Data Acquisition

Figure 4.3Timing chart of a memory-write / I/O-read DMA cycleFigure 4.4Timing chart of an I/O-write / memory-read DMA cycleYou may know that the XT used DMA to transfer data from the hard disk, while the AT doesnot. The AT’s disk transfer rates are much higher (over 500 kB/sec for a standard AT businterface) than the XT’s. The question arises: how can the AT achieve such high data transferrates without using DMA?The answer is found in new instructions added to the Intel 80286 (and higher) processors.These are the repeat input and output string instructions. On the 8088/8086, repeatinstructions are available for moving and processing strings in memory, but not to memoryadd-resses from I/O addresses (strings are just continuous sequences of bytes). Subsequentprocessors extended this to include I/O locations, from where a data acquisition board’s samplesoriginate.To obtain a series of data samples from a board, the program initializes a counter register,sets up a destination memory address register and executes the repeat input string wordinstruction (REP INSW), giving the I/O address of the board’s data register. The instructionautomatically reads a sample value, stores it at the memory address pointed to by the destinationaddress register, increments this register, decrements the counter and continues trans-