Prev: Netsilicon - Net + 50 hardware prob
Next: computer
From: Roman Mashak on 22 Jun 2008 16:08
Hello,

I'm trying to understand the mechanism of remapping on AT91SAM7 chips and
it's concerned the ARM assembly language. Basically what I wish to perceive
is the way C-startup is implemented. As far as I understand, it strongly
depends on a board involved and purpose of the firmware.

But there are some common parts of startup code as well, for example the
vector table: some examples I explored use "B .." instructions, while others
do various variations of "LDR pc ...". To my understanding, the first one
is doing _PC-relative_ jump, and "LDR pc, ..." populate PC with an
_absolute_ address.

So when is is preferable to use the first method (via 'B' instructions) and
whe the second one with absolute addresses? Are there any rules or common
practises for that?

Thanks.

Best regards, Roman Mashak


From: Not Really Me on 23 Jun 2008 11:18

"Roman Mashak" <mrv(a)tusur.ru> wrote in message
news:g3kj9a$uvt$1(a)relay.tomsk.ru...
> Hello,
>
> I'm trying to understand the mechanism of remapping on AT91SAM7 chips and
> it's concerned the ARM assembly language. Basically what I wish to
> perceive is the way C-startup is implemented. As far as I understand, it
> strongly depends on a board involved and purpose of the firmware.
>
> But there are some common parts of startup code as well, for example the
> vector table: some examples I explored use "B .." instructions, while
> others do various variations of "LDR pc ...". To my understanding, the
> first one is doing _PC-relative_ jump, and "LDR pc, ..." populate PC with
> an _absolute_ address.
>
> So when is is preferable to use the first method (via 'B' instructions)
> and whe the second one with absolute addresses? Are there any rules or
> common practises for that?
>
> Thanks.
>
> Best regards, Roman Mashak
>

You can use the relative addressing if all of your interrupt handlers fit in
the first 32K of code (14 bit address range). Some folks have a first level
handler in that space that dispatches to other relocatable code outside of
that range.

The second method allows you to put full 32 bit addresses in the table
(which usually immediately follows the vectors). Normally you boot from
flash and then set up RAM and do a remap to put the RAM version of the
vector table at 0. Once that is done you can modify the jump table to
install handlers.

For SAM7/9 chips they also have an advanced interrupt controller which can
provide handler addresses. You hard code the IRQ and FIQ vectors to do get
the jump address directly from the AIC.
ldr pc, [pc, #-0xF20] ; Branch to irq_handler via AIC IVR

Make sure to init the AIC and the handlers before enabling interrupts
though.

Scott



 | 
Pages: 1
Prev: Netsilicon - Net + 50 hardware prob
Next: computer