The X16 has a clever trick in that it has a memory mapped register that points to the next address to write to, and a register that causes anything you write to it to be written to that pointer in VRAM… and the pointer auto increments. So if you are doing a block copy it is as fast to write to VRAM.

One advantage of this scheme is that the CPU is not sharing access to RAM with the video processor so ordinary RAM access is fast.