Ada Programming/Types/mod

Types mod __TOC__ Description Unsigned integers in Ada have a value range from 0 to some positive number (not necessarily 1 subtracted from some power of 2). They are defined using the mod keyword because they implement a wrap-around arithmetic. mod Modulus where 'First is 0 and 'Last is Modulus - 1. Wrap-around arithmetic means that 'Last + 1 = 0 = 'First, and 'First - 1 = 'Last. Additionally to the normal arithmetic operators, bitwise and, or and xor are defined for the type (see below). The predefined package Interfaces (RM B 2) presents unsigned integers based on powers of 2 type Unsigned_n is mod 2**n; for which also shift and rotate operations are defined. The values of n depend on compiler and target architecture. You can use range to sub-range a modular type: type Byte is mod 256; subtype Half_Byte is Byte range 0 .. 127; But beware: the Modulus of Half_Byte is still 256! Arithmetic with such a type is interesting to say the least. Bitwise Operations Be very careful with bitwise operators and, or, xor, not, when the modulus is not a power of two. An example might exemplify the problem. type Unsigned is mod 2**5; -- modulus 32 X: Unsigned := 2#10110#; -- 22 not X = 2#01001# -- bit reversal: 9 ( = 31 - 22 ) as expected The other operators work similarly. Now take a modulus that is not a power of two. Naive expectations about the results may lead out of the value range. As an example take again the not operator (see the RM for the others): type Unsigned is mod 5; X: Unsigned := 2#001#; -- 1, bit reversal: 2#110# = 6 leads out of range The definition of not is therefore: not X = Unsigned'Last – X -- here: 4 – 1 = 2#011# See also Wikibook Ada Programming Ada Programming/Types Ada Programming/Keywords/mod Ada Reference Manual 4 4 Expressions 3 5 4 Integer Types Mod