Lab 3 - Assembly Implementation of C

1. Introduction & Overview

This laboratory exercise involves compiling a C application to analyze the resulting compiler-generated assembly code and the linker-generated map file.

2. Learning Outcomes

• Compiling a C program
• Reviewing the assembly output listing and the map file.

3. Requirements

This lab utilizes the following environment:

• Software: KEIL µVision5 MDK IDE.
• Hardware: STM32 Nucleo-F401RE development board.

4. Procedure

1. Open the CinAsmLab project workspace.

2. Ensure that the compiler is configured to generate assembly listings (Options -> Listing -> C Compiler Listing) and is set to maximum optimization (Options -> C/C++ -> Optimization Level 3 (-O3)).

3. Compile the source code, then review the output listing and map files to answer the questions below.

4.1 Assembly Code Listing

4.1.1 Evaluating the INIT_LIST Function

• Q4: Which context is saved upon entry?

  • Registers r4, r5, and lr are preserved upon entering the function.

• Q5: Which instruction restores context and returns?

  • The POP {r4, r5, pc} instruction is responsible for restoring the saved context and returning execution from the subroutine.

• Q6: Which register acts as the loop counter i?

  • Register r0 functions as the loop counter variable i.

• Q7: How is the operation i * 2000 performed in assembly? * The instruction MOVS r2, #0x7d loads the decimal value 125 (0x7d) into register r2 (or r1 depending on the exact build).

  • Next, LSLS r2, r2, #4 shifts the register left by 4 bits, effectively multiplying it by 16, resulting in $16\times 125=2000$.

  • Finally, MULS r2, r0, r2 multiplies the counter i (held in r0) by 2000, storing the final product back in r2.

• Q8: How is the operand offset[i] accessed in assembly?

  • LDR r4, |L1.104| assigns r4 the address of the .data section.

  • According to the map file, the .data section and the offset array begin at 0x2000_00d8, making r4 the base pointer.

  • The array stores integers, which are each 4 bytes in length.

  • To determine the specific element's address, a calculation is needed.

  • LSLS r3, r0, #2 shifts the index i (in r0) left by 2 bits, effectively multiplying it by 4 to account for the integer byte size.

  • The exact address is the base address (in r4) plus 4 times the index (in r3).

  • LDR r5, [r4,r3] retrieves the value at this calculated memory location into r5, representing the value of offset[i].

4.1.2 Evaluating the FIND_IN_LIST Function

• Q9: Which context is saved upon entry?

  • No context is preserved when entering this function.

• Q10: Which instruction returns from the subroutine, and why is it different?

  • The function uses BX lr to return.

  • This differs from init_list because no context was pushed to the stack, so nothing needs to be popped off.

• Q11: Which register holds the key argument?

  • Register r0 contains the key argument.

• Q12: How is the loop repeat test performed?

  • ADDS r0, r0, #1 increments the variable i by one.

  • CMP r0, #0xa evaluates r0 against 10 by performing a subtraction.

  • BCC |L1.52| loops back if the carry flag is cleared. This indicates the subtraction did not result in a borrow, meaning i remains less than or equal to 10.

4.2 Map File

4.2.1 Default Program

• Q13: Image Symbol Table (Global Symbols)

• Q14: Memory Map for main.o

  • .text: Size 0x6c bytes, Type: Code, Attributes: Read-only.

  • .data: Size 0x28 bytes, Type: Data, Attributes: Read-write.

  • .bss: Size 0x28 bytes, Type: Zero, Attributes: Read-write.

• Q15: STACK Size

  • The stack size is 0x0400 bytes, which equals 1024 bytes.

  • Yes, this aligns with the memory layout file declaration: ARM_LIB_STACK (0x20000000+4096) EMPTY - 0x0400.

• Q16: Image Component Sizes for main.o

4.2.2 Specifying C Data as Read-Only to Save RAM

Change the C source code to declare the offset array as const, then rebuild.

• Q17: Updated Image Symbol Table

• What changed and why?

  • The offset array shifted from RAM (0x2000_00d8) to ROM (0x0000_0a84).

  • Consequently, list moved to the first position in RAM (0x2000_00d8), as it no longer follows the offset array.

• Q18: Changes in main.txt

  • The offset array is relocated to .constdata instead of .data, represented by the assembly directive: AREA ||.constdata||, DATA, READONLY, ALIGN=2.

• Q19: Updated Memory Map for main.o

  • .text: 0x6c bytes, Code, Read-only.

  • .constdata: 0x28 bytes, Data, Read-only.

  • .bss: 0x28 bytes, Zero, Read-write.

• Q20: Updated Image Component Sizes

• What changed and why?

  • By declaring offset as const, the module requires 40 bytes of Read-Only (RO) data instead of Read-Write (RW) data.

  • This technique successfully minimizes RAM requirements.

  • Because microcontrollers typically possess significantly more Flash ROM capacity than RAM, this is a highly valuable optimization.