are entry-level and mid-range servers that succeeded the entry-level E-class servers and the mid-range G-, H-, I-class servers. The first models were introduced in late January 1996, consisting of the Model D200, D210, D250, D310 and D350. The Model D200 is a uniprocessor with a 75 MHz PA-7100LC microprocessor, support for up to 512 MB of memory and five EISA/HP-HSC slots. The Model D210 is similar, but it used a 100 MHz PA-7100LC. The Model D250 is dual-processor model and it used the 100 MHz PA-7100LC. It supported up to 768 MB of memory and had five EISA/HP-HSC slots. The Model D310 is a uniprocessor with a 100 MHz PA-7100LC, up to 512 MB of memory and eight EISA/HP-HSC slots. The Model D350 is a high-end D-class system, a dual-processor, it had two 100 MHz PA-7100LCs, up to 768 MB of memory and eight EISA/HP-HSC slots. In mid-September 1996, two new D-class servers were introduced to utilize the new 64-bit PA-8000 microprocessor, the Model D270 uniprocessor and the Model D370 dual-processor. Both were positioned as entry-level servers. They used the 160 MHz PA-8000 and supported 128 MB to 1.5 GB of memory. In January 1997, the low-end Model D220, D230, D320 and D330 were introduced, using 132 and 160 MHz versions of the PA-7300LC microprocessor. The D-class are tower servers with up to two microprocessors and are architecturally similar to the K-class. They sometimes masquerade as larger machines as HP shipped them mounted vertically inside a large cabinet containing a power supply and multiple disks with plenty of room for air to circulate. R-class The R-class is simply a D-class machine packaged in a rack-mount chassis. Unlike the D-class systems, it does not support hot-pluggable disks. N-class The N-class is a 10U rackmount server with up to eight CPUs and 12 PCI slots. It uses two Merced buses, one for every four processor slots. It is not a NUMA machine, having equal access to all memory slots. The I/O is unequal though; having one Ike IOMMU per bus means that one set of CPUs are closer to one set of I/O slots than the other. The N-class servers were marketed as "Itanium-ready", although when the Itanium shipped, no Itanium upgrade was made available for the N class. The N class did benefit from using the Merced bus, bridging the PA-8x00 microprocessors to it via a special adapter called DEW. The