Abstract: The system I/O interface and its data structure are designed to minimize the host CPU utilization in driving an adapter. The interface is also designed to reduce the system interference in processing I/O requests. To eliminate the need of using PIO instructions, the command/status blocks for exchanging messages between the system and the adapter reside in the system memory. The data structure is designed to avoid "share write" entries in order to further minimize the overhead of maintaining each coherency when updating an entry in the cache either concurrently or sequentially by both adapter and system CPU. Further, the data structure of the control and status blocks is resided in the system memory. The system CPU uses STORE instruction to prepare control blocks and LOAD instruction to read from completion status blocks; while the adapter will rely on its DMA engine to move data to/from system memory in accessing control/status blocks.

Abstract: A mixed-endian computer system enhanced to manage I/O DMA without a software DMA performance penalty. A mixed-endian computer system can change endian mode on a task by task basis if necessary. The mixed-endian system, as enhanced, performs one of two well-defined DMA operations based on control bits either in the DMA control register or in a bit vector associated with each page of processor storage. This invention also describes means for treating I/O registers as if they were of the endian of the executing processor, instead of the more typical need to have the register operate in a particular endian.