Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.
1 Notes on register bank usage in the kernel 2 ========================================== 3 4 Introduction 5 ------------ 6 7 The SH-3 and SH-4 CPU families traditionally include a single partial register 8 bank (selected by SR.RB, only r0 ... r7 are banked), whereas other families 9 may have more full-featured banking or simply no such capabilities at all. 10 11 SR.RB banking 12 ------------- 13 14 In the case of this type of banking, banked registers are mapped directly to 15 r0 ... r7 if SR.RB is set to the bank we are interested in, otherwise ldc/stc 16 can still be used to reference the banked registers (as r0_bank ... r7_bank) 17 when in the context of another bank. The developer must keep the SR.RB value 18 in mind when writing code that utilizes these banked registers, for obvious 19 reasons. Userspace is also not able to poke at the bank1 values, so these can 20 be used rather effectively as scratch registers by the kernel. 21 22 Presently 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)