Friday, October 14, 2011

64-Bit Architecture

The increase in size of both the data objects being operated on and physical memory and the need to hide disk and network access times by placing ever more information in the memory-based disk or network cache lead to severe pressure on the addressing ability of 32-bit processors and operating systems, which saturates at 4 GB. Some RISC processors—Alpha and PA from HP, SGI’s, MIPS, IBM’s Power, and Sun’s SPARC) support a much larger address space of 64-bits. Alpha was designed to be 64-bits right from its inception, while the others all began as 32-bit architectures and were extended later in life to 64-bits. In practice, Intel’s IA-32 architecture is limited to 32-bit addressing, although AMD has proposed a backwards-compatible extension to 64-bits. (It should be noted that, as defined, the IA-32 architecture allows addressing much more than 32-bits, but structured as 16,384 segments each of up to 4GB, but that no operating systems exist which make use of this feature). Intel’s new architecture, IA-64, is another architecture designed from inception to support 64-bit addressing. The first systems based on this architecture appeared on the market in the second half of 2001 with the Itanium processor (which had been codenamed Merced). The implementation of Itanium suffered so many delays and problems that many industry observers are of the opinion that the first commercially-attractive IA-64 implementation is Itanium’s follow-on, Itanium 2 (code-named McKinley). Systems based on, Itanium 2 appeared in the market in the second half of 2002.

Applications that can make good use of the vast address space offered by 64-bit addressing are primarily databases (particularly for decision support), with scientific applications and CAD programs also able to benefit. The advantages brought by a 64-bit architecture can be summarized as follow:

» The ability to address, per process and in virtual memory, a collection of objects representing more than 4 GB. This removes the need to manage the memory hierarchy explicitly within the application, and simplifies the ability to take advantage of the continuing increases in main memory size.

» The ability to support directly and efficiently files or filing systems whose size is greater than two to four GB.

» The ability to operate on large files placed directly in virtual memory. With a 64-bit address space, there is sufficient room for a substantial number of very large files to be placed mapped into virtual memory, where software can directly access them with load and store instructions rather than I/O operations and with the processor’s built-in address translation hardware doing all needed address translation. And, finally, the movement of data between memory and disk is handled automatically by the demand-paged virtual memory system itself.

» The ability to manage very large physical memories—larger than 4 GB. Memories of such size are principally used as cache for disk-based data; in particular, the performance of database management software depends in large part on their management of the memory hierarchy, which explains why database software generally takes responsibility for the management of disk caches. A DBMS can normally do better than a vanilla memory hierarchy manager, since the DBMS knows a lot about the data it is manipulating (whether a datum is data or index data, for example) and can act appropriately. Simply placing large amounts of (reused) data in memory provides a performance improvement, because it removes the need for the software to perform file-tomemory address mapping and also reduces I/O traffic.

It should however be noted that some 32-bit architectures (hardware and software system) can address physical memories of more than 4 GB and support file systems of more than 4 GB.

Clearly, to make full use of a 64-bit address space one needs—in addition to an appropriate processor—the necessary software: compilers, operating systems, applications. The vendors of 64-bit RISC systems (Compaq, Digital, HP, SGI, IBM, and Sun) offer such a suite. Some initiatives intended to adapt UNIX to IA-64 have fallen by the wayside. By mid 2004, Linux and Windows were the only operating systems planned for Itanium platforms and offered to system manufacturers, although there are 64-bit versions of AIX, HP-UX (available on both PA and Itanium for HP systems), Linux, Solaris, and Windows.

In the second half of 2001, IBM, with its z900 family of mainframe systems, introduced a 64-bit extension to the S/390 architecture, which had its roots in the S/360 system of the early 60s (with a 24-bit architecture). For servers, 64-bit architecture is a necessity, not a luxury. It acts as a foundation for the systems to support the changing needs of the application space and to take advantage of the opportunities offered by technology.

Source of Information : Elsevier Server Architectures 2005
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