 record is ad- functions, it is of minor significance how the function is im- dressed by a pointer. plemented, in hardware and/or software. To obtain software security and ease of handling, the control The heart of the AXE system structure is the function block with system supports this functional structure with: its function units. The function units are located within each func- tion block. Here, the distinction between hardware and software becomes evident. A typical function block consists of a hardware Programme unit, a regional software unit and a central software unit (see Figure 32.7). The interwork between function blocks and function units is carried out by means of software signals. Software signals are a set of data which precisely define the interface between the 'black boxes'. The use of software signals permits design modifications Pointer with minimum overall system impact, that is, without affecting other function blocks. Atypical AXE telephone exchange is built up of 400-500 function blocks. Common Individual 32.8.4 Software modularity Each software unit is programmed independently of all the others. Interaction between software units is by means of software signals and each software unit can only work with its own data. Conse- Figure 32.8 Central software unit structure quently, a programme fault in one software unit can not mutilate 32/14 Telephone exchanges 1. Special microprogrammed machine instructions used to trans- extracted from the reference store and finally the symbolic signal mit software signals between central software units. number is used to obtain the entry address to the task sequence from Protection against data access to data of other software units. In a signal distribution table located in the beginning of the pro- addition, all absolute address calculations are performed auto- gramme. In this way, programme changes made in a function block matically by microprogrammes. do not affect other function blocks as long as the software signal 3. Relocatable central software units;