January 9, 2007

Titan to Implement 64 Bit Addressing Mode

On January 14, 2007, CIT plans to implement 64-bit addressing mode on Titan. This should have no impact on users, and there should be no changes in how programs and applications function.

The Data Center has already been running tests of the new addressing mode on programs and applications such as Wylbur, RAMIS, and Librarian, where potential problems might have occurred. During the testing, no problems arose, and 64-bit has been successfully running on the test system for over a month at this point. Due to the way this support is implemented, there should be no compatibility issues or similar problems stemming from this upgrade.

Titan, by default, will be set to not allow use of this extra 64-bit storage unless the job specifically asks for it and its JCL is set to override the default. This storage can only be used by programs specifically designed to take advantage of it.

Further technical details

For those who require more in depth technical information, an IBM document describing the implementation can be downloaded as a PDF (2.77MB) here. The following is an excerpt from the document:

"As in the 31-bit address space, a virtual "line" marks the 16-megabyte address. The 64-bit address space also includes the virtual line at the 16-megabyte address; additionally, it includes a second virtual line called the bar that marks the 2-gigabyte address. The bar separates storage below the 2-gigabyte address, called below the bar, from storage above the 2-gigabyte address, called above the bar. The area above the bar is intended for data; no programs run above the bar. There is no area above the bar that is common to all address spaces, and no system control blocks exist above the bar. You can set a limit on how much virtual storage above the bar each address space can use. This limit is called the MEMLIMIT."

The initial setting of MEMLIMIT on Titan will be 0 (zero), meaning that this storage cannot be used unless the job specifically asks for it. Also note that this storage can only be used by programs specifically designed to take advantage of it.

Questions?

If you have questions regarding the 64-bit implementation, please contact the NIH Help Desk at 301-496-HELP (301-496-4357), 866-319-4357 (toll free), or 301-496-8294 (TTY).