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Hello Ted,
The operand_A path and operand_B path to on_chip memory is just a tentative design.
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OK, yes in reality your system will likely have additional components as well and they will all fit into the shared 32-bit address space seen by NIOS and SGDMA. It looks fine.
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(also please cross check that my understanding of the custom instruction is correct, i.e, the way it passes the addresses to the BIGDIGITS IP core)
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Yes I believe you have got it correct.
Good luck!