" When all your code and data together cannot be fit entirely within 256KB, you might need to use a "large" model. In this model, the PPE reserves chunks of effective address space for use by the SPE program, which accesses them through DMA. Memory mapping into effective address space is set up to give the SPE a secondary memory store which it can access using the DMA engine (MFC).
You can use this model in many ways. One is the streaming model: load a regular chunk of sequential data, modify it, write it back to main memory, and repeat. Another is to use the local store as a temporary buffer, copying random data back and forth as needed.
In some cases, the same techniques could be used for code, not just data; the primary code loaded on the SPE could use overlay segments located in main memory as-needed. The compiler can generate automatic overlaying code to handle this case. CESOF, and the toolchains in use, allow SPE code to refer to objects defined in the effective address space.
The simple LS-resident multitasking possible on a single SPE becomes somewhat more flexible when combined with automatic loading and storing of job code and data; a small kernel running on the SPE can load tasks, or swap them out when blocked or completed, allowing the SPE to manage multiple tasks from a job queue. Once again, this is still not preemptive."
