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 (Annotated)) 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