This document describes the Ampex magnetic tape system which is attached to Atlas, and its modes of use by the computer.
Provision has been left for attaching other tape mechanisms for special purposes; these are described elsewhere.
The tape mechanism is the Ampex TM 2 (improved FR 300) using one inch wide magnetic tape. There are 16 tracks across the tape, used as follows:
12 information tracks 2 clock tracks 1 block marker track 1 reference marker track (for Tape Addressing only)
Each clock digit is associated with a set of 6 information digits and the clock tracks are in the middle of the six associated information tracks as shown in. figure 1.
Information is stored on tape in blocks or sections of 512 48-bit words, followed by a 24-bit checksum. Each section is preceded by a leading block marker and a section address, and terminated by a trailing block marker and a zero address. Tapes are tested and pre-addressed by a special run on the machine before they are put into use, and the fixed position of addresses permits selective overwriting of sections. Moreover if at any time unusable sections of tape are discovered, these can be avoided by re-addressing the tape ignoring these sections. The pre-addressing and re-addressing operations are performed by built-in Supervisor routines, these being initiated by an operator request. Checksums are of 24 bits with end-around carry: they are used to check the accuracy of all reading and writing operations.
A 48-bit word is represented by four lateral stripes of 12 information bits, and a checksum by two stripes. Each 512 word section of information contains 2050 stripes and has an average length of 5.46 inches, with a gap of 2.3 inches between sections. Tapes are 3600 feet long and hold some 5000 sections or 2½ million 48-bit words.
The normal tape speed is about 120 inches per second and there are 375 binary digits per inch on each track. This gives an instantaneous transfer rate of 90,000 6-bit characters per second, or one 48-bit Atlas word every 89 microseconds. Allowing for the gaps between sections the effective transfer rate is about 64,000 characters per second. This is equivalent to one 512-word section every 64 milliseconds, or one word every 123 microseconds. There are also fast wind and rewind operations at about 180 inches per second, and these are used for long searches along the tape.
There are independent write and read heads separated by a gap of about 0.39 inches. When not operating the tape stops with the read head roughly mid-way between sections, ready to read the next section address. It is possible to read when the tape is moving either in the forward or reverse direction, but writing is only possible when the tape is moving forwards. All writing operations are checked by the read head.
Atlas may control a maximum of 32 magnetic tape mechanisms. Each mechanism is connected via one of the eight channels which can operate simultaneously, each controlling one read, write or short search operation. Wind, rewind and long search operations are autonomous and only need a channel to initiate and, if required, terminate them.
If only eight tape mechanisms are required for an Atlas installation then these can be connected directly to the eight channels. If more than eight mechanisms are required, at least some of them must be connected to the channels via tape switching units. A tape switching unit is connected to a pair of channels and may have up to eight mechanisms attached to it, each capable of being switched to either of the channels under supervisor control.
Transfer operations (read or write) are performed directly between the Tape Coordinator and the Core Store Coordinator and thus the execution of programs is not interrupted by them. The only way in which the execution of programs is affected during transfers is when a program and the tapes require access to the core store at tne same instant. In such cases the program hesitates momentarily.
The transfers are effected via a buffer containing two 48-bit words for each channel so that a core store access is required each time a word is transferred. The Core Store Coordinator has a priority system for interweaving core store accesses from the central computer (for the current program) and from the Tape and Drum Coordinators. The slowing down of the current program due to requests made by the drums and tapes for core store accesses depends in a complex way on the exact relative timing of these requests, and on whether they are for the same core store stacks. However it is estimated that the average slowing down of a program due to simultaneous transfers via all eight tape channels is about 12% (1.5% per channel).
A program is held up if it attempts to read from or write to a block of store which is involved in a magnetic tape transfer. The Supervisor program may then enter another program until the transfer is completed.
If a magnetic tape instruction cannot be obeyed immediately it is placed in a queue by the Supervisor. Only if the queue is already full is the program held up.
A write permit ring must be fitted to a reel of tape before that reel can be written on. Tapes containing permanent information will not have such a ring.
A write inhibit switch is also provided on each mechanism which the operator can use to isolate the tape. It is only possible to write on a tape when the write permit ring is on and the write current is switched on.
Tapes will be tested when they are addressed and faulty sections will be omitted. It will be possible to re-address tapes if particular sections give trouble subsequently.
Checksums will detect faulty recording or reading. The operation will then be repeated under the control of a fixed store program.
The address of the tape section is checked before all reading and writing operations.
The interlocks in the deck make it very unlikely that any stoppage of the mechanism will occur which could damage tape.
When a magnetic tape has useful information on it a descriptive title of the information is stored in section 0 of the tape. Before using such a tape a program or its Job Description must specify the correct name and allocate a number B (0 ≤ B ≤ 100) to that tape. Subsequent instructions refer to tape B of that program: normally B is written in the Ba digits of an instruction.
Tape numbers greater than 100 are reserved for special uses by subroutines.
The word Free, used as a title, indicates that the tape described has no useful information on it and is available for general use.Only the Supervisor can read or write block 0 of a tape.
Magnetic tapes on Atlas can be used in three different ways:-
The Supervisor uses magnetic tapes for general buffering of input and output for all programs, for the storage of compilers and other generally required routines,,and for general organizational purposes. For all these purposes the Supervisor normally requires three magnetic mechanisms, but it is arranged that, should any particular object program require more mechanisms simultaneously than would otherwise be available, the Supervisor can manage with two, one or no magnetic tapes, although at successively reduced efficiency and power. Tapes used by the Supervisor are called System Tapes.
Sets of programs can be loaded on to a magnetic tape by a satellite computer and the tape later mounted on the Atlas for the programs to be executed. Programs are unloaded and executed in order under Supervisor control. The Supervisor's scheduler routine reads the Job Description of a program on the satellite tape and schedules the program for execution in the same way as all the other programs fed in from the normal input peripherals. When the program is selected for execution by the Scheduler, it is read into the computer, its execution is initiated, and then the Job Description of the next program on the satellite tape is read and scheduled. These tapes are called Satellite Input Tapes.
The Supervisor arranges that the output from these satellite programs is fed to a Satellite Output Tape. This tape can then be transported to the satellite computer, for the printing or processing of the output.
Object programs may require magnetic tapes either simply as temporary working space to supplement the core and drum stores during program execution, or as a means of storing large quantities of information for use either by other programs or later runs of the same program, or as a means of transporting information between Atlas and a recording or processing device or another computer, or for the storage of the program itself. These tapes are called Private Tapes.
Tapes are mounted and dismounted under Supervisor Control. The Supervisor sends messages to a Magnetic Tape Operator via an on-line teleprinter.
The request for a Private Tape is made in the first instance either in the Job Description of that program or by an extracode during the execution of the program. In the former case the request for the mounting of the required tape is only given to the Magnetic Tape Operator when the Supervisor's Scheduler routine decides that the program's execution should be initiated shortly (which implies, amongst other things, that a mechanism is available or becoming available). In the latter case the request is passed immediately if a mechanism is available; otherwise the program is suspended automatically by the supervisor until a mechanism becomes available when it is automatically resumed. The dismounting of Private Tapes is initiated either by an extracode or by the Supervisor subsequent upon the ending of a program.
The mounting of Satellite Input Tapes is initiated by means of a request to the Supervisor from the Chief Operator. The Supervisor passes the request to the Magnetic Tape Operator as soon as a tape mechanism becomes free. The mounting and dismounting of Satellite Output Tapes is initiated automatically by the Supervisor when required.
The mounting of System Tapes is initiated either entirely within the Supervisor or, should an object program require information from a System Tape, by a request in a Job Description or by an extracode.
The basic tape operations transfer information between tape sections and 512-word blocks of the main store. The tape instructions available to the programmer are extracodes utilizing these operations. The programmer can use either block transfer or variable-length transfer extracodes. The block transfer extracodes transfer directly 512-word blocks of information between the programmer's program area of the main store and the private magnetic tape. The variable-length transfer extracodes permit, as far as the programmer is concerned, the transfer of pieces of information of arbitrary length between the program area and the private tapes, although in fact the programmer has to provide a buffer in the program area and thenceforward the extracodes perform transfers between the working area and the buffer, and the supervisor arranges transfers between the buffer and the private tapes in 512-word blocks when appropriate. Greater inherent efficiency is achieved by using the block transfer extracodes, and thus if the information to be stored on magnetic tape can conveniently broken up into units of one block, these extracodes are to be preferred. However for many purposes the variable-length transfers are much more suitable and convenient.
Details of the magnetic tape extracodes for private tapes are given in section 6.1 of GS 309A, Extracode Functions. Also in OS 309A (section 6.4.2) are details of the extracodes "1143, 4 and 7, which are used to obtain information from a System Tape.
