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The mem bootstring enables you to discover how much memory boot thinks your system has, and to reconfigure this if necessary. To find out how much memory boot thinks your system has, enter mem=/p.
To define your system as having 12MB of memory starting at 1MB in addition to its 640KB base memory, enter mem=1m+12m or, alternatively, mem=1m-13m You can use this to limit the memory size of your machine artificially -- for example, to test the performance of an application on a machine with a smaller amount of memory. You can only allocate memory as pages aligned on 4KB boundaries.
Memory above 16MB is not addressable by DMA for those peripheral controllers that only support 24-bit addressing. To mark memory above 16MB on a 24MB machine as non-DMAable, enter mem=1m-16m,16m+8m/n.
For more information about the mem bootstring, see the mem(HW) manual page.