Reading from Flash - when via ICODE/DCODE?

[Borys Jeleński]

Greetings,

in the system I develop I shall have a task resposible for fetching some amount of data from Flash to RAM. I should mention that the code will be executed from Flash with the instruction cache (I-Cache) enabled. As far I understand (after reading nRF52840 Product spec. 1.0), to avoid unnecessary bus stalls, it would preferrable, that when this task is executing, instructions should be fetched via ICODE bus while the actual data would be accessed by the CPU via DCODE bus. Please correct if I'm wrong about this idea but in any case, I would like to know what determines whether the CPU uses ICODE or DCODE for Flash readback. Do I assume correctly that ICODE is used for fetching from I-Cache while DCODE is utilized for any other Flash read operation?

By the way, if someone would be so kind to explain how I-Cache is populated, specifically which of the buses (ICODE/DCODE) is used to fetch the instruction from Flash in case of a cache miss.

Best regards,

Boris

[bjorn-spockeli]

HI Borys, 

The I-CODE bus is used to fetch instructions(either from the I-Cache or directly from the code memory region) and the D-CODE bus is used for data access in the code memory region, see Bus interfaces in the Cortex-M4 Technical Reference Manual. Hence, its the I-CODE bus will fetch the instruction regardless if there is a cache-hit or cache-miss. The links below are for the ARM Core

Best regards

Bjørn

[Borys Jeleński]

Thank you very much for the comprehensive answer,

Best regard,

Borys