Lesson 06: Create an ARM Assembly Language Program (Keil MDK-ARM)

ARM Assembly Template

ARM Assembly Sample Firmware code

A reset signal will be generated when power is applied to the board, or be triggered by pushing the reset button available on the board. After reset, ARM processor is in the foreground mode (thread mode), running at a privileged level, and using MSP stack pointer. The processor loads the 32-bit value at memory address $0000.0000 into the stack pointer R13(MSP), loads the 32-bit value at $0000.0004 into the program counter R15 (PC), and initializes the link register R14 (LR) to value $FFFF.FFFF. The value at address $0000.0004 is called the reset vector, and usually stored in flash ROM area.

The following assembly code are created for TI Tiva C Series ARM board. Please copy and paste the code into your main.s assembly source file.

;This is the first ARM Assembly language program you see in the lab
;This program template was created by Airs Lin @ 2017 California State University of Los Angeles.
;When you write your program, you could have your info at the top document block
;For Example:  Your Name, Student Number, what the program is for, and what it does etc.

; Constant Variable Here
STACK_ADDR_123G EQU     0x20008000

            THUMB

; Vector Table Mapped to Address 0 at Reset
; Linker requires __Vectors to be exported

            AREA    RESET, DATA, READONLY
            EXPORT  __Vectors
__Vectors
            DCD     STACK_ADDR_123G         ; stack pointer value when stack is empty
            DCD     Reset_Handler           ; reset vector

            ALIGN

; The program
; Linker requires Reset_Handler

            AREA    |.text|, CODE, READONLY
            ENTRY                           ; mark first instruction to execute
            EXPORT Reset_Handler
Reset_Handler
;;;;;;;;;; Your Main Code Starts from the next line ;;;;;;;;;;;;

            MOV     R0, #12


            ADD     R0, R0, #4
DeadLoop    B       DeadLoop

; The constant variable area
; The value will be stored on ROM segment
            AREA    ConstantVariables, DATA, READONLY

; The variables area
; The variables will be assigned to RAM area
            AREA    Variables, DATA, READWRITE

            END     ; End of the program

;This is the first ARM Assembly language program you see in the lab
;This program template was created by Airs Lin @ 2017 California State University of Los Angeles.
;When you write your program, you could have your info at the top document block
;For Example:  Your Name, Student Number, what the program is for, and what it does etc.

; Constant Variable Here
STACK_ADDR_1294 EQU     0x20040000

            THUMB

; Vector Table Mapped to Address 0 at Reset
; Linker requires __Vectors to be exported

            AREA    RESET, DATA, READONLY
            EXPORT  __Vectors
__Vectors
            DCD     STACK_ADDR_1294         ; stack pointer value when stack is empty
            DCD     Reset_Handler           ; reset vector

            ALIGN

; The program
; Linker requires Reset_Handler

            AREA    |.text|, CODE, READONLY
            ENTRY                           ; mark first instruction to execute
            EXPORT Reset_Handler
Reset_Handler
;;;;;;;;;; Your Main Code Starts from the next line ;;;;;;;;;;;;

            MOV     R0, #12


            ADD     R0, R0, #4
DeadLoop    B       DeadLoop

; The constant variable area
; The value will be stored on ROM segment
            AREA    ConstantVariables, DATA, READONLY

; The variables area
; The variables will be assigned to RAM area
            AREA    Variables, NOINIT, READWRITE

            END     ; End of the program

Four areas are created in the Sample Firmware code.

1. RESET Vectors Area (Line 14 ~ 18):
   This is very important step to build the reset vectors. It is created at flash ROM location $0000.0000. There are two 32-bit values in this area, first is the initialization value for stack pointer R13(MSP); second 32-bit value is pointed to the reset handler (main program)
2. Main Code Area (Line 25 ~ 35)
   The main program starts at this area.
3. Constant Variables Area (Line 39)
   The constant variables can be defined in this area. This area is created at flash ROM location, so the value can not be modified. All the variables in this area must have an initial value.
4. Variables Area (Line 43)
   This area can be used for variables, and it is created at RAM location from $2000.0000. Any variable in the RAM area needs initial value, the value must be assigned in the main code. 

Debug Interface

To debug the code, Keil MDK-ARM build-ins a very useful software simulator, and also fully supports JTAG adapters for in-circuit debugging.

• Software simulator providers complete instruction set simulation for all ARM7, ARM8, Cortex-M based microcontrollers. When debugging your program, op-codes are interpreted and executed as their corresponding instructions would be. This allows program execution without hardware within a fully controlled environment. The μVision simulator traps and reports any illegal memory access. It helps programmers to find out the serious problem that can grow and eventually crash embedded system (your program) without the hardware connection.

To set up simulator, follows the steps:

1. Right click on "Target 1" and select "Options for Target 'Target 1'..." or click on button to open options dialog.
   
2. Click on the "Debug" tab.
   
   • For software simulation: Click on "Use Simulator".
   • For in-circuit hardware debugger, click on "Use:" and select debugger interface to "Stellaris ICDI"
   
   
3. If you select "Use Stellaris ICDI" to debug your code, you must connect the Tiva board win the PC. After you connect the Tiva board, you can click on button "Settings" and make sure that "Stellaris USB ICDI(SN: XXXXXXXX)" is showed in "Attached Devices" field. If it is blank, it means that the Tiva in-circuit-debug-interface drivers are not installed on the PC. Please download the driver from TI, and then update the driver through the "Device Manager". The detail information about the driver can be found in "Lesson 0 - Installing Software and Drivers".
   

Software Simulation Pros and Cons (from Keil Web site)

Advantages:

• Early Access: Dynamic testing is available at every stage of software development
• No Hardware Required: Proof of concept testing before hardware design is done.
• Inject Dangerous Faults: Simulate conditions critical to hardware without risk.
• Full Visibility​: Simulation traces data, registers and instructions and gives a full insight into the system to analyze failures.

Limitations:

• Peripheral Access: Software access to peripheral registers cannot be tested in most cases or will behave differently.
• Memory Timing: Any simulation can only assume ideal conditions for memory access. It does not simulate wait states for data or code fetches.

Build Your Project

Click on the "Build" button or from the "Project" menu to select "Build Target", you will see the following screen.

The "Build Output" window displays errors, warnings, and build messages during the build process. Check the "Build Output" window for errors, if any error occurs, the compiler will not create the target machine code file. Scroll up the in that window to see the list of errors, fix all of the (syntax) errors in your code, and then rebuild the target again.

Debug the Program

Debug the Program

If there are no any error in "Build Output" window after built the project, then you can debug your program by using software simulator, or in-circuit debugger through the JTAG adapter.

1. Check the debug interface
   The debug interface is displayed in the bottom right corner of μVision main screen.
   
   • Simulation: You are using software simulator, so you do not need any hardware board.
   • Stellaris ICDI: You are using in-circuit debugger, you need to connect the Tiva board with your PC and make sure all drivers are already installed.
   
   
2. Start debugging
   You can debug the program by selecting "Start/Stop Debug Session" from the "Debug" menu or clicking on the Debug button .
   

   

   • If you do not have installed license keys for the Keil, a dialog will pop-up showing "EVALUATION MODE, Running with Code Size Limit: 32K". Just click on "OK" button.
     
   • Click on the "Maximize" button in the upper right corner of the μVision window to set the μVision to full-screen mode, so you can have a better and complete view. There are some useful windows open to help you debugging the program.
     
     
     • The "Registers" window shows you the registers, you can observe the change of the values in the registers when you step-by-step to run your program.
     • The "Disassembly" window shows the program execution in assembly code, or, intermixed with the source code. When the Disassembly windows is the active window, then all debug-stepping commands work on assembly level.
     • The "Memory" window displays the memory area element. Several memory windows can be used at a time. Enter an address or an expression that evaluates to a memory address into "Address" field. Double-click a value to change the content. The front color indicates the memory use:
       Black - CODE memory area or uninitialized RAM;
       Red - CONST data in Flash ROM that has been accessed at least once;
       Gold - memory location that has been initialized, but not accessed yet.;
       Green - memory location has been accessed at least once.
   • You can click on the "Debug" menu, or click on the buttons in the debug toolbar, or pressing shortcut-keys to debug your program.
     Debug Menu and Commands
     

     Debug Menu	Toolbar	Shortcut	Description
     Start/Stop Debug Session		Ctrl+F5	Starts or stops a debugging session.
     Reset CPU			Sets the CPU to RESET state.
     Run		F5	Continues executing the program until the next active breakpoint is reached.
     Stop			Stops the program execution immediately.
     Step		F11	Executes a single-step into a function; Executes the current instruction line.
     Step Over		F10	Executes a single-step over a function.
     Step Out		Ctrl+F11	Finishes executing the current function and stops afterwards.
     Run to Cursor Line		Ctrl+F10	Executes the program until the current cursor line is reached.
     Show Next Statement			Shows the next executable statement/instruction.
     Breakpoints		Ctrl+B	Opens the dialog Breakpoints.
     Insert/Remove Breakpoint		F9	Toggles the breakpoint on the current line.
     Enable/Disable Breakpoint		Ctrl+F9	Enables/disables the breakpoint on the current line.
     Disable All Breakpoints			Disables all breakpoints in the program.
     Kill All Breakpoints		Ctrl+Shift+F9	Removes all breakpoints in the program.

3. Stop Debugging
   Click on the "Start/Stop Debug Session" from the "Debug" menu or click on the debug button  to stop executing the program and to exit from debug mode.

Download Code

Download Code to Flash Memory on the Embedded Device

Follows steps to download the machine code to flash memory that are on your Tiva board.

1. You have to use in-circuit debugger. To check the settings, you can click on "Target 1" menu and select "Options for Target 'Target 1'..." or click on buttonto open "options" dialog. Then click on the "Debug" tab. Make sure that the "Use: Stellaris ICDI" is selected.
   
   
2. Connect the Tiva board to your PC.
3. Click on the "Download" from "Flash" menu or click on button .
   
4. Check any error message in the "Build output" window. If Keil shows "Flash Load finished..." message, then you can press "RESET" button on the Tiva board to start you program on the hardware.