Computer Science 237 Computer Organization Williams College Fall 2006

Agenda

• Announcements
• A quick look at possible solutions from Lab 1
• Lab 2 continues
• Assembly translation of sample code Translation: shiftyproduct.c example. A few things about our assembler:
  • register names are preceded by %
  • comment line starts with |
  Advice on approaching an assembly programming task:
  1. Assign variables to registers.
  2. Choose a size for integers (often .w)
  3. When necessary, allocate temporaries.
  4. Translate step-by-step.
  5. Don't be tricky.
  6. Don't optimize; you'll regret it later.
• Addressing Modes
  • Data and address register direct: %d3
  • Notion of effective address: ultimate location of value in memory
  • Address register indirect: (%a3) the effective address of the data is found in an address register; used for pointers
  • Address register indirect with displacement: 4(%a6) the effective address of the data is the contents of the register plus an offset; used for fields in a structure.
  • Address register indirect with displacement and indexing: 4(%a4,%d1.w) effective address is the address register plus the contents of the displacement register plus the displacement; useful for arrays
  • Address register indirect with predecrement: -(%a7) that is -(%sp) Suppose you're manipulating data with n bytes (1=b,2=w,4=l) Then
    1. the address register is decremented by n
    2. the effective address is now in the address register
  • Address register indirect with postincrement: (%a7)+ that is (%sp)+ Suppose you're manipulating data with n bytes (1=b,2=w,4=l) Then
    1. the effective address is in the address register
    2. the address register is incremented by n
  • Immediate: #4 We've seen and used this. There is no effective address.
  • Absolute: 0x4000
    • it's just a number
    • the effective address is in the operand
    Better know what you're doing. Danger:
    1. don't look at low addresses
    2. don't look at odd addresses (we're in a word-addressable world)
  • PC indirect with displacement: 4(%pc) or loop (a label) this allows relocatable code - jumps are relative to PC.

Examples

• shell (the first assembly example)
• multiply (the assembly version of shiftyproduct)