Re: [avr-gcc-list] use of specifin RAM location

[David Kelly]

On Tue, Nov 29, 2005 at 08:35:05AM +1000, address@hidden  
wrote:
> I have the same problem, and the point is that part of my external
> memory is inside an FPGA, basically under control of the FPGA (It's
> high-speed ADC data), while another part is external SRAM which I use
> for my heap etc.
>
> I access the FPGA memory at the moment using pointers which I  
> initialise
> to the right value. I don't yet have a method mapped out for the  
> SRAM -
> it's on the next version of the board (the present version has no
> external SRAM).
>
> I will have a need to allocate some large-ish arrays in the external
> SRAM in the next hardware version.
>
> Memory-mapped peripherals need to be accessed at particular addresses
> also, however these don't usually need allocation (and hence can be
> accessed via pointers initialised to the correct address).

The cleanest way to pull this off would be to lay a structure on the
memory-mapped device and allocate it in a named section. See
http://gcc.gnu.org/onlinedocs/gcc-3.4.3/gcc/Variable- 
Attributes.html#Variable-Attributes

Then in the linking stage define the location of the FPGA something like
this in Makefile:

LDFLAGS= -Wl,-Map,address@hidden \
        -Wl,--section-start=.eeprom=0x00810001 \
        -Wl,--section-start=.fpga1=0x6000 \
        -Wl,--section-start=.hsa2d=0x7000 \
        -Wl,--section-start=.sram=0x2000

Just use the __attribute__ to tag variables as section .sram and the
compiler will find places for them if you don't care about the order.

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.