Computer Science 237 Computer Organization Williams College Fall 2005

Announcements

• CS Colloquium today - more "What I did this summer" talks by CS students. 2:30 PM TCL 206.
• Turn in Lab 1 worksheets.
• Lab 2 Continues.

Agenda

• Assembly translation of sample code

  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 demonstrate tricky translation.
  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.

Practicalities

• A shell application is found in
  /usr/cs-local/share/cs237/examples/shell.tar.gz
• Untar the file:
  

      tar xvfz /usr/cs-local/share/cs237/examples/shell.tar.gz
   

• Rename the directory:

      mv shell multiply
   

• Change to directory; edit main.s. (Default application draws a vertical line.)
• The debugger is invoked using BRK or DEB n directives.

  Note you can see your registers.

  BRK just stops the program

  DEB n puts the value 0xDEB00000+n into D7 and stops

  These are defined in the cs237.h file, a simpler way for you to get into the Palm debugger.

  The values along the right are the contents of the stack - we'll worry about this later.

• Type make (also: make realclean)
• Start emulator & load application.
• Intricacies of running programs.

Examples

• multiply (the assembly version of shiftyproduct)