16. A decompression process of executable code by a microprocessor, saved in a program memory zone in compressed form, the process comprising: receiving from the microprocessor instruction requests of executable code comprising an instruction address to be executed, determining a reading address of an addressing table as a function of the instruction address to be executed, reading to the reading address in the addressing table addressing information of compressed words corresponding to the instruction to be executed, determining as a function of the addressing information read a reading address of the program memory, reading the program memory to the reading address, and decompressing the read words and transmitting them to the microprocessor, wherein each word of the executable code is compressed in the form of a part of predefined fixed length and a part of variable length whereof the length is defined by the part of fixed length, all the parts of fixed length and all the parts of variable length of the words of executable code being combined respectively into a block of parts of fixed length and into a block of parts of variable length, at least certain parts of variable length being localized in the block of parts of variable length by means of an addressing table, the decompression process further comprising: determining a reading address in the block of parts of fixed length as a function of the address of the instruction to be executed, reading the blocks of parts of fixed and variable length to the determined reading addresses, and decompressing the read parts of fixed and variable length to produce an executable instruction which is transmitted to the microprocessor, wherein the part of fixed length of each compressed instruction references a sub-table of a decompression table collecting at least part of the words of the executable code, the part of variable length giving the position in the sub-table of the word of executable code, the decompression of a part of fixed length and a part of corresponding variable length comprising: determining the length of the part of variable length as a function of the value of the part of corresponding fixed length, reading the part of corresponding variable length considering the determined length of the part of variable length, and if the part of fixed length references a sub-table of the decompression table, reading the word of executable code in the sub-table of the decompression table referenced by the read part of fixed length, to a position defined by the part of variable length read.
16. A decompression process of executable code by a microprocessor, saved in a program memory zone in compressed form, the process comprising: receiving from the microprocessor instruction requests of executable code comprising an instruction address to be executed, determining a reading address of an addressing table as a function of the instruction address to be executed, reading to the reading address in the addressing table addressing information of compressed words corresponding to the instruction to be executed, determining as a function of the addressing information read a reading address of the program memory, reading the program memory to the reading address, and decompressing the read words and transmitting them to the microprocessor, wherein each word of the executable code is compressed in the form of a part of predefined fixed length and a part of variable length whereof the length is defined by the part of fixed length, all the parts of fixed length and all the parts of variable length of the words of executable code being combined respectively into a block of parts of fixed length and into a block of parts of variable length, at least certain parts of variable length being localized in the block of parts of variable length by means of an addressing table, the decompression process further comprising: determining a reading address in the block of parts of fixed length as a function of the address of the instruction to be executed, reading the blocks of parts of fixed and variable length to the determined reading addresses, and decompressing the read parts of fixed and variable length to produce an executable instruction which is transmitted to the microprocessor, wherein the part of fixed length of each compressed instruction references a sub-table of a decompression table collecting at least part of the words of the executable code, the part of variable length giving the position in the sub-table of the word of executable code, the decompression of a part of fixed length and a part of corresponding variable length comprising: determining the length of the part of variable length as a function of the value of the part of corresponding fixed length, reading the part of corresponding variable length considering the determined length of the part of variable length, and if the part of fixed length references a sub-table of the decompression table, reading the word of executable code in the sub-table of the decompression table referenced by the read part of fixed length, to a position defined by the part of variable length read. 17. The decompression process as claimed in claim 16 , in which: if the part of fixed length indicates that the word of corresponding executable code is a second word of two identical words appearing consecutively in the executable code, the part of fixed length is decompressed by the word of executable code previously decompressed; if the part of fixed length specifies that the word of corresponding executable code does not feature in the decompression table, the process further comprises extraction steps of at least part of the word of executable code of the part of variable length corresponding to the part of fixed length; and if the part of fixed length specifies a stop point, a stop point is inserted into the decompressed executable code.