[otbn] Fix instructions that both read and write x1 The code was writing to the wrong address (one too high). This patch fixes things by defining stack_wr_idx to be the address to which we should write. It's normally equal to stack_wr_ptr_q (one above the top of the stack), but gets decremented by one if we're popping too. The patch also renames stack_rd_ptr to stack_rd_idx so that things of length StackDepthW are called "idx" and things of length StackDepthW+1 are called "ptr": hopefully this will make things a bit easier to understand. Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
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