It was the first Macintosh system software to have cooperative multitasking by default. It is widely considered to be best Mac System for older (non-PowerPC) machines, and there are hobbyist groups surfing the web using System 6 and elderly Macs to run it.
Cooperative multitasking actually made its Macintosh debut under System 2 with a program called Switcher, which allowed the user to launch multiple applications and switch between them. In System 4 (System Software 2.0) a much more seamless approach called MultiFinder was used, but was disabled by default. System 6 was the first Mac OS which included multitasking out of the box, enabled by default.
Multitasking under System 6 was optional — startup could be set to Finder or to "MultiFinder". If MultiFinder were selected, the Finder and its functions continued to run when an application was launched. The MultiFinder environment allowed users to see past the windows of running applications to view Finder icons such as the Trash, or the windows of other applications running in the background.
System 6 with MultiFinder differed from the later System 7 in the following principal ways:
- At the upper right hand corner, where System 7's application menu would later exist, was the icon of the currently running application; clicking on it would cycle through running applications rather than displaying a menu of running applications to choose from. However, a list of running applications was appended to the bottom of the Apple menu, and could be selected there.
- System 6 supported only 24 bits of addressable memory, rather than the full 32 bits, and so could address a maximum of 8 megabytes of RAM. There was no support for virtual memory.
- Under System 6, although you could drag a file or folder out onto the Desktop and the system would then remember that placement, you could not save directly to the Desktop, and there was no invisible Desktop Folder as there was under System 7 and subsequent classic Macintosh operating systems. In file "Open" dialogs, existing files on the Desktop would show up at the root of the drive.
- File "Open", "Save", and "Save As" dialog boxes would rotate through available disks when one used the Tab key. With the debut of System 7, the Tab key was re-purposed to shift the input focus from the file name field to the window showing the contents of the currently active disk or folder. These dialogs did not contain the "New Folder" button of later versions, although third party extensions made that functionality available.
- The Apple Menu was not as extensively editable as under System 7 and later classic Mac OS. The only items that could exist under the Apple Menu were mini-programs called Desk accessories (DAs), which were installed and removed with the aid of a program called Font/DA Mover. The Chooser, the Scrapbook, and the Control Panel counted as Desk Accessories, and only a maximum of 15 could be installed at one time. There was no "Apple Menu Items" folder.
- The Control Panel, as noted, was a DA rather than a folder. The actual items that would be known as Control Panels under System 7, were called control panel devices, or 'CDEVs', and were placed at the root of the System Folder rather than in a dedicated folder. Items with this file type were recognized by the Control Panel desk accessory, which would include them in a scrollable list of controls and settings. System behavior modifiers that did not include a control panel interface — these would be called "Extensions" under System 7 and beyond — were known as 'INITs' and were also placed at the root of the System Folder.
- The Trash, under System 6 and earlier, would empty itself automatically when one quit or, if MultiFinder were not running, when one launched an application. The Trash can changed appearance as a consequence of having contents — it appeared to bulge.
- System 6 did not have aliases; it did not have custom icons and although it supported color for applications, most of the Finder and System-dialog interfaces were in black and white. The Finder did support coloring of icon outlines on a color-capable Mac, and System 6 was the first to use this ability for Finder Labels. The Labels menu did not get displayed on a black and white Macintosh.
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