Investigation of a Lenovo ThinkPad A285 notebook's motherboard flash chip (stores UEFI firmware) revealed that the PSP core itself (as a device) is run before the main CPU and that its firmware bootstrapping process starts just before basic UEFI gets loaded. By using a few hand-written Python-based tools, they found that the off-chip firmware from the SPI ROM contained an application resembling an entire micro operating system. In 2019, a Berlin based security group discovered the off-chip firmware in ordinary UEFI image files (the code that boots up the operating system), which meant that it could be easily analyzed. The PSP contains on-chip firmware which is responsible for verifying the SPI ROM and loading off-chip firmware from it. The PSP itself represents an ARM core with the TrustZone extension which is inserted into the main CPU die as a coprocessor.
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