Patent GB-1132938-A (GB)
Title: Read only memories
IPC: G11C29/00
Filed: 1967-09-14
Abstract: 1,132,938. Read-only stores. INTERNATIONAL BUSINESS MACHINES CORP. 14 Sept., 1967 [17 Feb., 1967], No. 41884/67. Heading G4A. [Also in Division H4] The invention is a read-only store holding a plurality of word lists each containing check words in positions corresponding to the positions of words set in error in an associated list; an error word list and an error word address list; and, claimed separately, means for reading out the store to obtain the correct data. Type of store.-The read-only store is of the Lippmann photographic film type in which a bit is stored by establishing a standing wave of particular wavelength light in the film to store the bit 1 and not doing so to store the bit 0, then developing the film to produce reflecting layers in the film which will coherently reflect light of the same wavelength for read out but not light of other wavelengths. The store is in two sections M-0 and M-1 (Fig. 2) each consisting of 8 strips 0-7 and 0*-7* comprising a number of cells each. A multi-bit word is stored in one cell by using different wavelengths for the different bits. White light is used for read-out, the light reflected from store sections M-0 and M-1 being separated into component wavelengths by prisms 50, 46 respectively for reception by arrays of photo-cells 100, 200. Handling recording errors.-The strips are recorded on in the order 0, 0<SP>*</SP>, 1, 1<SP>*</SP>, 2, 2* . . . If a cell is recorded on incorrectly in any strip up to and including strip 6, the corresponding cell of the respective next strip is left blank, the word which should have been recorded in the cell left blank being recorded in the next cell instead (the strips are not normally filled, so as to provide room for this). The words which were incorrectly recorded in strips 0-6 are recorded in strip 6 <SP>*</SP> in the order of their addresses (strip and cell), each word being recorded in each of 3 cells which will be adjacent apart from the interpolation of blank cells for errors in strip 6. Each cell of strip 7 stores the address at which the word in the corresponding cell of strip 6<SP>*</SP> was originally incorrectly recorded and the number of the first cell in strip 6<SP>*</SP> where the word is correctly recorded (there might have been errors in recording strip 6<SP>*</SP>), except that if the corresponding cell of strip 6<SP>*</SP> was incorrectly recorded the cell of strip 7 is left blank. Each cell of strip 7<SP>*</SP> holds a blank if the corresponding cell of strip 7 was incorrectly recorded but otherwise holds a single colour (not representing data). This single colour (also used at the bottoms of other strips) is provided to permit the operation as below to be correct. For read-out, when a required cell is read the corresponding cell of the next strip is read at the same time and if neither is blank the contents of the required cell are passed to output. However, if either is blank the required address is compared with the address in the centre cell of strip 7 and depending on whether the former is larger or smaller the read-out position undergoes a " major step " to a position a quarter of the strip forward or backward for a further comparison, and so on, each major step being half the previous, until equality is obtained when the cell number stored in the cell of strip 7 thus obtained is used to obtain the required data from strip 6<SP>*</SP>. A count of the major steps is kept by a decrementable " scan register " and if it reaches 17 an auxiliary store (which may be erasable) is accessed to obtain the data since it was never correctly recorded in the read-only store. The major steps are performed by inverting one or two bits (selected by the scan register) of the address being used. " Minor steps " are also used to move from blank cells in strip 7 reached by major steps. Details of optical read-out arrangements (Fig. 2).-Deflectors 10, 20 receive horizontallypolarized light at the left and by means of birefringent crystals, interspersed with electrooptic switches selectively energizable to rotate the plane of polarization by 90 degrees, displace the beam in two dimensions. The resulting beam at 70 is used to read out a storage cell (selected by deflector 10) from one of the strips (selected by deflector 20) of one of two store sections M-0 and M-1 (selected by signal X 20 ), the strips of M-0 being labelled 0-7 and those of M-1 being labelled 0*-7*. If the selected cell is in section M-1, the same numbered cell of the next-higher-numbered strip of section M-0 is also read out, whereas if the selected cell is in section M-0 the same-numbered cell of the same-numbered strip of section M-1 is read out also. Fig. 2 shows the light paths for a situation of the second type. The beam at 70 will be horizontally or vertically polarized according as an even or odd number of electro-optic switches were operated in deflectors 10, 20. A modulo-2 adder 62 produces a 1 or 0 output according as the number is odd or even and, since X 20  is 1, one but not both of two quarter-wave electro-optic switches 32, 34 will be actuated to circularly polarize the beam whereupon a birefringent crystal 36 passes the horizontal component H undeflected and the vertical component V deflected. Quarter-wave electro-optic switch 38 is not energized so has no effect on the light, the horizontal component of which is reflected at boundary 40 of beam splitter 39 through a quarter-wave plate 42 which circularly polarizes it to read out store section M-1, the reflected light returning via plate 42 which converts it to vertically-polarized form so it passes through boundary 40 of beam-splitter 39 and is deflected by Wollaston prism 43 for its spectrum to be produced on photo-cell array 200 by prism 46. The vertical component V from birefringent crystal 36 passes straight through the beam-splitter 39 to be circularly polarized by quarter-wave plate 41 to read out store section M-0, the reflected light being horizontally-polarized by plate 41 so that it is reflected at boundary 40 and then deflected to prism 50 and photo-cells 100 by Wollaston prism 43. In a situation of the first type, i.e. same-numbered strips (and cells) to be read from both store sections M-0, M-1, X 2o  equals 0 and the light input to birefringent crystal 36 is made horizontally-polarized by energizing both quarter-wave electro-optic switches 32, 34 (under control of adder 62) if the light is not already horizontally-polarized at 70. The light passes through crystal 36 without change and is converted to circularly-polarized form in electro-optic switch 38. The horizontal and vertical components of this light are reflected and transmitted at boundary 40 to read out store sections M-1 and M-0 respectively as before.