linux/Documentation/sh/register-banks.txt
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   1        Notes on register bank usage in the kernel
   2        ==========================================
   3
   4Introduction
   5------------
   6
   7The SH-3 and SH-4 CPU families traditionally include a single partial register
   8bank (selected by SR.RB, only r0 ... r7 are banked), whereas other families
   9may have more full-featured banking or simply no such capabilities at all.
  10
  11SR.RB banking
  12-------------
  13
  14In the case of this type of banking, banked registers are mapped directly to
  15r0 ... r7 if SR.RB is set to the bank we are interested in, otherwise ldc/stc
  16can still be used to reference the banked registers (as r0_bank ... r7_bank)
  17when in the context of another bank. The developer must keep the SR.RB value
  18in mind when writing code that utilizes these banked registers, for obvious
  19reasons. Userspace is also not able to poke at the bank1 values, so these can
  20be used rather effectively as scratch registers by the kernel.
  21
  22Presently the kernel uses several of these registers.
  23
  24        - r0_bank, r1_bank (referenced as k0 and k1, used for scratch
  25          registers when doing exception handling).
  26        - r2_bank (used to track the EXPEVT/INTEVT code)
  27                - Used by do_IRQ() and friends for doing irq mapping based off
  28                  of the interrupt exception vector jump table offset
  29        - r6_bank (global interrupt mask)
  30                - The SR.IMASK interrupt handler makes use of this to set the
  31                  interrupt priority level (used by local_irq_enable())
  32        - r7_bank (current)
  33
  34
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