Less than a year after the SD Express standard was
announced, the fast PCIe-based technology has come to the tiny microSD form
factor. It will use the second row of pins of UHS-II style cards to enable NVMe
v1.3 connectivity.
microSD Express cards will deliver speeds of up to
985MB/s. For comparison, the fastest UHS-I cards go up to just over 100MB/s,
the fastest UHS-II push that to a bit more than 300MB/s.
With support for the low-power sub-states (L1.1 and
L1.2) of PCIe v3.1, the new cards will actually use less power than classic
microSDs, despite being much faster than them.
The Express standard is great at multitasking, so
there will be no performance hit to moving apps to external storage. To achieve
this, the microSD Express cards borrow a few tricks from PC SSDs.
- Multi Queue Support: each CPU core can store commands for the card in the host RAM, the cards will read and execute them
- Host Memory Buffer: this is a feature from SSDs without on-board DRAM that allows them to use the host RAM for cache
- Bus Mastering: this feature enables the ones above, by letting the card to access the host RAM (DMA); actually, this has other uses as it allows things other than the CPU (e.g. modem, camera, etc.) to send data to the storage device
Now for some bad news – as we said, the microSD
Express standard requires the extra UHS-II pins. And we are yet to see a phone
that supports UHS-II cards. Still, watch out for microSD cards with the
“Express” logo in the near future. Maybe phone makers will finally have a
reason to upgrade their microSD slots (the ones that still have them, that is).
