The SCU(System Control Unit) basically acts as a sort of Memory Management Unit for the various chips and RAM in the system. It also contains a rather nice DSP that can be used to handle complex math and DMA transfers between various RAM and registers.
Indirect DMA notes
The way you set up Indirect DMA is almost the same as regular DMA. You just have to set the DMA mode bit of the DXMD register to 1 instead. And then setup an array storing your transfer count, write address and read address(in that order) and write the address to this array to the DXW register.