By Slash Lane

Published: 01:25 PM EST

Adobe's plans to deliver 64-bit Photoshop support as part of the upcoming release of its Creative Suite 4.0 software bundle for Windows PCs, but Mac users will have to wait till version 5.0 to see the same treatment.

This has nothing to do with software maker's commitment to the Mac platform and is instead an unfortunate side affect of Apple's decision to scrap plans for a 64-bit version of its Carbon API set mid-course, said John Nack, Senior Product Manager for Photoshop applications.

Apple has long offered its developers two primary sets of programming interfaces (APIs) for writing Mac OS X applications: "Cocoa," which supports 64-bit development, and the legacy "Carbon" set, which only supports 32-bit. However, with a significant number of existing applications relying on Carbon, Apple at its 2006 developers conference said it had begun work to enable a 64-bit version of the API set.

As such, Adobe's original plan for its Creative Suite applications on the Mac was to add Intel support through the existing Carbon API set with the release of v3.0 and then deliver 64-bit support in v4.0 via the 64-bit Carbon API set, according to Nack.

"At the WWDC show last June, however, Adobe & other developers learned that Apple had decided to stop their Carbon 64 efforts. This means that 64-bit Mac apps need to be written to use Cocoa (as Lightroom is) instead of Carbon," he explained. "This means that we'll need to rewrite large parts of Photoshop and its plug-ins (potentially affecting over a million lines of code) to move it from Carbon to Cocoa."

Nack said Adobe immediately began adjusting its product development plans after learning of the change, but added that no one at the company "has ever ported an application the size of Photoshop from Carbon to Cocoa." Therefore, pushing for 64-bit support by v4.0 was just not feasible.

"It's a drag that the Mac x64 revision will take longer to deliver. We will get there, but not in CS4," he assured Mac users. "Our goal is to ship a 64-bit Mac version with Photoshop CS5, but we’ll be better able to assess that goal as we get farther along in the development process."

In a blog posting, Nack also took a stab at dispelling some myths about the benefits of 64-bit applications, specifically the notion that they instantly perform at twice the speed of 32-bit apps.

In its own tests, Adobe found the average 64-but app to run about 8 to 12 percent faster than a 32-bit one. But the primary advantage of 64-bit applications is their ability to address very large amounts of memory in excess of 4GB.

"This is great for pro photographers with large collections of high-res images," said Neff, who added that opening a 3.75 gigapixel image on a 4-core machine with 32GB RAM is about 10x faster in the 64-bit version of Photoshop currently under development than it is on the existing version.