GB1482819A - Addressing device for a programme stored data processing system - Google Patents

Abstract

1482819 Memory addressing INTERNATIONAL STANDARD ELECTRIC CORP 24 June 1975 [28 June 1974] 26711/75 Heading G4A An arrangement for selectively modifying memory addresses involving look up in read only memories is described. A processor supplies a program address having higher H and lower L order parts specifying a table base and a word location within a table in a memory 6 having a read only part for program instructions and a read/write part for data. The processor or a device including a comparator compares the high order part H with the address of the boundary between the read only and read/write parts of memory 6 and produces for each supplied address a signal DA or DA indicating data and an instruction respectively. For an instruction, signal DA enables read only memory 1 which is addressed by the high order part H to read out the corresponding page address in memory 6, which, together with the unchanged low order part L via selector 7, addresses memory 6. For data, signal DA causes read only memories 2 and 3 to be addressed with the high order part H to read out the page address in memory 6 containing the table which contains the desired word and the location in that page of the first word of the table respectively. The output of memory 3 is added at 4 to the low order part L of the address to form the low order part of the address of the required word to be supplied to memory 6, any carry R being added at 5 to the page address read from memory 2 to form the high order part of the address for memory 6. The read only memories 1-3 may be programmable devices which can be altered, e.g. as integrated circuits forming the ROM part of memory 6 are relocated. In a modification program addresses relating to, e.g. programs common to a number of users are supplied directly to memory 6. The address modification, with its attendant delays, is used when the supplied address relates to data or to a program particular to a particular user. In such cases a comparator acting on the high order part H of a supplied address determines whether address modification is required.