Control of the NXT I/O Maps is for advanced programmers.
There are about a dozen low-level modules within the NXT firmware to provide basic hardwre functionality support. The RobotC firmware is built on top of these modules. Each module exposes a variety of internal variables via an “I/O Map”. These are documented in the Development Tool Kits provided by LEGO. These functions provide read and write access to these variables.
nxtReadIOMap(sIOMapName, nIoResult, nBuffer, nOffset, nNumbOfBytes);
Reads the contents of the I/O Map for module named sIOMapName at offset nOffset into the variable nBuffer. nNumbOfBytes are read. The result of the operation is stored in nIoResult.
nxtWriteOMap(sIOMapName, nIoResult, nBuffer, nOffset, nNumbOfBytes);
Write into the I/O Map for module named sIOMapName at offset nOffset from the variable nBuffer. nNumbOfBytes are read. The result of the operation is stored in nIoResult.