System for performing DMA byte swapping within each data element in accordance to swapping indication bits within a DMA command

摘要

An apparatus and method for performing byte swapping using a direct memory access (DMA) controller is provided. In a computer system, a DMA controller for a peripheral component is coupled to system memory via a bus. The DMA controller receives a command pointer to initiate a memory access operation. The command pointer specifies the location of the first DMA command in a command list to be executed by the DMA controller. Each DMA command includes an address word giving the starting address and length word indicating the number of data words to be accessed in memory. Because the data stored in memory is double-word aligned, the two least significant bits of the length word are not needed to perform the memory access and are instead used to indicate any byte swapping that is to be performed on the data during the memory access. During a memory access, the DMA controller swaps the bytes in each double- word of data as specified by the two least significant bits of the length word.