1. A method for clearing address translation buffer entries of an address translation buffer in a computer system, the computer system comprising a plurality of page tables and a hierarchy of one or more translation tables, the hierarchy of one or more translation tables comprising a segment table having entries for locating one of said plurality of page tables, the method comprising the steps of: the computer system performing an address translation of a virtual address to a memory address, the performing address translation step comprising selectively accessing a page table and the hierarchy of one or more translation tables; the computer system saving translation information in said address translation buffer entries, wherein the translation information includes information for locating a specified translation table of said hierarchy of translation tables; the computer system determining from an opcode of a machine executable instruction to be executed that the instruction is configured to execute a clear operation; and executing the instruction, the executing step comprising: obtaining a specified translation table origin address of said specified translation table; determining each to-be-cleared entry of the address translation buffer entries, wherein said each to-be-cleared entry is characterised as comprising saved translation information associating said to-be-cleared entry with said obtained specified translation table origin address of said specified translation table; and selectively clearing said each determined to-be-cleared entries of the address translation buffer entries from the address translation buffer.