To receive a request from an application, that request must be copied from the application to TDP, so requires TDP virtual storage until sent to the Database. To receive a request's response from the Database, that response must be copied from I/O buffers to TDP virtual storage until returned to the application. Each request and response consists of three pieces: a piece representing work within TDP, a piece representing the request within TDP, and a piece containing the actual request or response data. Every request or response consists of one storage area for each of the first two pieces plus one or more storage areas for the third piece, depending on the size of the data.
All these areas are satisfied from TDP storage cells. Cells are allocated in different sizes to handle different uses. The first piece requires one 256-byte cell, the second one 416-byte cell, the third one or more 992-byte cells.
While the sizes are determined by TDP and cannot be changed, the number of cells for each size are controlled by the customer to balance the work required of TDP against the amount of virtual storage used. If there are insufficient cells, that processing waits until the needed cells are freed by other processing. If there is little other work or not enough cells were configured, that processing can never complete.
The TDP ADD CELLS command is used to add cells of specified sizes; the TDP DISPLAY CELLS command is used to obtain the status of the cells. There is no way to remove cells once added except to restart TDP.