Hello World! An Invitation to Computer Science

Lecture Slides

Translation from Assembly Language to Machine Code
Thursday March 13, 2008

Key ideas from last time

• Moore's Law
• von Neumann bottleneck
• assembly language:
    easier to read and write

Another conditional example

  input x
  if x < 5
      y ← x + 5
  else
      y ← x - 5
  output y

         .begin
         IN x
         -- test if x < 5
         LOAD five
         COMPARE x
         JUMPLT xlt 
         -- if not, set y ← x - 5
         LOAD x
         SUBTRACT five
         STORE y
         JUMP done
         -- if so, set y ← x + 5
   xlt:  LOAD x
         ADD five
         STORE y
         -- either way, output y
   done: OUT y
         HALT
   x:    .data 0
   y:    .data 0
   five: .data 5
         .end

Another loop example

Repeatedly have user enter numbers until they enter zero and then output sum of those numbers.

         .begin
         CLEAR sum
   loop: IN x
         LOAD x
         COMPARE zero
         JUMPEQ done
         ADD sum
         STORE sum
         JUMP loop
   done: OUT sum
         HALT
   sum:  .data 0
   x:    .data 0
   zero: .data 0
         .end

How does computer execute assembly program?

• translates to machine code via assembler
• assembler - an example of systems software
• computer programs to build and execute other programs
• 1-to-1 translation (much easier than full translation)
• two passes: build symbol table, then apply it
    1. if instruction has label - make entry in table
    2. if instruction references label - look it up
    (see flowcharts)
• could be lots of symbols: use binary search!

What does this do?

  .begin
a: JUMP a
  HALT
  .end

What we cannot do in this architecture

• more complex input/output
• interact with operating system
• fast multiplication, division
• indirection

What does this do?

        .begin
  loop:  LOAD three
         COMPARE i
         JUMPGT done
         LOAD sum
  inc:   ADD x
         STORE sum
         INCREMENT i
         INCREMENT inc
         JUMP loop
  done:  OUT sum
         HALT
  sum:   .DATA 0
  i:     .DATA 1
  three: .DATA 3
  x:     .DATA 7
         .DATA 2
         .DATA 4
         .END