The Mops window that opens when you doubleclick the PowerMops application normally appears like this:

The Mops window is divided through the middle. The lower half is a "Terminal" interface, which we will be discussing here. Comparing the Mops Terminal to the OSX Terminal, the Mops Terminal is limited to the Mops system rather than the POSIX commands of the OS X Terminal application. The lines of information beginning with "Room in data area ..." are optional and provide info about PowerMops' memory availability. The appearance of this information is controlled by the "Free Space" item of the Show Menu. As well as the Apple menu, you will see five menu titles. You won't be using all the menus right away, but acquaint yourself now with the contents of each menu.

The Apple menu contains the usual items. The File menu is much like the File menu in other applications, but with a special item (Load), which you'll use later for loading text files containing your program code.

The Edit menu has the usual items - cut, copy, paste and clear (delete), along with their keystrokes. Notice that Show>HFS Paths menu is identical to the OSX Terminal's $PATH command.

The remaining menus, List, Show and Utilities, contain many operations that will be useful in the writing and debugging of Mops programs.

Learn more about the menu below.

Sending Commands
For now, ignore the top part of the Mops window. In the lower part, you will see some information about how much memory is available in different areas—we'll explain these values below.

Below these lines, you will see a flashing cursor, like you might see in a text editor. Type something, and you will see that this window is indeed a text editing window. You may type and edit text here, as in any text editor.

Just for fun, type or select-drag this text into the lower half of the PowerMops window:

8 SPACES

36 emit CR

You can use multiple commands on one line:

36 EMIT  8 SPACES  38 EMIT  CR

About the «enter» Key
As we saw in the previous lesson, the use of the Enter key has a specialized use in Mops. In most text editors, Enter is treated the same as Return. At the hardware level, Enter and Return have different scan codes, but the Enter key has been dropped on many keyboards. In Mops, however, Enter causes Mops commands to be executed.

We said earlier that Mops behaves like a dictionary. When you opened the PowerMops Application, the Mac automatically loaded the basic Mops vocabulary into its memory. Each time you type a word—any group of text characters—and press «enter», Mops searches through its dictionary for that word and carries out whatever instructions are associated with it. If the word you type is not in the current Mops dictionary, a message appears on the screen to advise you that Mops could not find the word. We'll try it in a moment.

If you're familiar with another computer language, note that in Mops, as in other Forths, we use the word "word" in a different way from normal computer terminology. A Mops "word" is any group of text characters, terminated by white space (a space, tab or carriage return). In C you might say "if(a>b)foo(bar);" but in Mops that would be one word, because it contained no white space. Characters which are punctuation or special characters in other languages can be part of a Mops word, since the only thing that terminates a word is a white space character.

Now we'll demonstrate the difference between the Return and Enter keys.

First type a nonsensical word and press Return

gnarly «return»

The line just sits there in the Mops window, exactly as if you'd typed it into a text editor. Nothing else happens. Now try it again, but this time use «enter»:

gnarly 

Error # -13 : undefined word
gnarly
       ^
Current object: TW    Class:
Stack: empty
Return stack: Depth 42
 9511662 $9122EE ?NOTFOUND
 9502140 $90FDBC NUMBER
 9522950 $914F06 INTERPRET

A few things happened here. Since you used «enter», Mops tried to interpret "gnarly" as a command. The message coming back from Mops indicated that the word you typed in was undefined. This means that the name was not found in the dictionary—in that split second, Mops compared the name against over 1100 words in the Mops dictionary.

And finally, the flashing cursor appears below the message lines, indicating that Mops is waiting for you to type another command.

Now try typing the number 999, and then press «enter» . This time no message will appear from Mops—it has simply accepted the number you typed in. Where is it? It has been placed on a stack. You'll be able to see it in the upper part of the Mops window, which is a display of the stack:

Stack: depth 1
999  $    3E7

Stack: depth 2
888  $    378
999  $    3E7

You can now change the numeric base that Mops is using. Here you will change the base from decimal (the base that Mops starts in) to hexadecimal (base 16, hex for short). Type

hex «enter»

You have set your default base system to 16. To display the top number in hex, type . and press «enter». The top number (888) is taken off the stack and displayed in the Mops window as 378.

Any new numbers you enter are now assumed to be hex numbers. If you type in 3E7 «enter», it is displayed on the right side with the decimal 999 on the left. (If you are not very familiar with hex numbers, we give a fuller description in lesson 13). To change back to decimal, simply type

decimal «enter»

Now try the same thing, typing HEX and DECIMAL in upper case. The results should be exactly the same. That's because Mops makes no distinction between upper and lower case letters when it comes to words in its dictionary. Internally, everything is converted to upper case.

Now type an undefined word again. The error message lines will again appear, but also notice that the stack becomes empty. This is another part of the Mops error handling—the stack is emptied on an error.

The Menu Bar
PowerMops has a simple set of menus, yet the features built into them make writing code, compiling, and debugging rather easy. When you start PowerMops, a "front end" brings in the basic Mops menus - Apple, PowerMops, File, Edit, List, Show, and Utilities.

To help you understand the functions of each menu selection, we'll describe the action of each menu item. We'll also explain the built-in utilities, which can make you more productive in program creation and debugging. QuickEdit is a dedicated editor to interface with PowerMops.

Room in data area of dictionary:                200000
Room in code area of dictionary:               1499996
Distance to top of mainData range (neg is OK):   48460
Distance to top of mainCode range:             -352680
Total heap (no purge):                         20971520
Largest block (purge):                         20971520

PATHSMOD       modHdl    FA0D4 -> 804800 
seg#      10 
flags     2 
install?  false