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There are two schemes for accessing the packet buffer area (PMA) from the CPU: - 2 x 16 bit/word access scheme where two 16-bit half-words per word can be accessed. With this scheme the access can be half-word aligned and the PMA address offset corresponds therefore to the local USB IP address. The size of the PMA SRAM is usually 1024 byte. - 1 x 16 bit/word access scheme where one 16-bit half word per word can be accessed. With this scheme the access can only be word-aligned and the PMA address offset to a half-word is therefore twice the local USB IP address. The size of the PMA SRAM is usually 512 byte. Which access scheme is used depends on the STM32 model.