xilinx_zynq_zc702 vs. xilinx_zynq_zc706 memory map

[Chris Johns]

On 24/10/19 7:50 pm, Sebastian Huber wrote:
> On 24/10/2019 03:41, Jonathan Brandmeyer wrote:
>> On Wed, Oct 23, 2019 at 6:04 AM Thomas Doerfler
>> <thomas.doerfler at embedded-brains.de> wrote:
>>>
>>> Hi,
>>>
>>> most likely the RAM areas have been mapped to the lowest-possible
>>> non-NULL address, and they can be mapped to an address boundary matching
>>> the RAM size. zc702 has a 1MByte ram, mapped to the 1MByte boundary,
>>> zc706 has a 4MByte RAM mapped to the 4MByte boundary.
>>
>> I don't know what the actual rationale is, but this definitely isn't
>> it.  The DDR physical address mapping is fixed.  The lower 256k can be
>> mapped to DRAM or on-chip SRAM depending on system settings.  The
>> range from 256k - 1M is either inaccessible or mapped to DRAM.  The
>> range from 1M - 4M is always mapped to DRAM.  Its an
>> application-profile processor, which is why the typical sizes are 512M
>> or 1024M (SoC maximum) for DRAM.
>>
>> See also Xilinx UG585, section 4.1.
>>
>> The chip's reset defaults are for 192 kB of OCM to be mapped low, and
>> 64 kB to be mapped high with address filtering disabled.
> 
> 
> Yes, the DDR is surly greater than 4MiB. The unused 1MiB starting a 0 is there
> to catch NULL pointer access (read, write, and execute). Why the
> xilinx_zynq_zc706 has an unused 4MiB area is not clear to me.

It is not clear to me either. I am fine with the start address being the same
for all variants. If nothing else someone may speak up and we will find out the
reason.

Chris