Interpret the Assembly Output of the LC - 3 Compilers

Goals to understand the stack convention of the LC3 compiler:

• How the stack pointer and frame pointer are managed
• Where parameters, return values, return addresses, and local variables are stored
• What the stack looks like before, during, and after a function call.

Instructions

This assignment makes you interpret the assembly output of the LC3 compiler (lcc) in order to figure out the stack convention. We supply the original C file (stack.c) and the assembly code (stack.asm) generated from the compiler. The C program and a fragment of the assembly file with the function code are shown below, these should be sufficient. Note that we have modified lines zero and one to do separate decrements of the stack pointer, instead of combining them as the compiler does. Examine the C code and the assembly code and answer the questions, which are worth 5 points each. For the last question you must draw a picture of the stack at a certain point in the program, this problem is worth 30 points. This assignment does not require you to compile or run code using the LC3 compiler.

The Assignment

Here is the C code for the assignment, comments have been removed to save space:

int add(int param0, int param1)

{

    int result;

    result = param0 + param1;

    return (result);

}

int main(int argc, char *argv[])

{

    int local0 = 1234;

    int local1 = 2345;

    printf("Result: %d\n", add(local0, local1));

    return (0);

}

Here is the assembly code generated by lcc for the add function in stack.c

;;;;;;;;;;;;;;;;;;;;;;;;;;;;add;;;;;;;;;;;;;;;;;;;;;;;;;;;;

lc3_add

;; stack entry

 0:           ADD R6, R6, #­1

 1:           ADD R6, R6, #­1

 2:           STR R7, R6, #0

 3:           ADD R6, R6, #­1 

 4:           STR R5, R6, #0

 5:           ADD R5, R6, #­1

;; function body

 6:           ADD R6, R6, #­1

 7:           ldr R7, R5, #4

 8:           ldr R3, R5, #5

 9:           add R7, R7, R3

10:           str R7, R5, #0

11:           ldr R7, R5, #0

;; stack exit

12:                  STR R7, R5, #3

13:                  ADD R6, R5, #1

14:                  LDR R5, R6, #0

15:                  ADD R6, R6, #1

16:                  LDR R7, R6, #0

17:                  ADD R6, R6, #1

18:                  RET

Answer the following questions, using the variable names from the original program, or one of the following: stack pointer, frame pointer, and return address. When the answer is the frame pointer, identify whether it is the frame pointer from main() or add(). Do not tell me that R7 is getting pushed or R5 getting popped, we already know that from reading the code. Be specific with names from the original C program: local0, local1, param0, param1, etc. Assume that the main program has pushed param0 and param1 before calling the add () function.

Question 1: The code at line 0 is making room on the stack for which value?

Question 2: What is getting pushed at lines 1 and 2?

Question 3: What is getting pushed at lines 3 and 4?

Question 4: What value is being setup at line 5 for which function?

Question 5: The code at line 6 is making room on the stack for which value?

Question 6: Which parameter is loaded, and from what frame pointer offset at line 7?

Question 7: Which parameter is loaded, and from what frame pointer offset at line 8?

Question 8: What is the code at line 9 doing?

Question 9: What is being stored at line 10, and to which frame pointer offset is written?

Question 10: What is being load at line 11, and from which frame pointer offset is read?

Question 11: Is the instruction at line 11 redundant? If so why might the compiler generate it?

Question 12: What is being stored at line 12, and to which frame pointer offset is written?

Question 12: What is being done at line 13?

Question 13: What is getting popped at line 14 and 16?