Apple IIe - Manual (Page 22)

Page 22 of 74 IIe
Printed: Tuesday, March 4, 2003 10:40:15 AM
One of the most important chips on the main circuit board is the microprocessor. The
microprocessor is the brain of the computer system the chip that carries out the instructions
in the program one by one in the order that they appear.
The Apple IIe has a 65C02
microprocessor.
RAM
The row of small chips are called random access memory chips or RAM. These chips store both
the programs you load into the computer and the data you're working with (memos, budgets,
personnel files, students' grades, and so on).
The information in RAM is stored as electrical
impulses sort of like tiny light bulbs that can either be on or off. Each light bulb is called
a bit, and it takes eight of these bits to express each character on the keyboard. A series of
eight bits is called a byte. Because the information in RAM is stored electrically, it
disappears when the power is turned off. That's why it's important to think of RAM as a
temporary storage compartment.
The Apple IIe has 64K of RAM, which is roughly equivalent to 48 typed, double-spaced pages of
text.
You can add additional RAM by plugging an auxiliary memory card into the slot marked
AUX. CONNECTOR on the main circuit board.
K:
Computer memory is measure in K's, short for kilobyte. (A kilobyte is 1024 bytes.) As you
know, one byte equals one character. 64K is 65,536 characters. Part of that 64K is taken up by
the application program you're using. The rest is available to store the business report or
budget you're working on.
This part of the computer's memory is called random access memory because the microprocessor
can access any location within RAM at random.
It doesn't have to start at the first memory
location and plod through until it reaches the location it's after.
RAM is sometimes called main memory, or just plain memory. Think of it as a grid of thousands
of boxes, each identified by a number called its address.
Each box can hold one piece of code
an instruction for the microprocessor, a character (the H in Hello Mom, for example), or
another address in memory (which tells the microprocessor to jump back to square 1 or wherever
and follow the instruction it finds there).
Scrolling:
Of course there's a lot more room in memory than there is room on the screen to display it.
Picture the information in memory as being written on a paper scroll, and the monitor's screen
as a window in front of the scroll.
To see a different part of the scroll, move the cursor to
the bottom of the screen using the arrow keys. Press the arrow key again and a new line or
column will come into view. This is called scrolling.
ROM
Whereas RAM chips store both programs and information that change every time you use your
computer (one day a letter, the next day a budget), ROM chips contain information that never
changes.
And unlike RAM, the information in ROM stays intact when the power goes off, which is
just as well because one of the programs in ROM tells the microprocessor how to get started
when the power is turned on.
In addition to the program that gets the computer started when the power first comes on, ROM
contains a programming language called BASIC, an acronym for Beginner's All-purpose Symbolic
Instruction Code.
BASIC is easy to learn because it lets you give instructions to the computer