The Dynamically Mapped Shared Memory (DSHM) feature enables single instance databases to use very large (over 4GB) main memory on ix86 family systems.
You must set the DEDICATED_MEMORY parameter in order to use DSHM. All dedicated memory is available for DSHM use. You might also want to set the INVLPG_MAX parameter. Though they act independently, DEDICATED_MEMORY and INVLPG_MAX both affect DSHM. The INVLPG_MAX parameter is only used for DSHM.
The DSHM manual pages contain information about DSHM routines. Additional information about configuring and using DSHM, and writing programs to be DSHM-aware, is provided with the dshm package.