Let

1
Lets do a code walk-through

• Here we examine the design details for a short,
  but not trivial, assembly language example

2
A good way to get us going

• Its a short program (could fit on one page)
• Its self-contained (i.e., needs no libraries)
• It displays output that is helpful for CS
• It illustrates important assembly principles
• It implements a nested loop algorithm

3
Source-statement format

• Each statement in an assembly language programs
  source-file follows a standard format,
  comprised of four distinct fields

label opcode operand(s) comment
comma-separated list
whitespace
hashmark
colon
4
Labels

• The label field is allowed to be blank if the
  statement doesnt need a label (i.e., if its not
  referred to by any other statements)
• Where a label is needed, the programmer is
  responsible to making up the name
• It cannot begin with a digit-character (0..9)
• It can only contain letters and digits, or the
  three special characters ., _,

5
Two kinds of opcodes

• The opcode field holds a reserved word
• An Intel-x86 instruction, or
• A GNU-assembler directive
• This distinction will be very important!
• But its easy to recognize the directives
  because they begin with a dot-symbol (.)

6
Operands vary with the opcode

• Some opcodes require no operands
• Some opcodes need only one operand
• Most opcodes need two operands
• A few opcodes need three operands
• When more than one operand is needed, the
  comma-symbol is used a separator

7
Comment-field

• The comments are programmer-designed
• Their intent is to help other programmers
  understand the purpose of a statement in the
  overall flow of the program-algorithm
• They are ignored by the assembler as far as the
  language-translation is concerned
• But they will be displayed in an assembler
  listing-file (if in fact one is generated)

8
Typical counted-loop construct
Initialize some variables
Perform a loop-iteration
Adjust the iteration-counter
Done?
no
yes
9
A loop-within-a-loop
outer loop
inner loop
10
Demo nybbles.s

• You can download this source-file from our class
  website to your local directory
• cp /home/web/cruse/cs210/nybbles .
• You can assemble it and link it, like this
• as nybbles.s -o nybbles.o
• ld nybbles.o -o nybbles
• Then you can execute it, like this
• ./nybbles