The sake for Concurrency

06/01/2009

The idea of the E-BUS (borrowed from the PDP-11 UNIBUS) makes concurrency almost natural. However, it is not free: one have to plan for it.

I am planning to take advantage of DMA cycles. While the hard drive is transferring bulk data to Main Memory, the CPU could be doing something else, but what? Virtually nothing, because any single thing would require access to memory thus to the bus. The solution is to split the bus into different paths.

This is not new in computer engineering, of course, but I don't want to break the topology that I already have (based on the unified E-BUS). What could I do?

The solution is to keep the E-BUS outside (between units) but not inside (between cards). It occurred to me that the link from cards to E-BUS can be switched. One special card (or maybe a "mother board") can serve as "bus router" making different links between different cards inside the unit and only one of then going to the external E-BUS.

I would have the Main Memory distributed among two cards. While the hard disk is transferring bulk data to one, the CPU is using the other for doing something else.

It's a good idea... I will think about it.

I haven't mentioned it but it is obvious (I think) that I don't want to experiment with Pipe Lines, Prefetch, Cache, and that kind of things. No for now at least. So "concurrency" takes me to somewhere else.

The following diagram, for example, illustrates the idea of an auxiliary bus inside the Unit (unit bus or U-BUS) providing an extra path for concurrent processing.


The E-BUS gives access to the external E-BUS whereas the U-BUS provides only internal inter-cards connectivity. All cards would feature the standard ability to switch between the two buses. For doing that, a configuration register (REG) would allow the CPU to dictate the current path status at run-time.

In the diagram, MEM CARD #1 is connected to the external E-BUS possibly getting bulk data from a hard drive whereas the CPU CARD is using MEM CARD #1 for doing something else.

Here is another way for providing the extra path. Cards do not connect to the E-BUS directly but via a BUS ROUTER. This circuit is under the CPU control.