I have an SRAM controller Avalon slave we created which Nios can
successfully access to read and write SRAM.

My question is: I can not set the program or data memory address in
the Nios setup screen to this memory. Why not?

Also: How does Nios determine where a malloc() will allocate memory
from? Let's say I have GERMS on-chip in M4Ks, I have an on-chip
MegaRAM slave, and an SRAM controller. I want Nios to malloc() big
chunks from the SRAM. How do I set this up? Presently, a malloc of
even 4 bytes returns NULL for some reason.

PS. I can successully run Nios code from the SRAM.