4.1.5 Unused Bits in Passed Data

Whenever data is passed by value between two procedures either in registers, as is the case for the first six input arguments and return values, or in memory, as is the case for arguments after the first six, the bits not used by the data are usually sign extended or zero extended as appropriate.

The table below defines the rules for setting or clearing the unused bits.

Key:

Sign32 means sign extended to 32 bits —— The state of bits <63:32> is unpredictable.
Sign64 means sign extended to 64 bits.
Zero32 means zero extended to 32 bits —— The state of bits <63:32> is unpredictable.
Zero64 means zero extended to 64 bits.
Data32 means data is 32 bits —— The state of bits <63:32> is unpredictable.
2 * Data32 means two single-precision parts of the complex value are stored in memory as independent floating-point values (each handled as Data32).
Data64 means data is 64 bits.
2 * Data64 means two double-precision parts of the complex value are stored in memory as independent floating-point values (each handled as Data64).
Hard means passed in the layout defined by the Alpha System Reference Manual.
2 * Hard means two double-precision parts of the complex value are stored in a pair of registers as independent floating-point values (each handled as Hard).
Nostd means that the state of all high-order bits not occupied by the data is unpredictable across a call or return.

Table 4-2 Unused Bits in Passed Data

Data type	Type designator	Data size (bytes)	Register extension type	Memory extension type
byte logical	BU	1	Zero64	Zero64
word logical	WU	2	Zero64	Zero64
longword logical	LU	4	Sign64	Sign64
quadword logical	QU	8	Data64	Data64
byte integer	B	1	Sign64	Sign64
word integer	W	2	Sign64	Sign64
longword integer	L	4	Sign64	Sign64
quadword integer	Q	8	Data64	Data64
F floating	F	4	Hard	Data32
D floating	D	8	Hard	Data64
G floating	G	8	Hard	Data64
F floating complex	FC	2 * 4	2 * Hard	2 * Data32
D floating complex	DC	2 * 8	2 * Hard	2 * Data64
G floating complex	GC	2 * 8	2 * Hard	2 * Data64
IEEE floating single S	FS	4	Hard	Data32
IEEE floating double T	FT	8	Hard	Data64
IEEE floating extended X	FX	16	N/A	N/A
IEEE floating single S complex	FSC	2 * 4	2 * Hard	2 * Data32
IEEE floating double T complex	FTC	2 * 8	2 * Hard	2 * Data64
IEEE floating extended X complex	FXC	2 * 16	N/A	N/A
Structures	N/A		Nostd	Nostd
Small arrays of 8 bytes or less	N/A	≤≤ 8	Nostd	Nostd
32-bit address	N/A	4	Sign64	Sign64
64-bit address	N/A	8	Data64	Data64

Note Sign64 applied to a longword logical duplicates bit 31 through bits <63:32>, which may cause the 64-bit integer value to appear negative. However, careful use of 32-bit arithmetic and 64-bit logical instructions (but no right shifts) will preserve the 32-bit unsigned nature of the argument.

Because of the varied rules for sign extension of data when passed as arguments it is important that both calling and called routines agree on the data type of each argument. No implicit data type conversions can be assumed between the calling procedure and the called procedure.