Hardware description language - Software Development Case Studies in Java - page 114

[bus.command # ACK]

EXEC_FREAD
bus.command
# HD_READ

EXEC_FREAD2
bus.command
# RAM_WRITE
bus.address
# arg( IR )

code (IR) # FREAD

[bus.command # ACK]

DECODE FETCH

code (IR) # FOPEN [bus.command # ACK]

EXEC_FOPEN EXEC_FOPEN2

bus.command
# HD_OPEN

bus.command
# RAM_READ
bus.address
# arg( IR )

EXEC_FCLOSE

bus.command
# HD_CLOSE

code (IR) # FCLOSE [bus.command # ACK]

Figure 5.11 FSA for file system instructions

Bus , the constants must have package visibility and be accessible by all the
components.

class Bus {
final static String RAM_READ # "RAM_READ";
final static String RAM_WRITE # "RAM_WRITE";
final static String ACK # "ACK";
final static String INPUT # "INPUT";
final static String OUTPUT # "OUTPUT";
final static String FOPEN # "FOPEN";
final static String FREAD # "FREAD";
final static String FCLOSE # "FCLOSE";

}

The new classes added in this prototype are the device classes Keyboard ,
HardDisk and Display . They are all components of the computer and interact
through the Bus , thus they have an attribute bus implementing this

association, which is initialized by the constructor.
The Keyboard class is based on a BufferedReader that is used to read line
by line from the default input. The execute() method checks whether the
command bus contains the INPUT command, then reads a line from the input
and puts the string on the data bus; finally it acknowledges the command.