
what the hardware does at power up time to decide what code to execute
etc.

Is it a built-in part of the processor or is there a 'boot ROM'
somewhere the is executed at power up to decide what to do? Is this
documented anywhere?

http://www.ti.com/lit/ug/spruh73p/spruh73p.pdf

Use slides 7+ from my last TearDown presentation:

https://docs.google.com/presentation/d/1ylRm_dhX8jgWLHhoXH3zLXQgETY1eYh_PXyhtD6_vPo/edit#slide=id.g29152d7196_0_10

Regards,

On Mon, 25 Jun 2018 16:00:17 +0100, Chris Green
<cl@isbd.net> declaimed the following:

Is it a built-in part of the processor or is there a 'boot ROM'
somewhere the is executed at power up to decide what to do? Is this
documented anywhere?

  Beyond pages 67-69 of the BBB SRM? (though I suspect newer u-Boots have
modified some of the later stage behavior -- ie; based upon observed
behavior: u-Boot in eMMC will load a kernel from SD card without needing
the boot select button, boot select would be needed to bypass the eMMC
entirely; I've only needed to use boot select the first [and maybe second]
time I flashed the eMMC with a late Wheezy or early Jessie release image).

From the SRM
https://github.com/CircuitCo/BeagleBone-Black/blob/rev_b/BBB_SRM.pdf?raw=true
:
"""
The first row in Figure 39 is the default setting. On boot, the processor
will look for the eMMC on the MMC1 port first, followed by the microSD slot
on MMC0, USB0 and UART0. In the event there is no microSD card and the eMMC
is empty, UART0 or USB0 could be used as the board source.
"""

  The alternate (boot select held down) sequence is SPIO0, MMC0, USB0,
UART0. Note that the SD card is the second target in both cases, but the
eMMC is dropped completely, and the UART and USB swap position. (Hmmm --
wonder how the alternate sequence is affected if something is on SPIO0 that
doesn't support booting?)

Thanks Robert, that's just about exactly the sort of thing I was
looking for.