[dedicated chips]
      |||
[multiport RAM or a bus]
      |
["bridge" '51]--[multiport RAM or bus]==[dedicated chips]
      |
 [multiport RAM]
      |||
[CPU and fastest dedicated chips]

Yes, it's not pretty. But if we want DMA for some chips, we need it.

       [video]->out
         |
[CPU]--[multiport RAM]----[audio]-->out
         |
       [I/O]<---->in/out
        ||
        || bus
        ||
     [other devices]

Yet alternatively: Have some devices memory mapped for the CPU on the CPU-memory data and address bus. DMA possible with a separate chunk of memory (optional).

         [CPU]      [other devices]
           |             ||
        [decoder]---[multiport RAM]
           |
           |
[video]--[multiport RAM]----[audio]-->out
           |
         [I/O]<---->in/out

How was it done in Atari anyway?