From 0aabc206a1797fb839f45820dadcbc056953a6eb Mon Sep 17 00:00:00 2001
From: Aiken Harris <harraiken91@gmail.com>
Date: Mon, 1 Jun 2026 00:36:52 +0200
Subject: [PATCH] Add BSF and BSR instruction wrappers

---
 xtoskrnl/ar/amd64/cpufunc.cc          | 50 +++++++++++++++++++++++++++
 xtoskrnl/ar/i686/cpufunc.cc           | 50 +++++++++++++++++++++++++++
 xtoskrnl/includes/ar/amd64/cpufunc.hh |  4 +++
 xtoskrnl/includes/ar/i686/cpufunc.hh  |  4 +++
 4 files changed, 108 insertions(+)

diff --git a/xtoskrnl/ar/amd64/cpufunc.cc b/xtoskrnl/ar/amd64/cpufunc.cc
index f0c6933..df04cc9 100644
--- a/xtoskrnl/ar/amd64/cpufunc.cc
+++ b/xtoskrnl/ar/amd64/cpufunc.cc
@@ -617,6 +617,56 @@ AR::CpuFunctions::ReadWriteBarrier(VOID)
                      : "memory");
 }
 
+/**
+ * Performs a Bit Scan Forward instruction to locate the most significant set bit.
+ *
+ * @param Index
+ *        Receives the zero-based index of the highest set bit when one is found.
+ *
+ * @param Mask
+ *        Supplies the bitmap to scan.
+ *
+ * @return This routine returns TRUE when a set bit was found, otherwise FALSE.
+ *
+ * @since XT 1.0
+ */
+XTCDECL
+BOOLEAN
+AR::CpuFunctions::ScanForwardBit(OUT PULONG Index,
+                                 IN ULONG Mask)
+{
+    /* Defer to the BSF instruction */
+    __asm__("bsfl %[Mask], %[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask));
+
+    /* Report whether the input had any bit set */
+    return Mask ? TRUE : FALSE;
+}
+
+/**
+ * Performs a Bit Scan Reverse instruction to locate the most significant set bit.
+ *
+ * @param Index
+ *        Receives the zero-based index of the highest set bit when one is found.
+ *
+ * @param Mask
+ *        Supplies the bitmap to scan.
+ *
+ * @return This routine returns TRUE when a set bit was found, otherwise FALSE.
+ *
+ * @since XT 1.0
+ */
+XTCDECL
+BOOLEAN
+AR::CpuFunctions::ScanReverseBit(OUT PULONG Index,
+                                 IN ULONG Mask)
+{
+    /* Defer to the BSR instruction */
+    __asm__("bsrl %[Mask], %[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask));
+
+    /* Report whether the input had any bit set */
+    return Mask ? TRUE : FALSE;
+}
+
 /**
  * Instructs the processor to set the interrupt flag.
  *
diff --git a/xtoskrnl/ar/i686/cpufunc.cc b/xtoskrnl/ar/i686/cpufunc.cc
index 784d184..b1465e0 100644
--- a/xtoskrnl/ar/i686/cpufunc.cc
+++ b/xtoskrnl/ar/i686/cpufunc.cc
@@ -587,6 +587,56 @@ AR::CpuFunctions::ReadWriteBarrier(VOID)
                      : "memory");
 }
 
+/**
+ * Performs a Bit Scan Forward instruction to locate the most significant set bit.
+ *
+ * @param Index
+ *        Receives the zero-based index of the highest set bit when one is found.
+ *
+ * @param Mask
+ *        Supplies the bitmap to scan.
+ *
+ * @return This routine returns TRUE when a set bit was found, otherwise FALSE.
+ *
+ * @since XT 1.0
+ */
+XTCDECL
+BOOLEAN
+AR::CpuFunctions::ScanForwardBit(OUT PULONG Index,
+                                 IN ULONG Mask)
+{
+    /* Defer to the BSF instruction */
+    __asm__("bsfl %[Mask], %[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask));
+
+    /* Report whether the input had any bit set */
+    return Mask ? TRUE : FALSE;
+}
+
+/**
+ * Performs a Bit Scan Reverse instruction to locate the most significant set bit.
+ *
+ * @param Index
+ *        Receives the zero-based index of the highest set bit when one is found.
+ *
+ * @param Mask
+ *        Supplies the bitmap to scan.
+ *
+ * @return This routine returns TRUE when a set bit was found, otherwise FALSE.
+ *
+ * @since XT 1.0
+ */
+XTCDECL
+BOOLEAN
+AR::CpuFunctions::ScanReverseBit(OUT PULONG Index,
+                                 IN ULONG Mask)
+{
+    /* Defer to the BSR instruction */
+    __asm__("bsrl %[Mask], %[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask));
+
+    /* Report whether the input had any bit set */
+    return Mask ? TRUE : FALSE;
+}
+
 /**
  * Instructs the processor to set the interrupt flag.
  *
diff --git a/xtoskrnl/includes/ar/amd64/cpufunc.hh b/xtoskrnl/includes/ar/amd64/cpufunc.hh
index 0d0a43c..b717095 100644
--- a/xtoskrnl/includes/ar/amd64/cpufunc.hh
+++ b/xtoskrnl/includes/ar/amd64/cpufunc.hh
@@ -42,6 +42,10 @@ namespace AR
             STATIC XTCDECL ULONGLONG ReadTimeStampCounter(VOID);
             STATIC XTCDECL ULONGLONG ReadTimeStampCounterProcessor(OUT PULONG TscAux);
             STATIC XTCDECL VOID ReadWriteBarrier(VOID);
+            STATIC XTCDECL BOOLEAN ScanForwardBit(OUT PULONG Index,
+                                                  IN ULONG Mask);
+            STATIC XTCDECL BOOLEAN ScanReverseBit(OUT PULONG Index,
+                                                  IN ULONG Mask);
             STATIC XTCDECL VOID SetInterruptFlag(VOID);
             STATIC XTCDECL VOID StoreGlobalDescriptorTable(OUT PVOID Destination);
             STATIC XTCDECL VOID StoreInterruptDescriptorTable(OUT PVOID Destination);
diff --git a/xtoskrnl/includes/ar/i686/cpufunc.hh b/xtoskrnl/includes/ar/i686/cpufunc.hh
index 28cfdb1..3e5b3b3 100644
--- a/xtoskrnl/includes/ar/i686/cpufunc.hh
+++ b/xtoskrnl/includes/ar/i686/cpufunc.hh
@@ -41,6 +41,10 @@ namespace AR
             STATIC XTCDECL ULONGLONG ReadTimeStampCounter(VOID);
             STATIC XTCDECL ULONGLONG ReadTimeStampCounterProcessor(OUT PULONG TscAux);
             STATIC XTCDECL VOID ReadWriteBarrier(VOID);
+            STATIC XTCDECL BOOLEAN ScanForwardBit(OUT PULONG Index,
+                                                  IN ULONG Mask);
+            STATIC XTCDECL BOOLEAN ScanReverseBit(OUT PULONG Index,
+                                                  IN ULONG Mask);
             STATIC XTCDECL VOID SetInterruptFlag(VOID);
             STATIC XTCDECL VOID StoreGlobalDescriptorTable(OUT PVOID Destination);
             STATIC XTCDECL VOID StoreInterruptDescriptorTable(OUT PVOID Destination);