GB1574171A - Disc memory system - Google Patents

Description

(54) DISC MEMORY SYSTEM (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMITED of Abacus House 33 Gutter Lane, London, EC2V 8AH, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a memory system comprising a storage disc storing data information elements in individually addressable sectors of at least two concentroc information tracks in surface thereof, at least one accessing element for accessing said tracks, a drive unit constructed to rotate said disc at a substantially uniform rate to thereby move successive portions of a said track past a said accessing element at a substantially uniform speed, and a control unit having an input to which a control signal path is connected, which control unit is constructed to respond to the application of a sector address signal and a read command signal to said control signal path by causing a said accessing element to access the track which contains the sector to which the sector address signal corresponds, to locate that sector by means of signals read from the disc, and to read data information from that sector.

A known memory system of this kind is disclosed in United States Patent Specification 3348213. This known system operates on the so-called CKD (count-key-data) principle. The information elements (bits which are stored for later processing) are arranged in groups, each group being preceded by additional bits which are of importance only for controlling the selection of the relevant group. As shown in Figure ib of the said Patent Specification, the said additional bits form in succession a "start pulse," which is an interval without specific information but which is provided with a predetermined signal pattern which indicates the beginning of an information group (record) and which serves also, for example, for synchronization purposes, followed by a numeral (COUNT), which consists of an address character and two length indications for an identification or keyword (KEY) and the actual user information (DATA), respectively. The numeral is followed by the last-mentioned two items of data themselves. Furthermore, an initial address (home address) is present once per track. The start pulse can also serve as an address market for announcing the subsequent sector. As far as the lay-out of the groups of bits is concerned, the count information and/or the keyword may be omitted in given circumstances. The informationless interval of the start pulse can indicate the beginning of an information sector and can be used, by detection thereof, for controlling a starting situation for the reading of a sector, the signals written on the disc indicating the lay-out of the sector (soft sectoring).

In an alternative system it can be arranged that all sectors start at fixed angular positions, in such a case there being provided, for example, 24 sectors per track, each of which occupies a fixed angular interval of 15 ; additional indications corresponding to each sector may then be present on the memory disc, i.e. 24 indications which are used for all tracks together and which can be detected, for example optically. In contradistinction to the afore-said known system, this is an organization involving hard sectors. In such a system an additional starting angle indication which can also be detected, for example optically, will also be provided on the disc.

Errors are liable to occur in storage systems of these kinds, even if error correction mechanisms utilizing information codes containing redundant information are used. For example, selection of a predetermined information sector may appear to be impossible, for example because, an error has occured during the writing of the sector indentification information, so that the wrong identification number has been written. As another example synchronization with the starting signal may have been incorrect. As yet another example a component of the system which compares the required identification information with the actual indentification information may fail, for example only for a specific value of said information.

Another form of error is liable to occur when soft sectoring is employed, in that when an information sector is destructively read and subsequently rewritten for later use, a biased positional shift along the information track can occur. If this occurs a number of times, successive sectors may interfere with each other because of partial overlap, so that information is lost. The invention has for an object to enable the occurrence of errors in the selection of a sector to be reduced or, if this is not possible, to enable an indication of the cause of the error to be produced.

The invention provides a memory system comprising a storage disc storing data information elements in individually addressable sectors of at least two concentric information tracks in a surface thereof, which disc is provided with a starting angle indication for all the tracks together and a marking indication for sector, each track comprising the same number of sectors, at least one accessing element for accessing said tracks, a drive unit constructed to rotate said disc at a substantially uniform speed, a control unit having an input to which a control signal path is connected, which control unit is constructed to respond to the application of a sector address signal and a read command signal to said control signal path by causing a said accessing element to access the track which contains the sector to which the sector address signal corresponds, to locate that sector by means of signals read from the disc, and to read data information from that sector, and a detection unit having an input to which said control signal path is coupled, said detection unit comprising a first detector for said starting angle indication, which detector is constructed to produce an output signal each time said starting angle indication is in a predetermined position and hence each time the disc is in a predetermined angular position, a second detector for said marking indications, which detector is constructed to produce an output signal each time a marking indication which corresponds to a sector in a said track being accessed by a said accessing element is in a given angular position, a counter circuit to a count input of which the output of the second detector is coupled and to an acti vation input of which the detection unit input and the output of the first detector are coupled in such a manner that said counter circuit will be activated by the first output signal of the first detector to occur after the application of a sector address signal and a command signal to said control signal path at least if a sector should not have been located by said control unit in the meantime, and an output to which an output of the counter circuit and the output of the first detector are coupled in such manner that an error signal will be generated thereat, after such activation of said counter circuit, when the next output signal is produced by first detector if the count in said counter circuit should then be smaller than the number of sectors per track, at least if a sector should not have been located by said control unit before the occurrence of said next output signal.

The output of the counter circuit and the output of the first detector may be coupled to the detection unit output in such manner that an error signal of a different type will be generated at the detection unit output when said next output signal is produced by the first detector if the count in said counter circuit should then be equal to the number of sectors per track, provided that a sector is not located by said control unit in the interval between the application of the sector address signal and the read command signal to the control signal path and the occurrence of said next output signal. If this is so the two different types of error signal which can be generated and provide additional information to facilitate detection and removal of the error.

Each marking indication may be in the form of information elements stored in the relevant sector. This can give considerable flexibility to the information storage, and can lead to a higher storage density because the sectors need not then all be the same length.

Moreover, the length of an information sector can then be chosen as required, making it possible in certain circumstances to accommodate data information in a single information sector when two sectors would be required otherwise. This can in turn enable access to the information to be faster than it would otherwise be, particularly if the two sectors otherwise required would have to be sectors of two different tracks. It will be appreciated, however, that the length of a variable-length sector is usually still subject to upper and lower limits.

Each sector may comprise, for example independently of each other, a marking indication, sector identification information and data information elements in that order.

The data information elements may follow the sector identification information directly.

An embodiment of the information will be described in detail hereinafter, by way of example, with reference to the accompanying diagrammatic drawings in which Figure 1 shows a computer configuration including the embodiment, and Figure 2 shows an example of how infor mation may be provided in a sector of a track on a storage disc included in the configuration of Figure 1.

In Figure 1 a computer configuration comprises a central data processor 100 which is connected, via a multiple line 101, to a channel control unit 102. The latter comprises sections 103, each of which is reserved for a- respective peripheral apparatus.

These sections contain, for example, specific control words and location indications of a section of a main memory (not shown) which is reserved for the relevant peripheral apparatus. The units 100, 101, the main memory and any other channel control units may be interconnected via an interconnection unit (not shown). A multiple data-BUSline 104 and a control-BUS-line 105 connect the unit 102 to peripheral apparatuses 106, 107 and 108. The peripheral apparatus 106 is, for example, a data communication line. An adaptation unit 109 contains specific control words for controlling the data transfer and also a series-to-parallel convertor if the data communication line is of the single type. The peripheral apparatus 107 is, for example, an array of card readers which are cyclically interrogated by an adaptation unit 110. The peripheral apparatus 108 comprises a disc memory system including one or more discs with a disc surfaces 112, 113 for information storage. The disc surfaces are driven at a uniform rotary speed by a motor 3 by way of a transmission 2 (shown diagrammatically) on the shaft (shafts) 4 of the memory disc or discs. The rotary speed can be maintained substantially constant by way of a feedback mechanism (not shown). The memory disc (surface) 112 furthermore contains a starting angle indication 5 which passes a detection element 123 during each revolution. The angle indi cation may be, for example, a small reflective area on the memory disc, in which case the reflected light from a light source (not shown) is intercepted by the element 123 and gives rise to a signal on the output 7 thereof.

The memory disc 112 may comprise a magnetizable layer. The local value of the magnetization then represents information; this information may be binary, but also trivalent or even analog. The information can alternatively be stored in a different manner, for example in the form of photographic film density (in which case the memory will be a read-only memory) or in the form of magnetic bubbles which move with the disc.

The details of the information storage are immaterial. The output 7 is connected to an input of a tachometer 8 which produces an O.K. signal in the form of a logic "1" on its output 9 if a standard speed, or a speed which deviates therefrom by no more than plus or minus 3% is maintained. This signal can be used in known manner; for example the signal can be transported, v'a a connection (not shown) to the BUS-line 105, so that the absence of the said "1" initiates an alarm condition in the configuration.

In the Figure each of the disc surfaces comprises two information tracks with associated read/write heads 114 . . . 117. In practice a disc surface may contain, for example, 2200 tracks; in that case the track width of, for example, 0.25 mm will be negligibly small relative to the diameter of the disc which is, for example, 30 cm. The tracks are concentrically arranged. Information is stored in each track in a number of sectors thereof, the number of sectors per track being arbitrary, but being the same for all tracks. The sector organization on the disc is not determined by the material properties of the disc; these properties are preferably as uniform as possible. The sensitive parts of the read/write heads 114 . . . 117 move just over the disc surfaces. The heads in this embodiment also comprise read amplifiers. When an information sector is to be read, a register 118 receives, via the information BUS-line 104 and a switching element 133 which is controllable to conduct in either one of the two possible directions, an address signal which is generated by the central processor or the unit 102 on the BUS-line 104; this procedure is co-controlled by a control signal via the control-BUSline 105 and a line 134. The address signal consists of two portions, i.e. a track address portion in the register section 142, which portion is decoded in a decoder 119, and a portion in the register section 124, which portion contains information indicating the particular information sector requested. The output signal of the decoder 119 activates a corresponding one of the four read/write heads 114... 117. A marking indication (not shown) is provided on the disc(s) for each information sector, and a detector 121 for these marking indications is always activated, as is detector 123 for the starting angle indication. The latter can be activated by a fluxed, optically detectable indication on only one of the disc surfaces; provided that all discs rotate in synchronism with each other.

Hereinafter, one of the possible control modes will be considered, i.e. non-destructive reading without writing. This is controlled by a relevant signal, via the control BUS-line 105 and a line 135, from the central data processor 100. This signal activates the decoder 119 to in turn activate one of the heads in the read mode. As each sector passes the activated head a synchronization element 129 receives, via connection elements 130, 131, the information from the activated head in order to determine the interrogation instants of the signal, so that the effect of any deformation can be eliminated.

To this end, the read mechanism may comprise an oscillatory network which is tuned to the bit frequency or a higher harmonic thereof. The interrogated and digitized information is applied to the detector 121 via a switching element 132. This detector is constructed to specifically detect the marking indications of the sectors, for example by responding to a specific pattern of transitions in the magnetisation which does not occur elsewhere in the recorded information When a marking indication is detected, the detector 121 applies, via a line 144, a signal to a gate 143 and also to the switching element 132. As a result, the latter switches the output of element 129 to a register 122 which then receives the sector identification information from the information sector presently passing the activated head. Correspondence between the identification information read into the register 122 and the sector indication information received in the register section 124 from the BUS-line 104 gives rise to an O.K. signal on the output 136 of a comparison device 126. If an adequate interval exists between identification information and data information in the information track then read on the relevant disc surface, control can be effected so that the data information is read only if the OK. signal generated by the comparison device 126 is present. If the O.K. signal does not appear, for example within a predetermined period of time which may be determined by a monostable multivibrator (not shown), the switching element 132 is reset, so that the next information sector which passes the activated read head can again activate, after re-synchronization (because the information sectors are generally not synchronized relative to each other) of the detection instants, the detector 121 for the marking indication.

However, if the comparison device 126 generates an O.K. signal, i.e. if correspondence is detected, a signal is applied, via the line 136 and a signal channel (not shown) in the control BUS-line 105, to the channel control unit 102 in order to request the information BUS-line 104 for an information transport. Generally, the unit 102 will grant this request at the correct instant and will despatch an O.K. signal via the control BUS line and a line 137. This signal activates the switching elements 132, 133 to apply the data information BUS line 104, to the channel control unit 102 and the central- data plocessor 100. At the end of the transport, the lines 134, 135 and 137 receive, via the control BUS line 105, a signal which restores the original operating condition (rest) awaiting a subsequent request by the central data processor 100.

The signal on the line 135, which starts the read operation, furthermore sets an indication flipflop 139 to the state "1". If the O.K. signal appears on the line 136 this signal sets this flipflop to the state zero via an OR-gate 146. This can also be achieved by a further signal from control BUS line 105.

via a line 145, for example should the search have failed. (The lines 145 and 135 are connected to different conductors of the BUS line 105 which are not shown separately).

The state "1" of the flipfllop 139 and the output signal of the detector 123 for the starting angle information together set, via an AND-gate 140, a counter 125 to the zero count state; this signal also sets a second indication flipflop 147 to the state "1". The "1" output signal of the flipflop 147 unblocks the AND-gate 143 to allow passage of counting pulses from the detection element 121. When the output signal of the gate 146 occurs this signal also sets the flipflop 147 to the "0" state again. so that the input of the counter 125 is subsequently blocked by way of the gate 143. The second input of the gate 143 is connected to the output of the detector 121 for the marking indication, via the line 144, so that the counter 125, provided that it is not blocked by a "0"-state of the indication flipflop 147, counts the marking indications. The counter 125 has a capacity which corresponds to the (fixed) number of information sectors per track. This number may lie, for example, between 18 and 32. If the requested information sector cannot be found on the addressed information track, due to an error, there are two possible consequences. The first of these occurs if, upon the second passage of the starting angle indication 5 past the detector 123, all marking indications have been detected at least once and the counter 125 has thus reached its maximum count in such a case the resulting output carry signal on a line 148 generates, in conjunction with the signal on the line 7, an error signal on a line 150 via an AND-gate 149. Furthermore, under the control of the signal on the line 7 and the state "1" of the indication flipflop 147, an AND-gate 151 also supplies an error signal to an output 152, and the combination of these two signals forms a first type of error signal. On the other hand the output carry from the counter 125 may not occur, in which case a second type of error signal appears, i.e. on the line 152 only. (This signal on the line 152 does not occur immediately the flipflop 147 is set to the state "1", because this flipflop has a built-in delay which is longer than the pulse produced by the detection of the marking indication). The error signals on the lines 150, 152 are returned via the control BUS-line 105 (connections not shown), to cause the computer configuration to take steps to restore the information. These steps will be discussed hereinafter.

Figure 2 shows, by way of example, a possible format of an information sector on the memory disc, the said sector comprising fifteen separate sections 4 > 54 in this example. The section 40 which moves first past the read/write head does not contain information, for example because no transitions are present in the magnetization of the disc material; this section acts as the previously discussed sector marking indication and is called the address marker. The section 41 contains 7 octades which all have the value FF, F being a notation for a tetrade which has the decimal value 15 (i.e.

binary 1111). Consequently, the section 41 does not contain information either. (The sections 40 and 41 can alternatively be used together as the marking indication.) After the point 55, the sector identifiction information commences. The section 42 contains a preamble; the two octades thereof have, for example, the values F4 and 4B respectively, B being a notation for the decimal value 11. The octade 4B thus has the value 0 1 0 0 1 0 11. These two octades serve for the synchronization of the read mechanisms with the rotary speed of the memory disc.

The section 43 contains the actual indentification of the sector and consists of five octades (40 bits). The section 44 contains information, associated with the said identification, for the detection and correction of errors therein, and comprises four octades.

The section 45 contains further non-specific information; all three octades thereof have the value FF. At the point 56 the identification of the relevant sector terminates. The section 46 is an interval of 30 octades (240 bits) which all have the value FF and which thus do not contain specific information.

As a result, a specific delay is introduced during which a control device can compare the identification found with a desired identification and, depending on the result of this comparison, can determine whether the subsequent information is to be read or not.

After the point 57, te actual information content of the relevant memory sector commences. The section 47 again contains a preamble and its two octades have, for example, the values F4 and 4F. The sections 48, 49 and 50 together contain, for example, 1024 octades (8192 bits) as the actual information (data) content of the memory sector. Each of these sections contains an information quantity or logical record which may be used separately. These records need not all have the same length and their number may be different from three, for example one or more than ten. The section 51 contains, like the section 44, four octades for the detection and correction of errors in the sections 48, 49, 50. The section 52 corresponds to the section 45 as rewards content and function The information of the relevant memory sector terminates at the point 58. The section 53 is an infonnation- less section and, for a standard value of the rotary speed of the memory disc, it may have a length of, for example, 6% of that of the sections 46 to 52 i.e. for example, 64 octades. This section is provided so that there will still be space for the sector information in the case of a write operation in the relevant memory sector even if the rotary speed of the memory disc is up to 6% to low, so that overflow to the section 54 will never occur. A termination section 54 contains 36 octades which all have the value FF. The period of time, introduced thereby, during.

which no information is read, can be used to decide whether or not the directly subsequent sector of the memory disc will be read. The overall length of the memory sector amounts to 1187 octades, 1024 (86%) of which are available for information storage. The sections 41, 46 and 54 serve, during the time during which these sections pass the read/write head, to allow a decision to be taken as to whether or not the subsequent information will be read (section 46), as a preparation for an address to be received (section 41), for performing an error detection and/or correction operation (sections 46, 54) and as a preparation for a subsequent sector (section 54), respectively. Tlie shaded parts preceding the section 40 and following the section 54 may comprise other memory sectors or specific information which does not relate to a given memory sector, or a void space if, for example, the complete track length does not correspond to an integral number of memory sectors.

In the case of 14, 762 octades per track and 1187 octades per memory sector, for example, there will be a remainder of 518 octades after twelve sectors have been accommodated. If the lengths of the sectors are not fixed, more information bits can therefore be recorded on the track, because the said remainder can be assigned to one or more of the sectors.

In the foregoing description a stationary read/write head was assumed to be assigned to each information track. It is alternatively possible to provide a single read/write head per disc surface, in which case this head must be radially positioned, for example by a step motor, the track address portion in the register section 142 then being used to control the step motor until the head reaches the relevant track. An O.K. signal can be arranged to be generated when this is achieved, this signal then activating, for example the element 129.

Other control modes also exist. For example, information can be written on the disc surfaces: in that case first the correct write head is addressed, or the write head is moved to the correct position, and an era sing element provided in front of the rele vant head is also addressed. After the selection of the correct information sector, the data information present therein is erased and new information is written. The writing operation can be effected upon detection of the desired identification information; the presence of an informationless interval between identification information and data information is also useful in this control mode because the position of the data information relative to the identification information will be subject to tolerances as a result of the properties of the write mechanism. In order that such writing operations can be effected, the line 138 may be bidirectional, or a further signal path may be provided (which need not necessarily pass through the elements 132 and 129 but may be connected directly to the connection elements 130, 131).

It is alternatively possible, both in the case 6f a read operation and in the case of a write operation, to select a sector on the basis of count in the counter 125 (which counts the passing marking indications). In that case a comparison device must be provided for comparing the contents of the register section 124 and of the counter 125. Such a comparison operation can often be effected so quickly (as a result of the comparatively small value of the maximum possible count in counter 125), that the information-less intervals between identification information and data information become superfluous. If this is so the identification information can then always be written together with the data information and be used at a later stage as a verification when the information sector having the correct sequence number has been found, or alternatively the identification information can be completely omitted. It will be obvious that the omission of identification information (which is then superfluous) or the omission of the said information-less intervals saves space on the memory disc, so that its capactiy is increased. In the case of selection on the basis of the count in the counter, the error signals generated will again be of two kinds, corresponding to the cases (a) where the sector giving rise to the desired count has not been found (the comparison device could be defective just for this value), and (b) where a sector giving rise the desired count has been found, but at the end of a complete revolution there is a sisal that an error may have occured because the final count is too low (in the latter case the information sector read may be wrong one) respectively. In this and other cases it is sometimes useful for the line 138 to include an information buffer for the temporarv storage of the sector information until it is known that the standard number of information sectors has been detected on the addressed information track. The central data processor 100 can meanwhile execute other tasks and the information transport BUS line 104 may be active for other peripheral apparatus.

It is possible that only a single disc surface is present and that all sectors start at fixed angular positions, for example the same number of differ regularity in the magnetizable layer; this may be done if the final count is too low.

(h) If the error is localised in a hardware section other than the memory disc, this section (or, for example. the relevant printed circuit board and the electronic components or integrated circuits mounted thereon) are replaced.

The operations can thus be classified in three categories: trying to mitigate the consequences of the error, trying to find the cause of the error, and trying to remedy or avoid further occurrence of the error. The different operations can be performed in many possible combinations, it being obvious that the value of the final count can be made to govern the choice of a given combination, giving a shorter test program than would otherwise be the case. The error signals may be used as address bits for addressing an error correcting routine.

Furthermore, the counting of the marking indications constitutes an additional check as regards the detection mechanism in the read/write head (heads).

WHAT WE CLAIM IS:- 1. A memory system comprising a storage disc storing data information elements in individually addressable sectors of at least two concentric information tracks in a surface thereof, which disc is provided with a starting angle indication for all the tracks together and a marking indication for sector, each track comprising the same number of sectors, at least one accessing element for accessing said tracks, a drive unit constructed to rotate said disc at a substantially uniform rate to thereby move successive portions of said track past a said accessing element at a substantially uniform speed, a control unit having an input to which a control signal path is connected, which control unit is constructed to respond to the application of a sector address signal and a read command signal to said control signal path by causing a said accessing element to access the track which contains the sector to which the sector address signal corresponds, to locate that sector by means of signals read from the disc, and to read data information from that sector, and a defection unit having an input to which said control signal path is coupled, said detection unit comprising a first detector for said starting angle indication, which detector is constructed to produce an output signal each time said starting angle indication is in a predetermined position and hence each time the disc is in a predetermined angular position, a second detector for said marking indications, which detector is constructed to produce an output signal each time a marking indication which corresponds to a sector in a said track being accessed by a said accessing element is in a given angular position, a counter circuit to a count input of which the output of the second detector is coupled and to an activation input of which the detection unit input and the output of the first detector are coupled in such a manner that said counter circuit will be activated by the first output signal of the first detector to occur after the application of a sector address signal and a read command signal to said control signal path at least if a sector should not have been located by said control unit in the meantime, and an output to which an output of the counter circuit and the output of the first detector are coupled in such manner that an error signal will be generated thereat, after such activation of said counter circuit, when the next output signal is produced by the first detector if the count in said counter circuit should then be smaller than the number of sectors per track, at least if a sector should not have been located by said control unit before the occurrence of said next output signal.

2. A system as ,claimed in Claim 1, wherein the output of the counter circuit and the output of the first detector are coupled to the detection unit output in such manner that an error signal of a different type to the error signal of Claim 1 will be generated at the detection unit output when said next output signal is produced by the first detector if the count in said counter circuit should then be equal to the number of sectors per track, provided that a sector is not located by said control unit in the interval between the application of the sector address signal and the read command signal to the control signal path and the occurance of said next output signal.

3. A system as claimed in Claim 1 or Claim 2, wherein each marking indication is in the form of information elements stored in the relevant sector.

4. A system as claimed in Claim 3, wherein each sector comprises a marking indication, sector identification information and data information elements, in that order.

5. A memory system substantially as described herein with reference to the drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. regularity in the magnetizable layer; this may be done if the final count is too low. (h) If the error is localised in a hardware section other than the memory disc, this section (or, for example. the relevant printed circuit board and the electronic components or integrated circuits mounted thereon) are replaced. The operations can thus be classified in three categories: trying to mitigate the consequences of the error, trying to find the cause of the error, and trying to remedy or avoid further occurrence of the error. The different operations can be performed in many possible combinations, it being obvious that the value of the final count can be made to govern the choice of a given combination, giving a shorter test program than would otherwise be the case. The error signals may be used as address bits for addressing an error correcting routine. Furthermore, the counting of the marking indications constitutes an additional check as regards the detection mechanism in the read/write head (heads). WHAT WE CLAIM IS:-

1. A memory system comprising a storage disc storing data information elements in individually addressable sectors of at least two concentric information tracks in a surface thereof, which disc is provided with a starting angle indication for all the tracks together and a marking indication for sector, each track comprising the same number of sectors, at least one accessing element for accessing said tracks, a drive unit constructed to rotate said disc at a substantially uniform rate to thereby move successive portions of said track past a said accessing element at a substantially uniform speed, a control unit having an input to which a control signal path is connected, which control unit is constructed to respond to the application of a sector address signal and a read command signal to said control signal path by causing a said accessing element to access the track which contains the sector to which the sector address signal corresponds, to locate that sector by means of signals read from the disc, and to read data information from that sector, and a defection unit having an input to which said control signal path is coupled, said detection unit comprising a first detector for said starting angle indication, which detector is constructed to produce an output signal each time said starting angle indication is in a predetermined position and hence each time the disc is in a predetermined angular position, a second detector for said marking indications, which detector is constructed to produce an output signal each time a marking indication which corresponds to a sector in a said track being accessed by a said accessing element is in a given angular position, a counter circuit to a count input of which the output of the second detector is coupled and to an activation input of which the detection unit input and the output of the first detector are coupled in such a manner that said counter circuit will be activated by the first output signal of the first detector to occur after the application of a sector address signal and a read command signal to said control signal path at least if a sector should not have been located by said control unit in the meantime, and an output to which an output of the counter circuit and the output of the first detector are coupled in such manner that an error signal will be generated thereat, after such activation of said counter circuit, when the next output signal is produced by the first detector if the count in said counter circuit should then be smaller than the number of sectors per track, at least if a sector should not have been located by said control unit before the occurrence of said next output signal.

2. A system as ,claimed in Claim 1, wherein the output of the counter circuit and the output of the first detector are coupled to the detection unit output in such manner that an error signal of a different type to the error signal of Claim 1 will be generated at the detection unit output when said next output signal is produced by the first detector if the count in said counter circuit should then be equal to the number of sectors per track, provided that a sector is not located by said control unit in the interval between the application of the sector address signal and the read command signal to the control signal path and the occurance of said next output signal.

3. A system as claimed in Claim 1 or Claim 2, wherein each marking indication is in the form of information elements stored in the relevant sector.

4. A system as claimed in Claim 3, wherein each sector comprises a marking indication, sector identification information and data information elements, in that order.

5. A memory system substantially as described herein with reference to the drawing.