From f1517494407b1f1ca0063a756cc30d75e96d433c Mon Sep 17 00:00:00 2001 From: Paul Mundt Date: Wed, 27 Sep 2006 16:01:12 +0900 Subject: sh: Cleanup and document register bank usage. Initial register bank cleanup. Make SR.RB configurable, and add some preliminary documentation on register bank usage within the kernel. Signed-off-by: Paul Mundt --- Documentation/sh/register-banks.txt | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) create mode 100644 Documentation/sh/register-banks.txt (limited to 'Documentation/sh') diff --git a/Documentation/sh/register-banks.txt b/Documentation/sh/register-banks.txt new file mode 100644 index 000000000000..a6719f2f6594 --- /dev/null +++ b/Documentation/sh/register-banks.txt @@ -0,0 +1,33 @@ + Notes on register bank usage in the kernel + ========================================== + +Introduction +------------ + +The SH-3 and SH-4 CPU families traditionally include a single partial register +bank (selected by SR.RB, only r0 ... r7 are banked), whereas other families +may have more full-featured banking or simply no such capabilities at all. + +SR.RB banking +------------- + +In the case of this type of banking, banked registers are mapped directly to +r0 ... r7 if SR.RB is set to the bank we are interested in, otherwise ldc/stc +can still be used to reference the banked registers (as r0_bank ... r7_bank) +when in the context of another bank. The developer must keep the SR.RB value +in mind when writing code that utilizes these banked registers, for obvious +reasons. Userspace is also not able to poke at the bank1 values, so these can +be used rather effectively as scratch registers by the kernel. + +Presently the kernel uses several of these registers. + + - r0_bank, r1_bank (referenced as k0 and k1, used for scratch + registers when doing exception handling). + - r2_bank (used to track the EXPEVT/INTEVT code) + - Used by do_IRQ() and friends for doing irq mapping based off + of the interrupt exception vector jump table offset + - r6_bank (global interrupt mask) + - The SR.IMASK interrupt handler makes use of this to set the + interrupt priority level (used by local_irq_enable()) + - r7_bank (current) + -- cgit v1.2.3