64-bit data models

The 64-bit architecture has the following advantages:

• A 64-bit application can directly access 4 exabytes( 1 exabyte = 10¹⁸ bytes (or)  2⁶⁰ bytes ) of virtual memory, and the Intel Itanium processor provides a contiguous linear address space.
• 64-bit Linux allows for file sizes up to 4 exabytes, a very significant advantage to servers accessing large databases.

32-bit and 64-bit data models

	ILP32	LP64	LLP64	ILP64
char	8	8	8	8
short	16	16	16	16
int	32	32	32	64
long	32	64	32	64
long long	64	64	64	64
pointer	32	64	64	64

Note:
The disadvantage of the LP64 model is that storing a long into an int may overflow. On the other hand, casting a pointer to a long will work. In the LLP model, the reverse is true.

The difference among the three 64-bit models (LP64, LLP64, and ILP64) lies in the non-pointer data types. When the width of one or more of the C data types changes from one model to another, applications may be affected in various ways. These effects fall into two main categories:

• Size of data objects. The compilers align data types on a natural boundary; in other words, 32-bit data types are aligned on a 32-bit boundary on 64-bit systems, and 64-bit data types are aligned on a 64-bit boundary on 64-bit systems. This means that the size of data objects such as a structure or a union will be different on 32-bit and 64-bit systems.
• Size of fundamental data types. Common assumptions about the relationships between the fundamental data types may no longer be valid in a 64-bit data model. Applications that depend on those relationships will fail when compiled on a 64-bit platform. For example, the assumption sizeof (int) = sizeof (long) = sizeof (pointer) is valid for the ILP32 data model, but not valid for others.

Sample Operating Systems:

Microsoft Win64 (X64/IA64) => LLP64

Most Unix and Unix-like systems (Solaris, Linux, HP-UX Itanium , etc.) => LP64

Fujitsu-owned HAL Computer Systems => ILP64

Endianism

Endianism refers to the way in which data is stored, and defines how bytes are addressed in integral and floating point data types.

Little-endian means that the least significant byte is stored at the lowest memory address and the most significant byte is stored at the highest memory address.

Big-endian means that the most significant byte is stored at the lowest memory address and the least significant byte is stored at the highest memory address.

Table 3 shows a sample layout of a 64-bit long integer.

Table 3. Layout of a 64-bit long int

	Low address							High address
Little endian	Byte 0	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5	Byte 6	Byte 7
Big endian	Byte 7	Byte 6	Byte 5	Byte 4	Byte 3	Byte 2	Byte 1	Byte 0

For example, the 32-bit word 0x12345678 will be laid out on a big endian machine as follows:

Table 4. 0x12345678 on a big-endian system

Memory offset	0	1	2	3
Memory content	0x12	0x34	0x56	0x78

If we view 0x12345678 as two half words, 0x1234 and 0x5678, we would see the following in a big endian machine:

Table 5. 0x12345678 as two half words on a big-endian system

Memory offset	0	2
Memory content	0x1234	0x5678

However, on a little endian machine, the word 0x12345678 will be laid out as follows:

Table 6. 0x12345678 on a little-endian system

Memory offset	0	1	2	3
Memory content	0x78	0x56	0x34	0x12

Similarly, the two half-words 0x1234 and 0x5678 would look like the following:

Table 7. 0x12345678 as two half words on a little-endian system

Memory offset	0	2
Memory content	0x5678	0x1234

The following example illustrates the difference in byte order between big endian and little endian machines.

The C program below will print out "Big endian" when compiled and run on a big endian machine, and "Little endian" when compiled and run on a little endian machine.

Listing 2. Big endian vs. little endian

#include main () { int i = 0x12345678; if (*(char *)&i == 0x12) printf ("Big endian\n"); else if (*(char *)&i == 0x78) printf ("Little endian\n"); }

ref:

Porting Linux Applications to 64bit - http://www.ibm.com/developerworks/library/l-port64/index.html

Migrating to Solaris 64-bit: 32-bit Applications and Data Model - http://developers.sun.com/solaris/articles/solarisupgrade/64bit/Convert.html

Converting 32-bit Applications Into 64-bit Applications: Things to Consider - http://developers.sun.com/solaris/articles/ILP32toLP64Issues.html