diff --git a/libraries/AP_HAL/AP_HAL_Boards.h b/libraries/AP_HAL/AP_HAL_Boards.h
index 123642407b..99915ae9b1 100644
--- a/libraries/AP_HAL/AP_HAL_Boards.h
+++ b/libraries/AP_HAL/AP_HAL_Boards.h
@@ -19,6 +19,30 @@
 #define HAL_BOARD_LINUX    7
 #define HAL_BOARD_EMPTY    99
 
+
+/**
+   CPU classes, used to select if CPU intensive algorithms should be used
+
+   Note that these are only approximate, not exact CPU speeds. 
+ */
+#define HAL_CPU_CLASS_16   1   // 16Mhz, AVR2560 or similar
+#define HAL_CPU_CLASS_75   2   // 75Mhz, Flymaple or similar
+#define HAL_CPU_CLASS_150  3   // 150Mhz, PX4 or similar, assumes
+                               // hardware floating point. Assumes tens
+                               // of kilobytes of memory available
+#define HAL_CPU_CLASS_1000 4   // GigaHz class, SITL, BeagleBone etc,
+                               // assumes megabytes of memory available
+
+/**
+   operating system features:
+
+   HAL implementations may define the following extra feature defines
+   to 1 if available
+
+  HAL_OS_POSIX_IO    :  has posix-like filesystem IO
+ */
+
+
 /*
   define AP_HAL_BOARD_DRIVER to the right hal type for this
   board. This prevents us having a mess of ifdefs in every example
@@ -28,27 +52,45 @@
 #if CONFIG_HAL_BOARD == HAL_BOARD_APM1
 #define AP_HAL_BOARD_DRIVER AP_HAL_AVR_APM1
 #define HAL_BOARD_NAME "APM 1"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_16
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_APM2
 #define AP_HAL_BOARD_DRIVER AP_HAL_AVR_APM2
 #define HAL_BOARD_NAME "APM 2"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_16
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_AVR_SITL
 #define AP_HAL_BOARD_DRIVER AP_HAL_AVR_SITL
 #define HAL_BOARD_NAME "SITL"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_1000
+#define HAL_OS_POSIX_IO 1
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_FLYMAPLE
 #define AP_HAL_BOARD_DRIVER AP_HAL_FLYMAPLE
 #define HAL_BOARD_NAME "FLYMAPLE"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_75
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_PX4
 #define AP_HAL_BOARD_DRIVER AP_HAL_PX4
 #define HAL_BOARD_NAME "PX4"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_150
+#define HAL_OS_POSIX_IO 1
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_LINUX
 #define AP_HAL_BOARD_DRIVER AP_HAL_Linux
 #define HAL_BOARD_NAME "Linux"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_1000
+#define HAL_OS_POSIX_IO 1
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_EMPTY
 #define AP_HAL_BOARD_DRIVER AP_HAL_Empty
 #define HAL_BOARD_NAME "EMPTY"
+#define HAL_CPU_CLASS HAL_CPU_CLASS_16
+
 #else
 #error "Unknown CONFIG_HAL_BOARD type"
 #endif
 
+
 #endif // __AP_HAL_BOARDS_H__
 
diff --git a/libraries/AP_HAL_AVR/AP_HAL_AVR.h b/libraries/AP_HAL_AVR/AP_HAL_AVR.h
index 28e479ed0a..16579428c7 100644
--- a/libraries/AP_HAL_AVR/AP_HAL_AVR.h
+++ b/libraries/AP_HAL_AVR/AP_HAL_AVR.h
@@ -4,6 +4,7 @@
 
 #include <AP_HAL.h>
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_APM1 || CONFIG_HAL_BOARD == HAL_BOARD_APM2
 /**
  * This module exports AP_HAL::HAL instances only.
  * All internal drivers must conform to AP_HAL interfaces
@@ -14,5 +15,7 @@
 #include "HAL_AVR_APM2_Class.h"
 #include "AP_HAL_AVR_Main.h"
 
+#endif // CONFIG_HAL_BOARD
+
 #endif // __AP_HAL_AVR_H__
 
diff --git a/libraries/AP_HAL_AVR_SITL/AP_HAL_AVR_SITL.h b/libraries/AP_HAL_AVR_SITL/AP_HAL_AVR_SITL.h
index 961a709fb3..3222844f00 100644
--- a/libraries/AP_HAL_AVR_SITL/AP_HAL_AVR_SITL.h
+++ b/libraries/AP_HAL_AVR_SITL/AP_HAL_AVR_SITL.h
@@ -4,8 +4,12 @@
 
 #include <AP_HAL.h>
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_AVR_SITL
+
 #include "HAL_AVR_SITL_Class.h"
 #include "AP_HAL_AVR_SITL_Main.h"
 
+#endif // CONFIG_HAL_BOARD
+
 #endif // __AP_HAL_AVR_SITL_H__
 
diff --git a/libraries/AP_HAL_FLYMAPLE/AP_HAL_FLYMAPLE.h b/libraries/AP_HAL_FLYMAPLE/AP_HAL_FLYMAPLE.h
index 524e12da27..8c204c87d2 100644
--- a/libraries/AP_HAL_FLYMAPLE/AP_HAL_FLYMAPLE.h
+++ b/libraries/AP_HAL_FLYMAPLE/AP_HAL_FLYMAPLE.h
@@ -44,8 +44,12 @@
  * `void setup()` and `void loop()`, ala Arduino.
  */
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_FLYMAPLE
+
 #include "HAL_FLYMAPLE_Class.h"
 #include "AP_HAL_FLYMAPLE_Main.h"
 
+#endif // CONFIG_HAL_BOARD
+
 #endif //__AP_HAL_FLYMAPLE_H__
 
diff --git a/libraries/AP_HAL_Linux/AP_HAL_Linux.h b/libraries/AP_HAL_Linux/AP_HAL_Linux.h
index 5e0647d080..16f4f44fec 100644
--- a/libraries/AP_HAL_Linux/AP_HAL_Linux.h
+++ b/libraries/AP_HAL_Linux/AP_HAL_Linux.h
@@ -27,8 +27,11 @@
  * `void setup()` and `void loop()`, ala Arduino.
  */
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_LINUX
+
 #include "HAL_Linux_Class.h"
 #include "AP_HAL_Linux_Main.h"
 
+#endif // CONFIG_HAL_BOARD
 #endif //__AP_HAL_LINUX_H__
 
diff --git a/libraries/AP_HAL_PX4/AP_HAL_PX4.h b/libraries/AP_HAL_PX4/AP_HAL_PX4.h
index 0bb4ef203c..545acc3cc5 100644
--- a/libraries/AP_HAL_PX4/AP_HAL_PX4.h
+++ b/libraries/AP_HAL_PX4/AP_HAL_PX4.h
@@ -3,8 +3,11 @@
 #define __AP_HAL_PX4_H__
 
 #include <AP_HAL.h>
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
 #include "HAL_PX4_Class.h"
 #include "AP_HAL_PX4_Main.h"
 
+#endif // CONFIG_HAL_BOARD
 #endif // __AP_HAL_PX4_H__