Lab 2 : Writing in Assembly

Seneca College
SEH500 Microprocessors and Computer Architecture

Introduction

Cortex-M4 Memory Map

The 4GB address space of the Cortex®-M processors is partitioned into a number of memory regions (Figure 2.1). The partitioning is based on typical usages so that different areas are designed to be used primarily for:

• Program code accesses (e.g., CODE region)
• Data accesses (e.g., SRAM region)
• Peripherals (e.g., Peripheral region)
• Processor’s internal control and debug components (e.g., Private Peripheral Bus)

The architecture also allows high flexibility to allow memory regions to be used for other purposes. For example, programs can be executed from the CODE as well as the SRAM region, and a microcontroller can also integrate SRAM blocks in CODE region.

Figure 2.1 Memory map

In practice, many microcontroller devices only use a small portion of each region for program flash, SRAM, and peripherals. Some of the regions can be unused. Different microcontrollers have different memory sizes and peripheral address locations. This information is usually outlined in user manuals or datasheets from microcontroller vendors.

Cortex-M4 Registers

The register bank in the Cortex-M3 and Cortex-M4 processors has 16 registers. Thirteen of them are general-purpose 32-bit registers, and the other three have special uses, as can be seen in Figure 2.2.

Figure 2.2 Registers in the register bank

R0 – R12

Registers R0 to R12 are general-purpose registers. The first eight (R0 – R7) are also called low registers. Due to the limited available space in the instruction set, many 16-bit instructions can only access the low registers. The high registers (R8 – R12) can be used with 32-bit instructions, and a few with 16-bit instructions, like MOV (move). The initial values of R0 to R12 are undefined.

R13, stack pointer (SP)

R13 is the Stack Pointer. It is used for accessing the stack memory via PUSH and POP operations.

R14, link register (LR)

R14 is also called the Link Register (LR). This is used for holding the return address when calling a function or subroutine. At the end of the function or subroutine, the program control can return to the calling program and resume by loading the value of LR into the Program Counter (PC). When a function or subroutine call is made, the value of LR is updated automatically. If a function needs to call another function or subroutine, it needs to save the value of LR in the stack first. Otherwise, the current value in LR will be lost when the function call is made.

R15, program counter (PC)

R15 is the Program Counter (PC). It is readable and writeable: a read returns the current instruction address plus 4 (this is due to the pipeline nature of the design, and compatibility requirement with the ARM7TDMI™ processor). Writing to PC (e.g., using data transfer/processing instructions) causes a branch operation.

Program status registers

The Program Status Register is composed of three status registers:

• Application PSR (APSR)
• Execution PSR (EPSR)
• Interrupt PSR (IPSR)

These three registers can be accessed as one combined register, referred to as xPSR in some documentation. In ARM® assembler, when accessing xPSR (Figure 2.3), the symbol PSR is used.

Figure 2.3 Combined xPSR

The main 4 status bits of interest are:

• N: Negative flag - set to 1 when the result of an operation is negative
• Z: Zero flag - set to 1 when the result of an operation is zero
• C: Carry (or NOT borrow) flag
  • set to 1 if the addition produces a carry
  • set to 0 if the subtraction produces a borrow
• V: Overflow flag - set to 1 if the result of an operation is outside the range of a signed 32-bit integer

Reference: Yiu, Ch 4

Preparation

Have a look at the following instruction set and ensure you understand how to lookup an instruction.

Assembly Instruction

Documentation of the Cortex-M4 instruction set can be found here:

• Arm Cortex-M4 Processor Technical Reference Manual Revision
  • Table of processor instructions
• ARMv7-M Architecture Reference Manual

Procedures

1. Open MCUXpresso then start a new C/C++ project based on the Freedom board model that you have.

   

   Figure 2.3 New Project

2. In the new project configuration, rename the project then leave all other settings as default. Even though we will be programming in assembly language, we'll still like the IDE to use the SDK template to set up the Freedom board so we don't have to bother with the startup configuration (ie. vector table, ISR, stack address, heap address, etc.)

   

   Figure 2.4 New Project Name and Settings

3. Once the project is created, rename the C-code file from ".c" to ".s". If the IDE is not allowing you to rename, delete the C-code file and create a new file of the same name but with the extension ".s".

   

   Figure 2.5 Rename File

4. Replace the code within the file with the following:

   @ add directives
   .syntax unified             @ unified syntax used
   .cpu cortex-m4              @ cpu is cortex-m4
   .thumb                      @ use thumb encoding
   
   .text                       @ put code into the text section of memory
   .global main                @ declare main as a global variable
   .type main, %function       @ set main to function type
   
   main:                           @ start of main code with an label
       ldr     r0, =0x20000000     @ load 0x20000000 to R0
       mov     r1, #8              @ move #8 to R1
       add     r1, r1, #4          @ add #4 to R1 and write to R1
       str     r1, [r0]            @ store value of R1 to address of R0
   
   stop:                           @ define a new label called stop
       nop                         @ do nothing
       b       stop                @ jump back label stop to form a loop
   

   Your code should now look like this.

   

   Figure 2.6 Assembly Code

5. Build and run the code using the debug option. Open both the register view and the memory view and set to address 0x20000000.

   

   Figure 2.7 Register and Memory View

6. Step through (using Step Into or Step Over) the code and pay attention to the changes in the registers (register view) and memory (memory monitor). Step until you reach the stop label. Your code should not go any further as it will loop between the nop and b instructions. You can also see the program counter jumping back and forward.

7. Next, re-run the code but this time, expand the program status register and notice the flags that change especially after an arithmetics instruction.

   

   Figure 2.8 Program Status Register Flags

Lab Questions

Using the skills and knowledge acquired from this lab, answer the following post-lab question(s) on Blackboard.

1. Referring to the Cortex-M4 technical manual, how many "Add" instructions are there and what are they?

2. Write the instruction that loads the address 0x20000010 into register R0.

3. Write a functional assembly code that performs Exercise #2 from the module 3 lecture: Perform the calculation: A + B – C = D.

   • Use 0x20000010 for variable A = first 2 digits of your student #
   • Use 0x20000014 for variable B = next 2 digits of your student #
   • Use 0x20000018 for variable C = next 2 digits of your student #
   • Use 0x2000001C for variable D = 0

   We will not be using variables at this point. Run the code and verify that your answer is correct.

   Paste your assembly code into Blackboard along with a screenshot of the memory view showing the content of the four memory addresses.

4. Complete the code as necessary (ie. add all the necessary directives) and run it on your processor board. Comment out any line(s) that are invalid with "@" and explain why in the comment. Answer each question (indicated with ?) directly in the commment of your code and paste your code into Blackboard for submission.

   mov R2, #          @ use the first 2 digits of your student ID
   mov R3, #          @ use the last 2 digits of your student ID
   
   @Other examples to move immediate values
   mov     R5, #0x1234         @ R5 = ?
   movt    R5, #0x8765         @ R5 = ?
   movt    R5, #0x5678         @ R5 = ?
   movw    R6, #0x12345678     @ R6 = ?
   movw    R5, #0x5678         @ R6 = ?
   
   ldr     R7, =0x87654321     @ R7 = ?
   
   add     R1, R2, R3          @ R1 = ?
   movt    R3, #0xFFFF         @ R3 = ?
   adds    R1, R2, R3          @ R1 = ?
               @ What are the PSR (N, Z, C, V) flags now?
   
   subs    R1, R2, R3          @ R1 = ?
               @ How are the PSR flags affected?
   
   mov     R4, #0xFFFFFFFF     @ R4 = ?
   add     R1, R2, R4          @ R1 = ?
               @ How are the PSR flags affected?
   
   adds    R1, R2, R4          @ R1 = ?
               @ What happened to the PSR flags now?
   
   mov     R2, #0x00000002     @ R2 = ?
   adds    R1, R2, R4          @ R1 = ?
               @ again, what happened to the PSR flags?
   
   mov     R2, #0x00000001     @ R2 = ?
   mov     R3, #0x00000002     @ R3 = ?
   adds    R1, R2, R3          @ R1 = ?
               @ Add some small numbers again
               @ and check the PSR flags again, what happened?
   
   @ Add numbers that will create an overflow
   mov     R2, #0x7FFFFFFF     @ R2 = ?
   mov     R3, #0x7FFFFFFF     @ R3 = ?
   
   adds    R1, R2, R3          @ R1 = ?
               @ Check and see what happened to the PSR flags?
   

Reference

[1] Yiu, J. (2013). The Definitive Guide to ARM® Cortex®-M3 and Cortex®-M4 Processors. (3rd ed.). Elsevier Science & Technology.