Re: [Fx2lib-devel] Setupdata at 0xe000

[Maarten B. <sou...@ds...>]

> OK I see now. Sorry, I was unaware of the 512 bytes of RAM at 0xe000.

But these can only be used as RAM accordign to the 
datasheet.

> If we just go ahead and change the storage class of the string
> descriptors from "code" to "xdata" in setupdat.c[1] (i.e no preprocessor
> guards), will it conceivably break anything? It looks to me like
> changing it to "xdata" merely gives apps the option of relocating
> DSCR_AREA to 0xe000 (which may be used for "xdata" only, not "code"),
> but with NO downside.
> 
> Or am I missing something?

And how is that xdata filled? SDCC assumes the code 
memory is stored in a ROM-like memory and xdata is 
volatile. So during startup it copies initialized data 
from code to xdata. So you have two copies in memory and 
thus your memory footprint has grown. If you use --no-
xinit-opt it is not copied but initialized by lots of 
instructions, usually taking even more code memory.

I guess it works because SDCC's initialization code 
copied it from lower memory to xdata at 0xe000. I also 
guess you missed the downside.

SDCC does not support the concept of preinitialized 
xdata as the FX2 has when it gets the firmware from the 
driver or the I2C eeprom.

> FWIW, I just tried the code->xdata + DSCR_AREA=e000 change on the
> firmware in FPGALink[2] and it works fine.

Are you using C0 or C2 mode? C2 mode cannot load 0xe000 
according to the datasheet. Can the driver download to 
0xe000 directly?

My advice is to keep these constants in code memory. And 
if you do want to change them at runtime cast a code 
pointer to xdata pointer to access them.

If the driver really supports loading to 0xe000 you can 
choose to store the constants there. You can also choose 
to use 0xe000 as xdata as it was meant.

Maarten