Relocation Operands

Relocation operands are generally useful in three situations:

As support for independent expression of the high and low addresses
In application programs in which the programmer needs precise control over scheduling
In source code written for compiler development

Some macro instructions (for example, lda) require special coordination between the machine-code instructions and the relocation sequences given to the linker. By using the macro instructions, the assembler programmer relies on the assembler to generate the appropriate relocation sequences.

In some instances, the use of macro instructions may be undesirable. For example, a compiler that supports the generation of assembly language files may not want to defer instruction scheduling to the assembler. Such a compiler will want to schedule some or all of the machine-code instructions. To do this, the compiler must have a mechanism for emitting an object file's relocation sequences without using macro instructions. The mechanism for establishing these sequences is the relocation operand. A relocation operand is placed after the normal operand on an assembly language statement, as illustrated below:

opcode operand relocation_operand

The syntax of the relocation_operand is as follows:

![[relocation_type!]] sequence_number

Relocation type

The following table lists relocation_type values:

Relocation type	Windows NT description
braddr	Branch address.
hint	Jump hint.
poffset	Procedure offset.
refhi	High reference. Associated with an ldah instruction, this is normally accompanied by a pair relocation.
Sechi	Similar to refhi. This is for use by TLS.
Reflo	This is associated with a load, store, or lda instruction. The associated instruction may be the target of a pair relocation.
Seclo	Similar to reflo. This is for use by TLS.
Reflong	Longword relative reference. This is used when the address of a symbol is stored by a .long directive.
Refquad	Quadword relative reference. This is used when the address of a symbol is stored by a .quad directive.
Snum	Section number.
soffset	Section offset.

The relocation types must be enclosed within a pair of exclamation points (!) and are not case sensitive. Only refhi, sechi, reflo, and seclo are useful in the Alpha assembler.

Sequence number

The sequence_number value is a positive integer constant with a value range of 1 to 2147483647. The sequence number may be specified in decimal, hex, or octal notations.

The following examples contain relocation operands in the source code:

 # Equivalent C statement:
 # extern __int64 sym1, sym2;
 # sym1 += sym2;

Assembly statements containing macro instructions:

 ldq $1, sym1
 ldq $2, sym2
 addq $1, $2, $3
 stq $3, sym1

Assembly statements containing machine-code instructions requiring relocation operands:

In the Alpha assembler, an ldah instruction using a symbol without a relocation operand will produce a syntax error. By using the relocation operands, an ldah instruction is eliminated.

 ldah $4, sym1!refhi!1      # Load high value for sym1.
 ldah $5, sym2!refhi!2      # Load high value for sym2.
 ldq $1, sym1($4)!reflo!1   # Load sym1. Paired to ldah.
 ldq $2, sym2($5)!reflo!2   # Load sym2. Paired to ldah.
 addq $1, $2, $3
 stq $3, sym1($4)!reflo!1   # Store value in sym1 unpaired.