By Slash Lane

Published: 03:00 PM EST

Apple this week began testing Mac OS X 10.5.6, the next in a running series of updates planned for the Mac OS X 10.5 Leopard operating system, which could prove to be the most extensive yet.

The first test build, which reportedly arrive Thursday, is said to contain a sprawling laundry list of system components in need of evaluation. This list, which totals nearly six dozen, names almost every major component -- in addition to minor components like Calculator -- of the Mac OS X, according to people familiar with the matter.

A tally of fixes accompanying the pre-release is said to be similar in stature. Apple appears to be placing a particular emphasis on core audio technologies in the update, along with iCal.

Also listed among the 50+ code corrections are fixes targeting Safari, ColorSync, AddressBook, Printing, Syncing, iChat, and the Finder.

Earlier this week, Apple began widespread testing of QuickTime 7.6, which should improve handling of 5.1 channel audio, in addition to delivering improvements to AAC, MPEG-1, and Apple Lossless tracks.

In its bare delta form, Mac OS X 10.5.6 is said to weigh in just shy of 350MB. The current build number is believed to be 9G21.

Last week, Apple stepped up its testing efforts behind Mac OS X 10.6 Snow Leopard, providing a handful of its partners with a glimpse at new builds featuring a Finder completely re-written in Cocoa, the beginnings of Microsoft Exchange integration, and a new ImageBoot function.