Busmaster
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Busmaster On a Microchannel, EISA, or PCI machine, the I/O bus presents a full set of 32 address wires to every adapter card. The adapter can then generate a memory address to reference any location in RAM. A Busmaster adapter card has its own Direct Memory Access control chip on the adapter board. That chip can generate a sequence of memory addresses to read data from memory to the adapter, or to write data from the adapter to memory. Since DMA and I/O functions are combined on the same card, they can be tightly coordinated. A Busmaster EISA device can transfer data every other cycle, and PCI Busmaster cards can move data in every I/O cycle. The software on the PC builds a high level request to READ or WRITE an entire buffer of data. It passes the address and size of the buffer to the Busmaster card. Then the PC software does something else. Internally the Busmaster card moves data between itself and the buffer in memory. When the entire buffer has been processed, it generates an interrupt to the CPU indicating that the request is complete. In order to make effective use of a Busmaster adapter you need two things. First, you need to have something else to do while the adapter performs the I/O. Secondly, you need an operating system that will allow I/O to run in the background. Generally Busmaster adapters make sense only on Server machines running a multitasking operating system (Windows NT, OS/2, UNIX, or Netware). |
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