Direct Memory Access Controller
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The SH7305 used on the Prizm includes a Direct Memory Access Controller (DMAC), that can be used to perform high-speed transfers between external devices, without stressing the SH4A core. The DMAC on the SH7305 is identical to that on the SH7724, sharing the same base address, 0xFE000820.
The OS uses the DMAC at least for updating the LCD contents with the VRAM contents - see the Bdisp_PutDisp_DD syscall. How to use the DMAC directly is understood, to a certain point: see Non-blocking DMA.