The reason why many 8 bit home computers put the memory controller in the graphics chip was to give the graphics subsystem "first dibs" over bitmap data in RAM.

The reason why many 8 bit home computers put the memory controller in the graphics chip was to give the graphics subsystem "first dibs" over bitmap data in RAM. If the microprocessor has its own copy of RAM then the graphics subsystem may never update the display unless the cpu "writes through" all memory write operations (resulting in a significant performance slowdown).