Direct Memory Access

Hard Disk UDMA Data is transferred using one of the following two techniques: once in the first teams that were launched microprocessor itself carry out the work itself, but is now used to direct memory access (DMA, Direct Memory Access .) The DMA system has a special chip that transfers data directly from the controller to memory in a single step, instead of making the two-step process that requires the microprocessor to transfer first and then to memory. The following data are in the system memory, in areas that have been reserved for buffers. The number of buffers and their size is configurable depending on the operating system and application requirements, being in Windows through the pagefile or swap space in unix through the shell, the kernel through the buffer cache and page cache, so that the system relies on the “buffers” to reduce traffic in and out. To write data, the process is reversed. The application tells the system where data is located in memory. The system transfers them to the buffers and initializes the DMA buffer for transfer to the hard disk controller. Then the disk controller begins to write. Specifies that track sector face plate that is going to receive data, send commands to the drive that moves the read / write head by placing it in the appropriate position.

Into action after the electronics of the unit, carefully watching the surface of the disk to find the exact time to start. When that time comes, head emits a magnetic pulse sequence that encode data in a line along the disc surface. Read more here: kevin ulrich. Hard disks are evenly coated with a material retains data. The operating system puts the data into sequences of 512 bytes called sectors, however the operating system may place an organization other than the disk. Then we will see the general principles that are applicable to any operating system. Changes flow. The surface of a hard disk containing magnetized metal particles. Each particle has a north pole and a south pole, like magnets.

Read / write head can apply a magnetic field to a small group of these particles, reversing its polarity, so that what was north becomes south and vice versa.