At WWDC 2019, Apple unveiled its new modular Mac Pro, slated to be released later this fall. Hidden in the second developer beta of macOS Catalina is the return of the Expansion Slot Utility application, which allows users to manage the PCI expansion options of the Mac Pro.

Expansion Slot Utility was originally available on the early Intel-based Mac Pros, but support for the app was dropped with the 2008 revision. This was because the early 2008 model of the Mac Pro did not have a configurable PCIe setup as all lanes had fixed assignments.

With the all-new Mac Pro, however, Apple is bringing the Expansion Slot Utility application back. As first discovered by Steve Moser and Guilherme Rambo on Twitter, the Expansion Slot Utility is hidden in the second developer beta of macOS Catalina.

In the app, Mac Pro users will be able to manually assign bandwidth between pools or enable automatic bandwidth configuration. The latter option will allow the Mac Pro itself to dynamically assign bandwidth between PCI Expansion accessories.

The app will also tell users when their installed PCI cards are not arranged in a configuration that will produce the best performance. In this case, Expansion Slot Utility will offer suggestions on where to move specific cards in order to improve performance.

The 2019 Mac Pro features a total of eight PCI Expansion expansion slots, including double-wide slots, single-wide slots, and a single half-length slot. Here’s what Apple says about the Mac Pro’s expandability:

Two MPX Modules or up to four PCI Express card slots

Each MPX bay provides:

  • x16 gen 3 bandwidth for graphics
  • x8 gen 3 bandwidth for Thunderbolt
  • DisplayPort video routing
  • Up to 500W power for an MPX Module

Alternatively, each MPX bay can support:

  • One full-length, double-wide x16 gen 3 slot and one full-length, double-wide x8 gen 3 slot (MPX bay 1)
  • Or two full-length, double-wide x16 gen 3 slots (MPX bay 2)
  • Up to 300W auxiliary power via two 8-pin connectors

Three full-length PCI Express gen 3 slots

  • One x16 slot; two x8 slots
  • 75W auxiliary power available

One half-length x4 PCI Express gen 3 slot with Apple I/O card installed

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