Memory Organization
Chapter 7
7-11
The second major part of memory is the user program (Figure 7.1). It is
divided into two areas:
This Area:
Stores this:
main program
The program is a group of ladder diagram instructions that control an
application that guides the processor. These instructions can examine
or change the status of bits in the memory of the processor. The
status of these bits determines the operation of your output devices.
When you write a program, you specify the things you want done in
your application and the conditions that must be met before those
things should be done.
subroutine area
This area stores small programs that are periodically accessed. The
subroutine area is not scanned unless you program the processor to
jump to this area (which is located in the memory between the main
program and the message store areas). This area acts as the end of
program statement for the main program.
The user program area begins at the end of the data table.
The third major part of memory is the message storage area (Figure 7.1).
You are able to print messages in hard copy. You can store up to 70
messages using the 1770-T3 terminal, or 198 messages using the 1770-RG
module and an RS-232-C device.
Message storage follows the end statement of your program and is limited
by the number of unused words remaining in memory. Each word stores
two alphanumeric characters. A character is any alpha or numerical figure
(this includes blank spaces).
User Program
Message Storage
