Computer Systems: A Programmer's Perspective Plus Mastering Engineering With Pearson Etext -- Access Card Package (3rd Edition)
Computer Systems: A Programmer's Perspective Plus Mastering Engineering With Pearson Etext -- Access Card Package (3rd Edition)
3rd Edition
ISBN: 9780134123837
Author: Randal E. Bryant, David R. O'Hallaron
Publisher: PEARSON
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Chapter 12.5, Problem 12.7PP

Explanation of Solution

Instruction ordering of “badcnt.c”:

  • When performing instruction “H1”, the value of “%rdx1” and “%rdx2” is null and “cnt” value is “0”.
  • When performing instruction “L1”:
    • The instruction “L1” means loads the shared variable “cnt” into register “%rdx1”.
      • Hence, the value of “%rdx1” contains the value of “cnt” variable.
      • Therefore, %rdx1=0
  • When performing instruction “H1”, the value of “%rdx1” and “%rdx2” is null and “cnt” value is “0”.
  • When performing instruction “L2”:
    • The instruction “L2” means loads the shared variable “cnt” into register “%rdx2”.
      • Hence, the value of “%rdx2” contains the value of “cnt” variable.
      • Therefore, %rdx2=0
  • When performing instruction “U2”:
    • The instruction “U2” means update value in register “%rdx2”.
      • Update value means increment the value of “%rdx2” by “1”.

        %rdx2=%rdx2+1=0+1%rdx2=1

      • Therefore, %rdx2=1 and the value of “cnt” is “0”. Here, the “cnt” will be not be update. It only update after the instruction stored.
  • When performing instruction “S2”:
    • The instruction “S2” means store the updated value of register “%rdx2” into “cnt”.
      • Hence, %rdx2=1 and “cnt” value is “1”.
  • When performing instruction “U1”:
    • The instruction “U1” means update value in register “%rdx1”.
      • Update value means increment the value of “%rdx1” by “1”

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