CS208, Monday 24 Jan 2022

Questions

• Let's talk about 0xFFFFFFC3 vs. 0x000000C3
  • typecasting
  • automatic typecasting / "promotion" (Yikes!)
  • %x, %d
• Other questions?

Learning about assembly language

• Backwards compatibility has made a giant mess of related terminology
• Reading is slow and weird as you build up vocabulary
• Gotta use those liberal arts critical thinking skills to evaluate credibility and bad writing
• Keep a list of questions
• Experiment (gcc -S and Compiler Explorer can help a lot)

Architecture

• Registers
• Memory
• Status register FLAGS: note the bits CF, ZF, and SF
• Some registers are special: SP and BP, but also SI, DI

Watch out

• The "Aside" on p.172 of your textbook ("The ever-changing forms of generated code") is important
• Textbook + Wikipedia + Wikibooks + miscellaneous boring blogs as references
• Watch out: operand order varies; names vary across the decades (and linger in software)
• Code Explorer vs. gcc -S

Some instructions

• Easy: MOV, ADD, SUB, MUL, NEG
• Weird but easy: LEA
• Conditionals: CMP, TEST
• Jumps: JLE, JS
• Functions: CALL, RET
• Stack: PUSH, POP (and registers SP, BP)

New assignment

• Walk-through puzzle 0
• Wednesday: work on assignment; more x86
• Friday: what's the exam going to be like? short lab on gdb