From 779939b2553d7aff642f2ca81021b53a967935c9 Mon Sep 17 00:00:00 2001
From: Ralf Corsepius <ralf.corsepius@rtems.org>
Date: Tue, 15 Feb 2005 17:43:25 +0000
Subject: (_CPU_Bitfield_Find_first_bit, _CPU_Priority_Mask,
 _CPU_Priority_bits_index): New.

---
 cpukit/score/cpu/powerpc/rtems/score/cpu.h | 88 ++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

(limited to 'cpukit/score')

diff --git a/cpukit/score/cpu/powerpc/rtems/score/cpu.h b/cpukit/score/cpu/powerpc/rtems/score/cpu.h
index 90563f09d6..6f1ac9b02f 100644
--- a/cpukit/score/cpu/powerpc/rtems/score/cpu.h
+++ b/cpukit/score/cpu/powerpc/rtems/score/cpu.h
@@ -275,6 +275,94 @@ void _CPU_Context_Initialize(
 /* end of Context handler macros */
 #endif /* ASM */
 
+#ifndef ASM
+/* Bitfield handler macros */
+
+/*
+ *  This routine sets _output to the bit number of the first bit
+ *  set in _value.  _value is of CPU dependent type Priority_Bit_map_control.
+ *  This type may be either 16 or 32 bits wide although only the 16
+ *  least significant bits will be used.
+ *
+ *  There are a number of variables in using a "find first bit" type
+ *  instruction.
+ *
+ *    (1) What happens when run on a value of zero?
+ *    (2) Bits may be numbered from MSB to LSB or vice-versa.
+ *    (3) The numbering may be zero or one based.
+ *    (4) The "find first bit" instruction may search from MSB or LSB.
+ *
+ *  RTEMS guarantees that (1) will never happen so it is not a concern.
+ *  (2),(3), (4) are handled by the macros _CPU_Priority_mask() and
+ *  _CPU_Priority_Bits_index().  These three form a set of routines
+ *  which must logically operate together.  Bits in the _value are
+ *  set and cleared based on masks built by _CPU_Priority_mask().
+ *  The basic major and minor values calculated by _Priority_Major()
+ *  and _Priority_Minor() are "massaged" by _CPU_Priority_Bits_index()
+ *  to properly range between the values returned by the "find first bit"
+ *  instruction.  This makes it possible for _Priority_Get_highest() to
+ *  calculate the major and directly index into the minor table.
+ *  This mapping is necessary to ensure that 0 (a high priority major/minor)
+ *  is the first bit found.
+ *
+ *  This entire "find first bit" and mapping process depends heavily
+ *  on the manner in which a priority is broken into a major and minor
+ *  components with the major being the 4 MSB of a priority and minor
+ *  the 4 LSB.  Thus (0 << 4) + 0 corresponds to priority 0 -- the highest
+ *  priority.  And (15 << 4) + 14 corresponds to priority 254 -- the next
+ *  to the lowest priority.
+ *
+ *  If your CPU does not have a "find first bit" instruction, then
+ *  there are ways to make do without it.  Here are a handful of ways
+ *  to implement this in software:
+ *
+ *    - a series of 16 bit test instructions
+ *    - a "binary search using if's"
+ *    - _number = 0
+ *      if _value > 0x00ff
+ *        _value >>=8
+ *        _number = 8;
+ *
+ *      if _value > 0x0000f
+ *        _value >=8
+ *        _number += 4
+ *
+ *      _number += bit_set_table[ _value ]
+ *
+ *    where bit_set_table[ 16 ] has values which indicate the first
+ *      bit set
+ */
+
+#define _CPU_Bitfield_Find_first_bit( _value, _output ) \
+  { \
+    asm volatile ("cntlzw %0, %1" : "=r" ((_output)), "=r" ((_value)) : \
+		  "1" ((_value))); \
+  }
+
+/* end of Bitfield handler macros */
+
+/*
+ *  This routine builds the mask which corresponds to the bit fields
+ *  as searched by _CPU_Bitfield_Find_first_bit().  See the discussion
+ *  for that routine.
+ */
+
+#define _CPU_Priority_Mask( _bit_number ) \
+  ( 0x80000000 >> (_bit_number) )
+
+/*
+ *  This routine translates the bit numbers returned by
+ *  _CPU_Bitfield_Find_first_bit() into something suitable for use as
+ *  a major or minor component of a priority.  See the discussion
+ *  for that routine.
+ */
+
+#define _CPU_Priority_bits_index( _priority ) \
+  (_priority)
+
+/* end of Priority handler macros */
+#endif /* ASM */
+
 /* functions */
 
 #ifndef ASM
-- 
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