[Oberon] Project Oberon running from LPDDR memory on Pipistrello

[Magnus Karlsson]

FYI, I have worked the last week with Jörg Straube on getting Project 
Oberon running from DRAM on the Pipistrello board and I just checked in 
the code on github:
See 
https://github.com/Saanlima/Pipistrello/tree/master/Projects/Oberon_lpddr

It's using a 128KB cache organized as a 2-way 256-set cache with 256 
byte cache lines.  The interface between the cache and the memory 
controller is 128 bits wide @100 MHz, and the memory controller itself 
has 800 MB/s peak memory bandwidth.
On power-up the lower 128KB of the 1MB address space is covered by the 
cache.  The main code for the cache is a simple state machine using 
about 150 lines of code with 4 states.

The cache actually covers the lower 8MB address space so in theory we 
could have an 8MB Oberon system, but due to a bug somewhere in the 
Oberon code it crashes after about 1.25 sec with a trap (C2H on the 
LEDs) with more than 1MB of memory.  The solution is to have the non-io 
address space aliased to the first 1MB.  It seems like there are memory 
access cycles above 1MB that needs to actually affect the memory at the 
first 1MB.  I did discover this way back when I did the 2MB version of 
Pepino and had the tie address bit 20 to 0 for it to boot (i.e. alias 
the second MB to the first MB) but never got around to pinpoint the 
problem.  The same problem popped up again and I was able to trace it down.

Video memory is in dual-port BRAM and can be relocated by writing to a 
new register.  At power-up the video is a the normal spot (0E7F00H) but 
can be moved up above the 8MB boundary to have a continuous 8MB memory 
section for program and data by simply writing to a register with the 
new video base address.

Cheers,
Magnus