Macintosh Quadra 900

The Macintosh Quadra 900 is a personal computer designed, manufactured, and sold by Apple Computer from October 1991 to May 1992. It was introduced alongside the Quadra 700 as the first computers in the Quadra family of Macintosh computers using the Motorola 68040 processor. It is also the first computer from Apple to be housed in an 18.6 inch (47 cm) tall mid-tower form factor, which by 1991 had gained momentum with PC manufacturers as a suitable design for departmental servers. 
The Quadra 900 had a short lifespan; it was discontinued about six months after the first shipments in favor of the more powerful Quadra 950.
Hardware.
The Quadra 900 was more expandable than the Quadra 700 but cost US$7,200. The Quadra 900 could be upgraded to 256 megabytes of RAM, an astronomical amount for the time, when a typical midrange system would come equipped with 2–4 MB. The standard, as-shipped configuration for the 900 was 4 MB. The high RAM and storage capacity, along with the expandability of five NuBus 90 slots and fast 25 MHz processor, made it a very useful computer for scientific or design work. The mid-tower case design features a single 5.25-inch drive bay initially intended to hold a tape backup drive, that was often repurposed to hold a CD-ROM or SyQuest drive during the model's operational lifespan. Featuring 16 slots for 30-pin SIMMs (installed in groups of four), Quadra 900s with full complements of RAM were exceedingly rare due to the high cost (many thousands of dollars) of the SIMMs at the time.
A new three-mode key-lock system on the front panel can be set to Off, On, and Secure. When set to Off, the machine can't be powered up. In Secure mode, the floppy drive and ADB port are disabled, offering a degree of local protection.
System 7.0.1 was included and supports up to Mac OS 8.1. It is the earliest Macintosh model with the ability to run Mac OS 8.
With the Power Macintosh Upgrade Card installed, the Quadra 900 runs at and its name is reported in the System Profiler as the Power Macintosh 900.