CS208, Friday 11 Feb 2022

Exam and grades (10)

• Scores out of 52: wide range (19-51), lots of 30's and low 40's
  
• Exam as snapshot + study guide
• Common: high homework scores, lower exam score
• First pass through a course: hard to calibrate point totals
• Later today: I'll send a grade report with a rough grade estimate to you
• Specific issues
  • the nature of char pointers
  • two's complement
• Monday: start with Q&A on the exam

Why are we mucking about in x86 code? (5)

• What are the instructions our computer hardware can execute directly? (arithmetic (numerical and logical), copy data between registers and memory, compare two numbers, jump to a different instruction based on a comparison, and [we haven't done these instructions] copy data between registers and I/O devices)
• How do our familiar high-level programming constructs get implemented with such tiny instructions?
  • types
  • conditionals
  • loops
  • functions
• How do those implementations affect running time and the nature of our bugs?
• Big question: how can one processor run many programs at once?
• Big question: what services do operating systems and hardware need to provide to programs to enable them to do their jobs?

Stuff you need

• Addressing modes
  • (%r)
  • 0x8(%r)
  • 0x8(%r1, %r2, 4)
• Sections 3.7, 3.8 of the book
• Revisit the stack: what happens at the beginning and end of a function call?

Walkthrough

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