 as the programme and data interfaces are maintained. parts of a software unit can be stored in any free area of the The major part, i.e. all central software units, of the programmes programme and data store, each software unit can be handled for AXE are written in a high level language, PLEX (Programming separately, when faulty software units are replaced, or when Language for Telephone Exchanges) that takes the modularity at- new functions are introduced to the system. tributes, function blocks and software signals into account. Each function block in an exchange is assigned a unique function 32.8.5 Hardware modularity block number. Only when the function block number register (one of the programme control registers) contains the function block AXE hardware is composed of magazines, mechanical frames hold- number for a certain block, access can be obtained to the programme ing printed board assemblies, which are housed in cabinets. Hard- and data. ware units can be added, modified or deleted while the AXE system Access to data which is only permitted within the function block is operational. All connections between frames are made by plug in uses a base address method, in which every separate piece of data is cables. The AXE hardware structure is shown in Figure 32.10. given its own base address. The base address is found in the From the smallest component on the printed board assembly up to reference store. This base address contains all the information about the cabinets containing full equipment, a building block philosophy the location of the data in the data store and how addressing is is applied. carried out. In this way address calculations are made in hardware and not in software, which simplifies programming considerably. 32.8.6 By using this base address method, all data in the data store is Technological modularity completely relocatable, that is, it can be placed in or moved to any idle area in the data store. Technological modularity, in turn, permits the adoption of improved The programme for a f