CSE142
29. Mar. 2001
QOTD 1.
When Compilers Were People
Imagine you have a sister in grade school. She's just learning to count. She knows how to count numbers, say, how to add one. Just imagine her as a "human compiler", and explain how she could express the problem of adding 6 and 9 to a problem requiring adding only one at a time.
6 + 9 -> counting problem
Solution:
A compiler translates a program in a high level language to a machine language program that can be executed. In some sense, a statement in machine language, which we generally call a machine language instruction, is a kind of atomic operation. Generally each statement of the high level language is mapped to a set of these atomic machine language instructions by the compiler.
So
let's make an analogy between the compiler and your sister.
|
|
Compiler |
Sister |
|
Input |
High level language |
Complex computation like 6+9 |
|
Process |
Express the high level language statement as a set of machine language instructions |
Express the 6+9 as a set of "add one" operations |
|
Output |
A set of simpler machine language instructions |
A set of simpler "add one" operations to get the result of 6+9 |
Express 6+9 as a set of ˇ°add oneˇ± operations:
Perform ˇ°add oneˇ± 6 times and
starting from that result, continue to perform 9 more ˇ°add oneˇ± operations,
the final number you have counted is the result of 6+9.
Express x = y + 3 as a set of machine language instructions: (Don't worry about the details, this is just here to give you a sense of the process, you will learn more from later lectures)
1. A ˇ°Loadˇ± instruction to load the value of y into a special memory place, which is called a ˇ°registerˇ±.
2. An ˇ°Addˇ± instruction to add the value in the register with 3, and store the result to another register.
3.
A ˇ°Storeˇ± instruction to store the result in the register to x
Observation: The number of expressed machine language instruction is generally larger than the number of corresponding high level language statements. Just like the number of ˇ°add oneˇ± operations (15) is larger than the one ˇ°6+9ˇ± operation (1).
What other observations can you make? Think of themˇ If you're not sure whether it is true for a compiler, discuss them on the mailing list or ask your TA or instructor.
ˇˇ
Self-Check Question:
Adapted from Self-Check p. 21
What do you think these five high-level language statements mean? Use simple plain language to explain them.
x = a * b * c;
Solution: x is equal to the product of a, b and c.
x = z / y;
Solution: x is equal to the quotient of z divided by y.
d = c + b - a;
Solution: d is equal to the value of c plus b minus a.
z = z + 8;
Solution: z's value is increased by 8.
Note: The value of z in the right side has the old value, but the value of z in the left side has the new value.
celsius = kelvin - 273.15;
Solution: the Celsius value is attained by subtracting 273.15 from Kelvin value.