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this post was submitted on 02 Sep 2023
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Programming
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I think one of the motivations for having separate modes like this, with (some) separate registers for each, is to reduce the time taken to switch contexts between modes. If they didn't have separate registers, the data in the user mode registers would have to be saved somewhere when making a switch into kernel mode, and then copied back again when switching back to user mode.
There are no separate registers, every call to kernel mode takes extra time precisely because it has to save all the caller's registers, then restore them again before returning.
It involves even more registers than what's visible to the user, because the kernel also has to change the ones related to memory and device access permissions.