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A comprehensive example in Operand Manipulation

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• Suppose we have the following variables defined:

       int   x[100];
       short y[100];
       byte  z[100];
  
       int   i;
       short j;
       byte  k;
  
       class List
       {
          int   value1;
  	short value2;
  	short value3;
  	List next
       }
  
       List head;   (head contains the start of a linked list,
  		   assume the linked list has been created and is
  		   not empty)
  

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• Write an equivalent assembler program for:

  y[i + j] = z[ x[ k ] ] + head.value1 + head.next.value3;

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• How to compute the expression:

  Get z[ x [ k ] ] in register r10 (I use the register r10 to store the running computation) Steps: Get k Use k to get x[k] Use x[k] to get z[ x[k] ] in r10 Get head.value1 in r9: Get head Use head to get head.value1 in r9 Add r9 to r10 (running sum is in r10) Get head.next.value3 in r9: Get head Use head to get head.next Use head.next to get head.next.value3 in r9 Add r9 to r10 (running sum is in r10) Store r10 into y[i + j]: Get i Get j Add i and j to i+j Use i+j to get the memory address of y[i + j] Store r10 in this memory address

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• The assembler code for this assignment statement is:

  /* *********************************** Get z[ x[k] ] in register r10 *********************************** */ // Get byte k movw r0, #:lower16:k movt r0, #:upper16:k // r0 = addr(k) ldrsb r0, [r0] // r0 = k // Get int x[k] movw r1, #:lower16:x movt r1, #:upper16:x // r1 = base address of (int) array x add r0, r0, r0 // r0 = 2k add r0, r0, r0 // r0 = 4k (= offset to x[i]) ldr r0, [r1,r0] // r0 = x[k] // Get byte z[ x[k] ] = z[ r0 ] movw r1, #:lower16:z movt r1, #:upper16:z // r1 = base address of (byte) array z ldrsb r10, [r1,r0] // r10 = z[ x[k] ] /* **************************************** Get int head.value1 in register r9 **************************************** */ // Get pointer head movw r0, #:lower16:head movt r0, #:upper16:head // r0 = addr(head) ldr r0, [r0] // r0 = head // Get int head.value1 ldr r9, [r0, #0] // r9 = head.value1 add r10, r10, r9 // r10 = z[ x[ k ] ] + head.value1 /* ****************************************** Get short head.next.value3 in register r9 ****************************************** */ // Get pointer head movw r0, #:lower16:head movt r0, #:upper16:head // r0 = addr(head) ldr r0, [r0] // r0 = head // Get pointer head.next ldr r0, [r0, #8] // r0 = head.next // Get short head.next.value3 ldrsh r9, [r0, #6] // r9 = head.next.value3 add r10, r10, r9 // r10 = z[ x[ k ] ] + head.value1 + head.next.value3 /* ************************************* Store r10 in short y[i + j] ************************************* */ // Get int i movw r0, #:lower16:i movt r0, #:upper16:i // r0 = addr(i) ldr r0, [r0] // r0 = i // Get short j movw r1, #:lower16:j movt r1, #:upper16:j // r1 = addr(j) ldrsh r1, [r1] // r1 = j // Compute i+j add r0, r0, r1 // r0 = r0 + r1 // Get address of short y[i + j] (base = y, offset = 2*(i+j) movw r1, #:lower16:y movt r1, #:upper16:y // r1 = base address of array y add r0, r0, r0 // r0 = 2*(i+j) (offset) // Address of short y[i+j] is ready !!! // Store r10 in y[i+j] strh r10, [r1, r0] // Stores r10 in base address + offset // If you use str, you will also write the next array element !!!

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• Example Program: (Demo above code)                                                
  • Prog file: /home/cs255001/demo/asm/5-all-struct/summary-example.s

  How to run the program:

  To compile:   as255 summary-example To run: use EGTAPI