diff --git a/Pal/lib/Makefile b/Pal/lib/Makefile
index 5cb8ee76..6139cad2 100644
--- a/Pal/lib/Makefile
+++ b/Pal/lib/Makefile
@@ -8,7 +8,7 @@ CFLAGS	= -Wall -fPIC -O2 -std=gnu99 -fgnu89-inline -U_FORTIFY_SOURCE \
 	  -fno-stack-protector -fno-builtin
 ARFLAGS	=
 
-include ../src/host/$(OS)/Makefile.am
+include ../src/host/$(PAL_HOST)/Makefile.am
 
 CFLAGS += -I. -I../include -I../src
 subdirs = string stdlib network graphene
diff --git a/Pal/src/Makefile b/Pal/src/Makefile
index 0b4b3457..9ff478e3 100644
--- a/Pal/src/Makefile
+++ b/Pal/src/Makefile
@@ -5,9 +5,9 @@ include Makefile.Host
 -include Makefile.Status
 
 ifeq ($(DEBUG),1)
-BUILD_TARGET = $(OS)-debug
+BUILD_TARGET = $(PAL_HOST)-debug
 else
-BUILD_TARGET = $(OS)
+BUILD_TARGET = $(PAL_HOST)
 endif
 
 # Customizable PAL Targets
@@ -19,9 +19,9 @@ pal_lib_deps =
 pal_lib_post =
 pal_static =
 
-include host/$(OS)/Makefile.am
+include host/$(PAL_HOST)/Makefile.am
 
-CFLAGS	+= -I. -I../include -Ihost/$(OS) -I../lib
+CFLAGS	+= -I. -I../include -Ihost/$(PAL_HOST) -I../lib
 
 ifeq ($(PROFILING),1)
 CFLAGS	+= -DPROFILING=1
@@ -30,13 +30,13 @@ endif
 files_to_build = $(pal_lib) $(pal_lib_post) $(pal_static) \
 		 $(pal_loader) $(pal_sec)
 
-defs	= -DIN_PAL -D$(OS_MACRO) -DPAL_DIR=$(PAL_DIR) \
+defs	= -DIN_PAL -D$(PAL_HOST_MACRO) -DPAL_DIR=$(PAL_DIR) \
 	  -DRUNTIME_DIR=$(RUNTIME_DIR)
 objs	= $(addprefix db_,streams memory threading semaphore events process \
 	    object main misc ipc exception rtld) slab printf
 graphene_lib = .lib/graphene-lib.a
-host_lib = host/$(OS)/libpal-$(OS).a
-headers	= $(wildcard *.h) $(wildcard ../lib/*.h) host/$(OS)/pal_host.h
+host_lib = host/$(PAL_HOST)/libpal-$(PAL_HOST).a
+headers	= $(wildcard *.h) $(wildcard ../lib/*.h) host/$(PAL_HOST)/pal_host.h
 
 ifeq ($(SYS),x86_64-linux-gnu)
 files_to_build += user_start.o user_shared_start.o
@@ -48,14 +48,14 @@ CFLAGS += -DDEBUG
 endif
 export DEBUG
 
-runtime_loader = $(RUNTIME_DIR)/pal-$(OS)
+runtime_loader = $(RUNTIME_DIR)/pal-$(PAL_HOST)
 
 ifneq ($(pal_lib),)
-	runtime_lib += $(RUNTIME_DIR)/libpal-$(OS)$(suffix $(pal_lib))
+	runtime_lib += $(RUNTIME_DIR)/libpal-$(PAL_HOST)$(suffix $(pal_lib))
 endif
 
 ifneq ($(pal_sec),)
-	runtime_sec += $(RUNTIME_DIR)/pal_sec-$(OS)
+	runtime_sec += $(RUNTIME_DIR)/pal_sec-$(PAL_HOST)
 endif
 
 files_to_install = $(runtime_loader) $(runtime_lib) $(runtime_sec)
@@ -81,11 +81,11 @@ endif
 $(graphene_lib): host_endian.h
 	$(MAKE) -C ../lib target=$(shell pwd)/.lib/
 
-host_endian.h: host/$(OS)/host_endian.h
+host_endian.h: host/$(PAL_HOST)/host_endian.h
 	ln -sf $< $@
 
 $(host_lib): $(graphene_lib)
-	@[ ! -d host/$(OS) ] || $(MAKE) -C host/$(OS)
+	@[ ! -d host/$(PAL_HOST) ] || $(MAKE) -C host/$(PAL_HOST)
 
 $(pal_loader) $(pal_sec): $(host_lib)
 
@@ -103,7 +103,7 @@ endif
 
 ifneq ($(pal_sec),)
 $(pal_sec): $(graphene_lib)
-	@[ ! -d security/$(OS) ] || $(MAKE) -C security/$(OS)
+	@[ ! -d security/$(PAL_HOST) ] || $(MAKE) -C security/$(PAL_HOST)
 
 $(runtime_sec): $(pal_sec)
 	cp -f $< $@
@@ -111,7 +111,7 @@ endif
 
 ifneq ($(pal_lib_post),)
 $(pal_lib_post): $(pal_lib)
-	$(MAKE) -C host/$(OS) ../../$@
+	$(MAKE) -C host/$(PAL_HOST) ../../$@
 endif
 
 $(pal_static): $(addsuffix .o,$(objs)) $(host_lib) $(graphene_lib)
@@ -140,9 +140,9 @@ user_shared_start.o: user_start.S
 	@$(CC) $(CFLAGS) $(defs) -E $< -o $@ $(CFLAGS-suffix)
 
 clean:
-ifneq ($(SAVED_OS),$(OS))
-	$(MAKE) clean OS=$(SAVED_OS)
+ifneq ($(SAVED_OS),$(PAL_HOST))
+	$(MAKE) clean PAL_HOST=$(SAVED_OS)
 endif
 	rm -rf $(addsuffix .o,$(objs)) $(files_to_build) host_endian.h .lib Makefile.Status
-	@[ ! -d host/$(OS) ] || $(MAKE) -C host/$(OS) clean
-	@[ ! -d security/$(OS) ] || $(MAKE) -C security/$(OS) clean
+	@[ ! -d host/$(PAL_HOST) ] || $(MAKE) -C host/$(PAL_HOST) clean
+	@[ ! -d security/$(PAL_HOST) ] || $(MAKE) -C security/$(PAL_HOST) clean
diff --git a/Pal/src/Makefile.Host b/Pal/src/Makefile.Host
index 4aa55402..9742ac6e 100644
--- a/Pal/src/Makefile.Host
+++ b/Pal/src/Makefile.Host
@@ -1,11 +1,11 @@
 all_hosts = Skeleton Linux Linux-SGX FreeBSD
 
-ifeq ($(OS),)
+ifeq ($(PAL_HOST),)
 SYS := $(shell gcc -dumpmachine)
 ifeq ($(SYS),x86_64-linux-gnu)
-OS := Linux
+PAL_HOST := Linux
 else ifeq ($(findstring freebsd,$(SYS)),freebsd)
-OS := FreeBSD
+PAL_HOST := FreeBSD
 else
 $(error Unsupported platform: $(SYS))
 endif
@@ -16,15 +16,16 @@ ifeq ($(SGX)$(SGX_RUN),11)
 	$(error "Do not use SGX=1 and SGX_RUN=1 at the same time")
 endif
 ifeq ($(SGX)$(SGX_RUN),1)
-	OS := $(patsubst %-SGX,%,$(OS))-SGX
+	PAL_HOST := $(patsubst %-SGX,%,$(PAL_HOST))-SGX
 endif
 
-ifeq ($(findstring $(OS),$(all_hosts)),)
-$(error Unsupported platform: $(OS))
+ifeq ($(findstring $(PAL_HOST),$(all_hosts)),)
+$(error Unsupported platform: $(PAL_HOST))
 endif
 
-export OS
-export OS_MACRO = $(shell echo $(OS) | tr '[:lower:]' '[:upper:]' | tr '-' '_')
+export PAL_HOST
+export PAL_HOST_MACRO = $(shell echo $(PAL_HOST) | tr '[:lower:]' '[:upper:]' | tr '-' '_')
 
+.DEFAULT_GOAL := all
 print_host:
-	@echo $(OS)
+	@echo $(PAL_HOST)
diff --git a/Pal/src/Makefile.Test b/Pal/src/Makefile.Test
index 37476596..45db1608 100644
--- a/Pal/src/Makefile.Test
+++ b/Pal/src/Makefile.Test
@@ -2,4 +2,4 @@ DIR = $(dir $(lastword $(MAKEFILE_LIST)))
 expand_target = $(1)
 
 include $(DIR)/Makefile.Host
--include $(DIR)/host/$(OS)/Makefile.Test
+-include $(DIR)/host/$(PAL_HOST)/Makefile.Test
diff --git a/Pal/src/host/Linux-SGX/Makefile.am b/Pal/src/host/Linux-SGX/Makefile.am
index 92d7d0e2..b4ab37af 100644
--- a/Pal/src/host/Linux-SGX/Makefile.am
+++ b/Pal/src/host/Linux-SGX/Makefile.am
@@ -1,5 +1,5 @@
 # Add host-specific compilation rules here
-HOST_DIR = host/$(OS)
+HOST_DIR = host/$(PAL_HOST)
 
 CC	= gcc
 AS	= gcc
@@ -12,13 +12,13 @@ CFLAGS	= -Wall -fPIC -O2 -maes -std=gnu99 -fgnu89-inline -U_FORTIFY_SOURCE \
 ASFLAGS = -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
 	  -x assembler-with-cpp -DIN_ENCLAVE
 LDFLAGS	= -shared -nostdlib -z combreloc -z defs \
-	  --version-script $(HOST_DIR)/pal.map -T host/$(OS)/enclave.lds \
+	  --version-script $(HOST_DIR)/pal.map -T $(HOST_DIR)/enclave.lds \
 	  --hash-style=gnu -e enclave_entry
 ARFLAGS	=
 
-pal_loader = host/$(OS)/pal-sgx
+pal_loader = $(HOST_DIR)/pal-sgx
 pal_sec =
 pal_lib = libpal-enclave.so
-pal_lib_deps = host/$(OS)/enclave.lds host/$(OS)/pal.map
+pal_lib_deps = $(HOST_DIR)/enclave.lds $(HOST_DIR)/pal.map
 pal_lib_post = libpal.so
 pal_static = libpal.a
diff --git a/Pal/src/host/Linux/Makefile.am b/Pal/src/host/Linux/Makefile.am
index 8f2275eb..2cb3419c 100644
--- a/Pal/src/host/Linux/Makefile.am
+++ b/Pal/src/host/Linux/Makefile.am
@@ -1,6 +1,6 @@
 # Add host-specific compilation rules here
-HOST_DIR = host/$(OS)
-SEC_DIR = security/$(OS)
+HOST_DIR = host/$(PAL_HOST)
+SEC_DIR = security/$(PAL_HOST)
 
 CC	= gcc
 AS	= gcc