64-bit Photoshop CS4 for Windows only; no 64-bit Mac version until Photoshop CS5

“What does 64-bit computing mean, practically speaking? In a nutshell, it lets an application address very large amounts of memory–specifically, more than 4 gigabytes,” John Nack, Senior Product Manager, Adobe Photoshop, writes on his blog John Nack on Adobe.

Nack writes, “It’s also important to say what 64-bit doesn’t mean. It doesn’t make applications somehow run twice as fast. As Photoshop architect Scott Byer writes, “64-bit applications don’t magically get faster access to memory, or any of the other key things that would help most applications perform better.” In our testing, when an app isn’t using a large data set (one that would otherwise require memory swapping), the speedup due to running in 64-bit mode is around 8-12%.” (more…)