In many cases, it is desirable to restrict the operation of a computer to known modes. For example, a parent may wish to restrict gaming time while allowing word processing. In another example, a company may wish to limit the use of an expensive peripheral, such as a 3-D printer, only to authorized users. In another example, a pay-per-use computer may have an unlimited use mode when the terms of an associated contract are satisfied and a restricted use mode that only allows input of additional usage time or points when terms of the associated contract are not met.
Direct memory addressing (DMA) allows a peripheral device to directly access memory so data updates and interchange may occur while allowing the computers main processor to continue other operations. However, DMA creates another vulnerability when attempting to restrict computer operation to a core set of functions by creating an opportunity for a peripheral device using DMA to alter or replace a security program running in main memory.
However, because DMA is an integral part of the operation of most computers and even fundamental operations may require the use of DMA-oriented peripherals, it may not be possible to simply turn off DMA when restricted use operation is desired.