Modify main.s to use _start not main

Finding programs and setting cmake variables

cmake/blackboard.cmake

@@ -0,0 +1,14 @@
+cmake_minimum_required(VERSION 3.13)
+
+set(TARGET_TRIPLET arm-none-eabi)
+set(CMAKE_SYSTEM_NAME Generic)
+
+set(BLACKBOARD_MACHINE_FLAGS "-mcpu=cortex-a9 -mfpu=vfpv3 -mfloat-abi=hard")
+
+set(CMAKE_ASM_FLAGS_INIT "${BLACKBOARD_MACHINE_FLAGS}")
+set(CMAKE_C_FLAGS_INIT "${BLACKBOARD_MACHINE_FLAGS}")
+set(CMAKE_CXX_FLAGS_INIT "${BLACKBOARD_MACHINE_FLAGS}")
+set(CMAKE_EXE_LINKER_FLAGS_INIT "${BLACKBOARD_MACHINE_FLAGS} -ffreestanding -nostdlib -nostdlib++")
+
+include("${CMAKE_CURRENT_LIST_DIR}/clang.cmake")
+

cmake/clang.cmake

@@ -0,0 +1,42 @@
+cmake_minimum_required(VERSION 3.13)
+
+find_program(CMAKE_ASM_COMPILER clang)
+find_program(CMAKE_C_COMPILER   clang)
+find_program(CMAKE_CXX_COMPILER clang++)
+
+# Get host triplet, in case the target isn't set we'll use it as a default
+execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpmachine OUTPUT_VARIABLE HOST_TRIPLET)
+message(STATUS "Host target triplet: ${HOST_TRIPLET}")
+
+if(NOT TARGET_TRIPLET)
+	message(WARNING "TARGET_TRIPLET not set, defaulting to host system (${HOST_TRIPLET})")
+	set(TARGET_TRIPLET ${HOST_TRIPLET})
+endif()
+
+# Get target processor from target triplet (first in triplet, ex. for arm-none-eabi -> arm)
+string(REGEX MATCH "^[^-]*" TARGET_ARCHITECTURE ${TARGET_TRIPLET})
+
+# We need to tell clang what our target is
+set(CMAKE_ASM_COMPILER_TARGET ${TARGET_TRIPLET})
+set(CMAKE_C_COMPILER_TARGET ${TARGET_TRIPLET})
+set(CMAKE_CXX_COMPILER_TARGET ${TARGET_TRIPLET})
+
+if(NOT CMAKE_SYSTEM_NAME)
+	# Cmake uses Generic as system name for embedded systems, use that here if
+	# working on embedded systems
+	message(WARNING "CMAKE_SYSTEM_NAME name not set, defaulting to host system name (${CMAKE_HOST_SYSTEM_NAME})")
+	set(CMAKE_SYSTEM_NAME ${CMAKE_HOST_SYSTEM_NAME})
+endif()
+
+if(NOT CMAKE_SYSTEM_VERSION)
+	set(CMAKE_SYSTEM_VERSION ${CMAKE_HOST_SYSTEM_VERSION})
+endif()
+
+if(NOT CMAKE_SYSTEM_PROCESSOR)
+	# Getting architecture from target triplet, should not be setting CMAKE_SYSTEM_PROCESSOR
+	set(CMAKE_SYSTEM_PROCESSOR ${TARGET_ARCHITECTURE})
+endif()
+
+message(STATUS "Target: ${TARGET_TRIPLET}")
+message(STATUS "Target system name: ${CMAKE_SYSTEM_NAME}")
+

src/main.s

@@ -1,5 +1,5 @@
 .text
-.global main
+.global _start
 
 @define constants, these can be used as symbols in your code
 .equ LED_CTL, 0x41210000
@@ -7,7 +7,7 @@
 
 @the set and equ directives are equivalent and can be used interchangeably
 
-main:
+_start:
 	ldr r1,=SW_DATA	@load switch address from constant
 	ldr r2,=LED_CTL	@load LED address from constant
 loop: