Yesterday's refreshes of the iMac and MacBook Pro were highlighted by the switch from Intel sixth-generation Skylake Core processors to seventh-generation Kaby Lake CPUs, but models with discrete graphics cards also get a boost to AMD Radeon Pro 500-series graphics chips. Let's have a look at the details of these refreshed Polaris parts.
Most MacBook Pro machines come with Intel IGPs, but higher-end 15" units come with AMD Radeon Radeon Pro 555 or Radeon Pro 560 graphics chips onboard. Both chips are likely based off the Polaris 21 chip inside the desktop Radeon HD 560. For reference, that chip has 1024 stream processors at a 1275 MHz boost clock.
The Radeon Pro 555 packs 1.3 TFLOPS of peak compute power courtesy of 768 SPs packed into 12 compute units. Those figures would imply a boost clock of 850 MHz. The bigger Radeon Pro 560 has 1024 SPs in an array of 16 CUs, offering up a total of 1.9 TFLOPS, indicating that the boost clock is presumably set at 925 MHz. Both graphics options list 81 GB/s of memory bandwidth, which works out to 5 GT/s GDDR5 memory on a 128-bit-wide memory bus. The Radeon Pro 555 comes with 2 GB of its own memory, while the Radeon Pro 560 doubles that figure at 4 GB.
Updated iMacs are available with three new AMD graphics chip options: the Radeon Pro 570, Radeon Pro 575, and Radeon Pro 580. All three appear to be built around the same Polaris 20 chip found in the desktop Radeon RX 570 and Radeon RX 580 cards. Memory bandwidth is set at 217 GB/s for the whole trio, a measure that works out to 6.8 GT/s GDDR5 on a 256-bit bus. For reference, the standard Radeon RX 570 packs 2048 SPs boosting to 1244 MHz and the Radeon RX 580 has 2304 at a 1340 MHz boost clock.
The Radeon Pro 570 has been knocked down to 1792 SPs (in 28 CUs) offering up to 3.6 TFLOPs, which should mean a 1.0 GHz base clock according to a little napkin math. The Radeon Pro RX 575 delivers up to 4.5 TFLOPS from its 2048 SPs or 32 CUs, suggesting a 1100 MHz boost clock. Both those chips get 4 GB of RAM. Lastly, the Radeon Pro 580 has 2304 SPs (thus 36 CUs) cranking out up to 5.5 TFLOPS, indicating a 1.2 GHz boost clock. Much like its desktop counterpart, the Pro 580 has 8 GB of RAM on tap.
The 768 SP configuration in the Radeon Pro 555 is not entirely novel. The same figure was found in the Radeon RX 455, though the 455 was built around older Bonaire silicon. Similarly, the 1792 SP layout in the Radeon Pro 575 has also been seen before in the Polaris 10 Radeon HD 470D sold exclusively in East Asian markets. All Radeon Pro 500-series chips appear to be Apple-exclusive, at least for now.