Java Reference
In-Depth Information
Unit
Bytes
Which is approximately
10
3
(1024 bytes exactly)
1 kilobyte (KB)
1024 bytes
10
6
(1,000,000 bytes)
1 megabyte (MB)
1024 kilobytes
10
9
(1,000,000,000 bytes)
1 gigabyte (GB)
1024 megabytes
10
12
(1,000,000,000,000 bytes)
1 terabyte (TB)
1024 gigabytes
10
15
(1,000,000,000,000,000 bytes)
1 petabyte (PB)
1024 terabytes
10
18
(1,000,000,000,000,000,000 bytes)
1 exabyte (EB)
1024 petabytes
10
21
(1,000,000,000,000,000,000,000 bytes)
1 zettabyte (ZB)
1024 exabytes
Fig. 1.4
|
Byte measurements.
Level Languages
Programmers write instructions in various programming languages, some directly under-
standable by computers and others requiring intermediate
translation
steps. Hundreds of
such languages are in use today. These may be divided into three general types:
1.
Machine languages
2.
Assembly languages
3.
High-level languages
Machine Languages
Any computer can directly understand only its own
machine language
,
defined by its
hardware design. Machine languages generally consist of strings of numbers (ultimately re-
duced to 1s and 0s) that instruct computers to perform their most elementary operations
one at a time. Machine languages are
machine dependent
(a particular machine language
can be used on only one type of computer). Such languages are cumbersome for humans.
For example, here's a section of an early machine-language payroll program that adds over-
time pay to base pay and stores the result in gross pay:
+1300042774
+1400593419
+1200274027
Assembly Languages and Assemblers
Programming in machine language was simply too slow and tedious for most program-
mers. Instead of using the strings of numbers that computers could directly understand,
programmers began using English-like abbreviations to represent elementary operations.
These abbreviations formed the basis of
assembly languages
.
Translator programs
called
as-
semblers
were developed to convert early assembly-language programs to machine lan-
guage at computer speeds. The following section of an assembly-language payroll program
also adds overtime pay to base pay and stores the result in gross pay:
load basepay
add overpay
store grosspay
