| As you can see the
PowerVR2DC graphics chip is the heart of the system, in that it contains
specific circuitry for connecting all the devices together. This extra
circuitry is called the System ASIC (Application Specific Integrated Circuit).
Internal Bandwidth
The diagram above shows the bandwidth
between the processors, and their individual dedicated memories. This follows
the segmented memory architectural design. The SMA design allows each major
component (CPU, Graphics, Sound) to do it's 'job' without any data contention
interference from each other.
|
Data Path
|
Bandwidth
|
|
CPU SH-4 to 16 MB Main Memory
|
64-bits x 100 MHz = 800 MB/s
|
|
PVR2DC to 8 MB Graphics Memory
|
64-bits x 100 MHz = 800 MB/s
|
|
ARM7 Sound to Sound Memory
|
16-bits x 66 MHz = 132 MB/s
|
|
Total:
|
1732
MB/s
|
Note also that a game can have textures
stored in main memory and those textures can be moved into graphics memory
at 64-bits x 100 MHz = 800 MB/s, as indicated on the diagram. The Dreamcast
has exceptionally high internal bandwidth, and is even more impressive
when paired with the incredibly bandwidth efficient 'tile' based rendering
of the PowerVR graphics chip!
External Bandwidth
Controller Ports: These ports have
a rated bandwidth of 2 Mbits/s, or 250 KBytes/s. This is 1/6th the speed
of a PC USB (Universal Serial Bus) port, which can transfer data at 12
Mbits/s. Even though it is quite a bit slower then USB, it still more then
fast enough for any data transfer that has to take place between the controllers
and the main unit. A 128 KB VMS memory unit can transfer it's entire memory
contents to the Dreamcasts main memory in half a second.
The port coming off of the SH-4 is
the rear serial port, that has the same performance characteristics as
the controller ports, which is about 2 Mbits/s, or 250 KBytes/s. This port
can be used for networking two DC's together, or attaching external devices
like a keyboard, etc.
Modem Port: This port is attached
to a bus that is rated at 16-bits x 25 MHz = 50 MB/s, and this same bus
is also used to transfer sound data from the GD-ROM to the sound system.
The speed of this port clearly shows that it cannot support any processor
or memory expansion at all, which is contrary to what others have suggested.
It seems quite clear by this diagram that the Dreamcast has no external
ports that can support a processor or memory expanision, as the ports are
not fast enough.
(Source: Joe Musashi by email, Hitachi's
SuperH Japanese website) |
