Statements in a high level programming language

There are 3 kinds of statements in any high level programming language:

  1. Assignment statements

  2. Conditional statements (if, if-else, ...)

  3. Loop statements (while, for, ....)

 

You can prove that programming languages that contain these 3 statement types are Turing complete.

 

We have learned enough assembler programming to show you how a compiler translates assignment statements that only using simple (= unstructured) variables

C program with simple (global) variables and an assignment statement

I can now show you how the (C) compiler translates this C program: 

   int   a = 4;     
   short b = 5;     
   char  c        // 1 byte;         

   int main( )
   {
     c = a + 5*b;                   
   }

Jave program with simple (class) variables and an assignment statement

Translating the variable definitions: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                   
   }

We have discussed variable definitions before: 

     .data
 a:  .4byte  4    // a initialized with the value 4
 b:  .2byte  5    // b initialized with the value 5
 c:  .skip   1    // c is not initialized

DEMO: /home/cs255001/demo/asm/2-mov/add.s

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

How does the compiler translate the above statement into ARM assembler code: 

  .text


  // Always translate the RHS first


   (Teaching note: explain register allocation and de-allocation)

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

Load a into a register: 

  .text
  // r1 = (copy of) a
  movw    r0, #:lower16:a  // Moves the address of memory
  movt    r0, #:upper16:a  // variable a into register r0
  ldr     r1,[r0]          // Load value in int var a to r1

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

Then load b into another register: 

  .text
  // r1 = (copy of) a
  movw    r0, #:lower16:a 
  movt    r0, #:upper16:a
  ldr     r1,[r0]          

  // r2 = (copy of) b
  movw    r0, #:lower16:b // Moves the address of memory
  movt    r0, #:upper16:b // variable b into register r0
  ldrsh   r2,[r0]         // Load value in short var b to r2

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

Multiply b with 5 to obtain 5*b: 

  .text
  // r1 = (copy of) a
  movw    r0, #:lower16:a         mov   r3, #5
  movt    r0, #:upper16:a         mul   r2, r3, r2 // r2=5*b
  ldr     r1,[r0]          

  // r2 = (copy of) b
  movw    r0, #:lower16:b 
  movt    r0, #:upper16:b 
  ldrsh   r2,[r0]

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

Add a with 5*b --- the result is in a register... 

  .text
  // r1 = (copy of) a
  movw    r0, #:lower16:a         mov   r3, #5
  movt    r0, #:upper16:a         mul   r2, r3, r2 // r2=5*b
  ldr     r1,[r0]                 add   r1, r1, r2 // r1=RHS

  // r2 = (copy of) b
  movw    r0, #:lower16:b 
  movt    r0, #:upper16:b 
  ldrsh   r2,[r0]

Jave program with simple (class) variables and an assignment statement

Translating the assignment statement: 

   int   a = 4;               
   short b = 5;               
   char  c        // 1 byte;  

   int main( )
   {
     c = a + 5*b;                  
   }

Store the result into variable c in memory: 

  .text
  // r1 = (copy of) a
  movw    r0, #:lower16:a         mov   r3, #5
  movt    r0, #:upper16:a         mul   r2, r3, r2 // r2=5*b
  ldr     r1,[r0]                 add   r1, r1, r2 // r1=RHS

  // r2 = (copy of) b             movw  r0, #:lower16:c
  movw    r0, #:lower16:b         movt  r0, #:upper16:c
  movt    r0, #:upper16:b         strb  r1, [r0] 
  ldrsh   r2,[r0]

Jave program with simple (class) variables and an assignment statement

The complete ARM assembler program: 

        // Move a into r1
        movw    r0, #:lower16:a  // Moves the address of memory
        movt    r0, #:upper16:a  // variable a into register r0
        ldr     r1,[r0]          // Move value in int var into r1

        // Move b into r2
        movw    r0, #:lower16:b  // Moves the address of memory
        movt    r0, #:upper16:b  // variable b into register r0
        ldrsh   r2,[r0]          // Move value in short var into r2

        // Compute 5*b
        mov     r3, #5           // r3 = 5
        mul     r2, r3, r2       // r2 = r3 * r2 = 5*b

        // Compute a + 5*b
        add     r1, r1, r2       // r1 = a + b

        // Store sum (in r1) to var c
        movw    r0, #:lower16:c  // Moves the address of memory
        movt    r0, #:upper16:c  // variable c into register r0
        strb    r1,[r0]

DEMO: /home/cs255001/demo/asm/2-mov/add.s