SIMPL

Tom Kelliher, CS23

May 3, 1996

Evaluation.

Well, if it's SIMPL, what is there to discuss?

From Wednesday:

• = l, r // Assignment; lvalue, rvalue.
• + l, r // Increment.

Combine:

• + l, vr, c // lvalue, variable rvalue, constant

+target,source,45
meaning: target = source + 45

+a,b,
meaning: a = b

+bob,,0
meaning: bob = 0

+i,i,-12
meaning: i -= 12

Limits on variable names?

Other operations?

• Branching:

  ?i,j,30
  

  Meaning: If i < j, branch to statement numbered 30, otherwise execute ``next'' statement.
• Print:

  <"mesg",i
  <"Hello world!\n",
  <,j
  

  Meaning?
• Read:

  >i
  

  Meaning?
• Stop:

  !
  

How can we distinguish statements?

What state information do we need?

• Symbol table for variables:
  1. Create Entry(name, value)
  2. bool Query Value(name, value)
  3. bool Set Value(name value)
  4. Destroy Entry()
  Use a hash table
• Program statements:
  1. Base class: statement.
  2. Derived classes: one for each operation.
  3. Create()
  4. Perform()
  5. Destroy().
  Again, a hash table.
• Program counter.

Is the language any good? Can we:

• Compare i and j for equality?
• Output 1, 2, ..., n?
• Multiply 2 numbers?