Chapter 38 The RW-400-a new polymorphic data system 483

module's storage and may not be in buffer cells addressed by the computer at execution time. The extended addressing and buffer register indexing may be used to materially simplify repetitive data acquisition operations.

The primary function of a Buffer Module is not, however, that of an auxiliary computer storage unit. The drum and tape modules more aptly serve this function in the RW-400 system. A Buffer Module is capable of operating autonomously and of controlling other modules such as Tape Modules, Drum Modules, Peripheral Buffers, Display Buffers, Printers or Plotters. This capability enables the Buffer Modules in a system to perform routine tape searching and data transferral tasks thereby freeing the Computer Modules to do more computing. In its "self-instruction" mode, the buffer executes its own internally stored program in much the same fashion as a computer. The memory of a Buffer Module will therefore be occupied by its own control programs as well as blocks of data which it is holding for transmission to other units. The buffer is used to acquire information from the relatively slower auxiliary storage and communication modules while the computer proceeds at high speed. Blocks of information retrieved in advance of computer need by the buffer may then be rapidly transferred to the computer's own storage or operated upon as they stand in the buffer via the indirect addressing capability of the computer. Another feature of the buffer is its switching capability. Each Buffer Module is composed of two buffer units tied together. A unit function switching feature permits the employment of the two units together in an alternating mode of operation. Continuous information transfer from tape to computer, for example, may be accomplished without stopping the tape unit. A switching instruction executed simultaneously by both units of a Buffer Module causes whatever devices were connected to the first unit to be connected to the second and vice versa.

Now that the functional controlling modules and the module interconnection concept have been discussed, the more conventional auxiliary storage modules available with the system may be described to round out the processing capability of the system.

A Tape Module consists of an altered Ampex FR-300 tape transport plus the necessary power supplies and control circuitry to effect information reading, writing and control. One inch mylar tape is used. Information is written on 16 channels-two of which are clock channels. The remaining 14 channels consist of 13 information bits plus parity. The information reading or recording rate is 15,000 computer words per second. Data may be recorded on tape in variable blocks up to a maximum of 1024 words per block (the size of the storage available to hold the data in a sending or receiving module). Each block is preceded by a block identification which permits selective tape information searching by a Buffer Module. Single blocks imbedded in a tape file of other blocks can be overwritten. A two-stack head permits automatic verification of each block as it is written. Readback parity errors are automatically detected during the writing process. Thus drop-out areas may be determined while the data is still available in a computer or buffer for recording elsewhere.

A description of the RW-400's tape handling capability would not be complete without mentioning the Tape Adapter (TA) module. This is a self-contained unit capable of performing the reading and writing of magnetic tapes in a format acceptable to the IBM 704 and 709 systems. The TA consists of an Ampex FR-300 half-inch digital tape transport, including dual gap head and servo control system; reading, writing and control circuits; and a module housing with its own blower and power supply.