Direct Access Storage Devices

Abstract

The direct access storage device, or DASD, as it will be referred to, is a generic name for peripheral computer storages that have the approximate data access characteristic shown in Fig. 7. This characteristic is also compared in the figure with those corresponding to magnetic tape and core. No time scale is implied by this diagram but, as drawn, would be roughly logarithmic. The access time of core memories is independent of the addressing distance between accessions, and is on the order of 1 to 10 microseconds. The access time of magnetic tape is very nearly linear over a single reel of tape, which spans an addressing distance corresponding to approximately 15 million bytes, or 3 to 4 million words. The typical slope of this line is around 10^-5 sec per byte for a 90 KB/S tape drive. The DASD, in contrast has linear portions of its characteristic, which are separated by vertical jumps, where the number and height of these jumps are determined by the mechanical construction of the device. In fact, the DASDs can be further classified according to the number and height of such breaks in their characteristic, as shown in Table 3. It is also necessary, in the case of the DASD, to redefine the notion of distance between addresses, as will be shown after the discussion of mechanical construction.