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Bare Metal rev two wip

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dwelch
2014-09-19 17:53:44 -04:00
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@@ -143,9 +143,99 @@ http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pd
(might be an old link, find the one on the wiki page)
And the schematic for the board
http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf
http://www.raspberrypi.org/wp-content/uploads/2012/10/Raspberry-Pi-R2.0-Schematics-Issue2.2_027.pdf
(might be an old link, find the one on the wiki page)
As well as some documents from ARM.
The Raspberry Pi is centered around the Broadcom BCM2835 media
processor. ARM does not make chips they sell/license the source code
to their processor design, which is normally integrated into what is
called an SoC or System on Chip. Which means some useful peripherals
are added to the chip that might historically have been on separate
chips like a DDR (memory) controller, or a USB controller, PCIe, etc.
For power or size or economy of scale reasons the folks that buy ARM
processor cores generally need a processor to add to their chip and
it is easier sometimes to buy than make your own. Most folks dont
realize it and think that because almost every big box computer (server,
desktop or laptop) is Intel x86 based (or a clone) that x86 processors
dominate the world, not realizing that that same box has many other
processors inside, not all of them ARM's but some. For every x86 you
own or use you likely own or use many many ARM based products. This
chip from Broadcom is one of the myriad of ARM based products out there
fighting for a space in the various niche markets.
Be it an ARM based chip or some other the first thing a bare metal
programmer needs to do is figure out which processor you have. Simply
stating it is an ARM processor is not remotely enough. ARM has an ever
growing array of processor products. Some chip vendors are more
helpful than others at figuring this out. The BCM2835 document
mentioned above would normally be the place where you would find this
out, but in this case it does say ARM in the document but doesnt even
say ARM11 much less arm1176jzfs. Fortunately the Raspberry Pi
creators and community has the wiki page above which provides the
information we need. ARM has at least four different cores in the
ARM11 category this one is the ARM1176 specifically arm1176jzfs a bunch
of letters that mean something to ARM as to the features included. For
us that means wse can find one of the two documents we need from ARM.
Generally you start at
http://infocenter.arm.com
And along the left side you find the processor series, in this case
ARM11 processors. Expand that and see the ARM1136, ARM1156, ARM1176
and the MPCore. We want ARM1176. Our first goal here is to find
the Technical Reference Manual, TRM, for the core we are using. For
the moment this is an accurate link directly to that document
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0301h/DDI0301H_arm1176jzfs_r0p7_trm.pdf
In the preface of the TRM it gives us a hint as to the ARM ARM we need
(ARM ARM = ARM Architectural Reference Manual).
ARM Architecture Reference Manual (ARM DDI 0406)
There used to be only one ARM ARM for the whole ARM world but the
architectural differences were such that they left the original ARM ARM
with the last architecture it supported and started creating new ones.
So back on the left of the page expand ARM Architecture and then expand
Reference Manuals.
Unfortunately the didnt tell usin the TRM which architecture name to
look for, so we have to fumble around a bit or do some Googling to find
that we need the ARMv7-AR Reference Manual. From that page it shows
This manual describes the instruction set, memory model, and programmers'
model for ARMv7 (A&R profile) compliant processors, including:
Cortex-A series
Cortex-R series
Qualcomm Scorpion.
It also describes the later ARMv6 architecture releases for ARM11
processors, and describes Thumb-2 and the TrustZone security
extensions.
If you get the manual through ARMs website they appear to require a
login. It is free other than giving up an email address which no doubt
you have or can create a gmail one or whatever.
https://silver.arm.com/download/download.tm?pv=1603196
So the r0p7 nomenclature means rev 0.7 the r is rev and the p is a period.
Now hopefully the Raspberry Pi folks who provided that link gave us the
right rev. Just because ARM has fixed some bugs in some rev and the
currently selling rev is some other number, any ARM based chip you are
using is built from a specific rev of that product and there are times
where a rev change generates different internal addressing or features
in the chip (certainly if you have access to the errata, you need to
be very careful to apply the correct errata to the right rev, far too
often are workarounds applied improperly to arm code causing more
problems for that software than solutions). The ARM1176JZF-S has only
the r0p7 rev of TRM. But look at the ARM11 MPCore TRM and see there
is an r1p0 and r2p0 and I know that if you use the wrong one there
you can have stuff not work. When in doubt take the newest one and
hope for the best, if you know for sure, then even if the ARM web page
marks that doc as Superseded, use that doc.
To add to the confusion wikipedia shows that the ARM1176 is architecture
version ARMv6Z. The part we care about is the ARMv6 part as you will
see soon.
So what was the point of that exercise?