A method for managing a dynamic address translation DAT table, characterised by the method comprising steps of: determining, by a processor, from an opcode of a machine executable instruction to be executed, that the instruction is a compare and replace DAT table entry instruction, which is used to replace a valid DAT table entry with a new entry, the instruction comprising an R 1 field and an R 2 field, the R 1 and R 2 fields each designating an even-odd pair of general registers R 1 and R 1 +1 and R 2 and R 2 +1, and must designate an even-numbered register, wherein general register R 1 is the even register of the pair and contains a first operand called the compare value, general register R 1 +1 is the odd register of the pair and contains a replacement value, and wherein general registers R 2 and R 2 +1 contain addressing information such as designated-table type and effective index into a table to locate an original second operand from a second operand memory location; and executing, by the processor, the instruction comprising: comparing the first operand and the original second operand; based on the first operand being equal to the original second operand, storing the replacement value in the second operand memory location as a new second operand; and selectively clearing all local Translation Lookaside Buffer entries corresponding to entries formed from DAT using translation table entries based on a translation table entry designated by a value of the original second operand. determining, by a processor, from an opcode of a machine executable instruction to be executed, that the instruction is a compare and replace DAT table entry instruction, which is used to replace a valid DAT table entry with a new entry, the instruction comprising an R 1 field and an R 2 field, the R 1 and R 2 fields each designating an even-odd pair of general registers R 1 and R 1 +1 and R 2 and R 2 +1, and must designate an even-numbered register, wherein general register R 1 is the even register of the pair and contains a first operand called the compare value, general register R 1 +1 is the odd register of the pair and contains a replacement value, and wherein general registers R 2 and R 2 +1 contain addressing information such as designated-table type and effective index into a table to locate an original second operand from a second operand memory location; and executing, by the processor, the instruction comprising: comparing the first operand and the original second operand; based on the first operand being equal to the original second operand, storing the replacement value in the second operand memory location as a new second operand; and selectively clearing all local Translation Lookaside Buffer entries corresponding to entries formed from DAT using translation table entries based on a translation table entry designated by a value of the original second operand. comparing the first operand and the original second operand; based on the first operand being equal to the original second operand, storing the replacement value in the second operand memory location as a new second operand; and selectively clearing all local Translation Lookaside Buffer entries corresponding to entries formed from DAT using translation table entries based on a translation table entry designated by a value of the original second operand.