Ada Programming/Attributes/'Ref

Description

’REF denotes the effective address of the first of the storage units allocated to the object. ’REF is not supported for a package, task unit or entry. The two forms for this attribute are X’REF and SYSTEM.ADDRESS’REF(N). Only use X’REF in machine code procedures. Use SYSTEM.ADDRESS’REF(N) anywhere to convert an integer expression to an address.

X’REF

The attribute generates a reference to the entity to which it is applied. In X’REF, X must be either a constant, variable, procedure, function, or label. The attribute returns a value of the type MACHINE_CODE.OPERAND. Only use it to designate an operand in a code-statement. Precede the instruction generated by the code-statement in which the attribute occurs by additional instructions needed to facilitate the reference, such as loading a base register. If the declarative section of the procedure contains pragma IMPLICIT_CODE (OFF), a warning generates if additional code is required.

SYSTEM.ADDRESS’REF(N)

The effect of this attribute is similar to the effect of an unchecked conversion from integer to address. However, use SYSTEM.ADDRESS’REF(N) instead in the following listed circumstances. In these circumstances, N must be static. In SYSTEM.ADDRESS’REF(N), SYSTEM.ADDRESS must be type SYSTEM.ADDRESS. N must be an expression of type UNIVERSAL_INTEGER. The attribute returns a value of type SYSTEM.ADDRESS, which represents the address designated by N.

• With any of the runtime configuration packages: Use of unchecked conversion in an address clause requires the generation of elaboration code, but the configuration packages are not elaborated.
• In any instance where N is greater than INTEGER’LAST: Such values are required in address clauses that reference the upper portion of memory. To use unchecked conversion in these instances requires the expression be given as a negative integer.
• To place an object at an address, use ’REF.

Example

In the following example, the integer_value converts to an address for use in the address representation clause. The form avoids UNCHECKED_CONVERSION and is useful for 32-bit unsigned addresses.

--place an object at an address
for object use at ADDRESS’REF (integer_value)
--to use unsigned addresses
for VECTOR use at SYSTEM.ADDRESS’REF(16#808000d0#);
TOP_OF_MEMORY : SYSTEM.ADDRESS := SYSTEM.ADDRESS’REF(16#FFFFFFFF#);