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It then instructs the DMA hardware to begin the transfer. When the transfer is complete, the device interrupts the CPU. Scatter-gather or vectored I/O DMA allows the transfer of data to and from multiple memory areas in a single DMA transaction. It is equivalent to the chaining together of multiple simple DMA requests.
The first use of channel I/O was with the IBM 709 [2] vacuum tube mainframe in 1957, whose Model 766 Data Synchronizer was the first channel controller. The 709's transistorized successor, the IBM 7090, [3] had two to eight 6-bit channels (the 7607) and a channel multiplexor (the 7606) which could control up to eight channels.
The Word DMA (WDMA) interface is a method for transferring data between a computer (through an Advanced Technology Attachment (ATA) controller) and an ATA device; it was the fastest method until Ultra Direct Memory Access (UDMA) was implemented.
Some examples of "complex" instructions include: transferring multiple registers to or from memory (especially the stack) at once; moving large blocks of memory (e.g. string copy or DMA transfer) complicated integer and floating-point arithmetic (e.g. square root, or transcendental functions such as logarithm, sine, cosine, etc.)
In computing, bus mastering is a feature supported by many bus architectures that enables a device connected to the bus to initiate direct memory access (DMA) transactions. It is also referred to as first-party DMA , in contrast with third-party DMA where a system DMA controller actually does the transfer.
Memory-mapped I/O is preferred in IA-32 and x86-64 based architectures because the instructions that perform port-based I/O are limited to one register: EAX, AX, and AL are the only registers that data can be moved into or out of, and either a byte-sized immediate value in the instruction or a value in register DX determines which port is the source or destination port of the transfer.
In these cases the I/O systems can access their data memory while the processor is using a different bank. One example is the Zilog Z80, whose M1 line indicates that the processor is reading instructions; if those instructions are in a different bank, or more commonly ROM, the I/O systems can access RAM without interfering with the processor.
In contrast, in direct memory access (DMA) operations, the CPU is uninvolved in the data transfer. The term can refer to either memory-mapped I/O (MMIO) or port-mapped I/O (PMIO). PMIO refers to transfers using a special address space outside of normal memory, usually accessed with dedicated instructions, such as IN and OUT in x86 architectures.