Processor direct slot

A processor direct slot (PDS) is a slot incorporated into many older Macintosh models that allowed direct access to the signal pins of a CPU, similar to the functionality of a local bus in PCs. This would result in much higher speeds than having to go through a bus layer, such as NuBus, which typically ran at a slower 10 MHz speed.

History
The "SE" expansion slot, introduced in the Macintosh SE in 1987, was the first processor direct slot, interfacing with the Motolola 68000 processor. The L2 cache slot of the Macintosh IIci, introduced in 1989, was another version of the PDS which supported the 68030 processor. These allowed for 3rd party companies, such as DayStar, to develop processor upgrades that did not require the removal of the CPU.

The LC slot began as a PDS for the 68020 / 68030 processors in the Macintosh LC / LC II, but proved to be so popular for Apple's Performa line that later PowerPC-based versions were essentially emulating the 68030 pin signals for the LC slot that they inherited. More recent Macs have such high processor speeds that a PDS would not be practical and instead adopted PCIe.