Loadstar 237

home *** CD-ROM | disk | FTP | other *** search

/ Loadstar 237 / 237.d81 / t.programming1 < prev next >

Wrap

Text File | 2004-01-01 | 17.8 KB | 697 lines

u P R O G R A M M I N G The Great Adventure of Creativity and Logic by Dave Moorman We are entering into a strange world of symbolic logic. Process Logic, to be exact. If you do those logic problems found in puzzle magazines, you know Classic Logic. If you found Geometry enjoyable in hight school, you know Proof Logic. And if you studied philosophic logic in college, you are aquainted with Symbolic Logic. Process Logic is a combination of Proof Logic and Symbolic Logic -- plus three wonderful additional features. Like Proof Logic, you will be arranging statements and commands in a particular order -- not to prove some truth, but to affect some action in the computer. In the process, you will use Symbolic Logic to manipulate values. The three additional features are symbolic value holders (called variables and arrays), loops, and conditional commands. It is the ability to make conditional changes in the flow of logic that gives a computer its ability to "think." The C-64 includes a built in BASIC interpreter. Computers are controlled with three types of language. At its very heart, the computer processor recognizes certain values as "instructions." This is built into the machine itself, and is called Machine Language. EVERYTHING the computer does is done by means of ML. ML is nothing but numbers, that is, numeric values. Remembering such values and the tasks they perform is difficult for humans. We need at least some easily recognizable code words to remind us about what is going on. The ML programmer writes this code and the computer uses a program to ASSEMBLE that code into ML -- which is what the computer actually understands. The code the programmer writes is called ML, or more correctly, Assembly Source Code. But a computer can be smarter than that. Assembly ML source code has a one-to-one relationship with the code the computer understands. However, the computer can be programmed to read words, numbers, and other characters and translate them into complex groups of ML instructions. Such a language is called Compiled. The program that translates Compiler Code into ML Code is called a Compiler. A compiler compiles an entire program or routine at a time. If the programmer has made a mistake the compiler cannot understand, it reports errors -- but only after chugging through the whole source code. So the programmer writes, compiles, debugs, recompiles, executes, rewrites, recompiles, etc, etc. This is an arduous task, to say the least. It was even mor frustrating back in the 1960's when the programmer had to punch cards with each line of the program and take the "batch" to the computer room. The operator would run the batch and return a paper print-out to the programmer in a few hours. Or days! At that time, computers were finally becoming fairly fast and powerful. A terminal could be directly connected to the computer so the programmer did not have to wait. But the computer then did a lot of waiting -- for the programmer's input. The concept of time-sharing was developed, where the computer could switch between many different terminals, running different programs at apparently the same time. To take advantage of time sharing and to provide a language that was easy for students to learn and use, BASIC (standing for Beginner's All Purpose Symbolic Instruction Code) was written (invented) in 1963, at Dartmouth College, by mathematicians John George Kemeny and Tom Kurtzas. The commands and math the programmer typed looked enough like English to make reading the code relatively easy. But the big advantage of BASIC was that it was -- and is -- an Interpreted Language. During the user's tiny slice of processor time, a single BASIC statement would be read, turned into the ML Code necessary to execute the command, and processed. Then the processor turned to another terminal and program to process. On the BASIC program's next turn, the next BASIC statement would be interpreted and executed. The great thing about an interpreted language like BASIC is that the program runs until an error occurs. Then it stops and delivers an error message. The programmer can fix the error and rerun the program. This made BASIC very interactive. The programmer did not have to get everything right before seeing how at least SOME of the program performed. In December of 1974, the January issue of Popular Electronics published news about the first home computer -- the Altair 8800. Two Harvard students, Paul Allen and Bill Gates saw the magazine -- and their future. They dropped out of college and rushed to Albuquerque, NM, where the Altair was being built. They realized that these home computers needed an "operating system" -- a simple way for users to interact with the machine. Bill Gates wrote Altair BASIC using a mainframe computer with an emulator that made it act like the Intel 8008 microprocessor used by the Altair. His BASIC included ML code to read the keystrokes and put the BASIC program into memory. Other code would read BASIC commands and jump to ML routines that performed them. The whole thing fit in just 4 kilobytes of memory (which was rather expensive at the time). Gates and his newly founded company -- MicroSoft -- went on to write BASIC for nearly every home computer. BASIC 2 used about 16K of memory, but was remarkably powerful. Most anything a programmer wanted to do could be done in BASIC. True -- it was slower than straight ML. But it was easy to learn, faster to write, and more-or-less portable between different makes of computers. When Commodore produced the Personal Electronic Transactor -- the PET -- they turned to MicroSoft for BASIC. Legend has it that Commodore CEO Jack Tramiel bought BASIC 2.0 outright for $7,000 from cash- strapped MicroSoft. So, in the fall of 1981 when Commodore designed the C-64, they already owned the BASIC 2.0 operating system. The C-64 has color video and other features for which BASIC 2.0 had no commands. But that was OK. Game designers would certainly use fast ML for their code. And BASIC 2.0 has commands which can directly read or write information to places in memory that can control these features. On the up side, the C-64 was designed to be modified with ML code. Though BASIC 2.0 was in Read Only Memory (ROM) and could not be changed, certain critical jump locations were in Random Access Memory (RAM). By changing the jump addresses, a programmer could add new commands to BASIC and perform all sorts of miracles the designers never dreamed of. The designers did include "paddle controls" for then popular games like Break Out. These controls proved perfect for adding a mouse. All in all, a C-64 was a fantastic machine in 1982 when it was unveiled at the January Consumer Electronics Show in Las Vegas. Its capabilities -- especially as a "game machine" -- and its incredible price (that dropped below $200 in 1984) kept it in production through 1992. Over its decade of manufacture, some 27 million units were sold, making the C-64 the "Best Selling Computer of the 20th Century," according to the revered Guinness Book of World Records (2000-2001). And to top of a remarkable (if often ignored) history, a C-64 Direct-to-TV game joystick was marketed in 2004 through QVC shopping channel. Some 200,000 units were sold between Thanksgiving and Christmas. Thanks to the designer -- Jeri Ellsworth -- the computer inside the joystick is a real, honest-to- goodness C-64. With nine wires soldered to the credit-card-sized board, a user can connect a PS2 keyboard, Commodore disk drive, and an external power supply. The Commodore 64 -- more than any other first-generation, 8-bit computer -- has proved itself as THE computer for gamesters and hobbyist programmers all over the world. INSIDE the C-64 Every computer has three essential parts -- 1. A processor which executes ML instructions and does math and logic operations. 2. Input/Output capabilities -- for keyboard, mouse, joystick, printers, and disk drives. 3. Memory -- "itty-bitty boxes" called bytes which can each hold a value of 0 through 255. Today's computers can handle up to 8 bytes at a time, making them incredibly fast. Such speed is necessary for processing sound recording, photo-quality images, and real-time videos. The first generation computers (such as the C-64) had processors that could handle only one byte at a time. A byte is composed of eight bits -- little switches which are on or off, 1 or 0 -- hence these are called 8-bit computers. A two-byte value is used to point to a particular byte in memory, which means that an 8-bit computer is limited to 256 x 256 or 65536 bytes of memory. One kilobyte is actually 1024 bytes, so 65536/1024 equals 64. Thus the name Commodore 64. But the C-64 has more than 64K of memory. The ROM which holds all the ML code to make BASIC work is "banked" on top of RAM. By "flipping" certain bits in the computer, an ML programmer can set ROM aside and use the RAM "underneath." Indeed, most everything about the built-in operating system can be ignored, giving the expert programmer a full 64K of memory with which to work. As mentioned before, even though BASIC 2.0 is powerful, ML programmers have created a number of BASIC extensions and ML modules to add features for BASIC programmers. In 2004, LOADSTAR featured DotBASIC which adds 72 commands to BASIC 2.0 -- including full mouse control and Event Driven programming. Other modules play music and sound effects in the background, enable easy access to bitmap graphics, and even let ordinary BASIC programmers operate sprites (movable screen objects) and do split screen effects. THE LIMITATIONS The 6510 microprocessor in the C-64 operates at 1 Megahertz -- which is terribly slow when compared to the 2-3 Gigahertz Pentiums now on the market. However, 1 Mhz is 1,000,000 clock cycles per second (which is really pretty quick!) and the 6510's instructions are quite efficient compared to Pentium instructions. The screen is comprised of 64,000 pixels -- 320 x 200. In multi-color mode, two bits determine which of four colors will be displayed as double-wide pixels (160 x 200). The result is a bit grainy. Text characters are all 8 x 8 pixels in size, and can display the character "cell" color or the background color. In multi-color text mode, 4x8 double-wide pixels can present the character color or one of three "universal" colors. The font includes 256 characters, but the programmer is not limited to the two built-in 256 character fonts. With a font editor, each of the 256 characters can show any 8 x 8 combination of pixels. Moreover, the programmer has eight 24 x 21 pixel sprites -- movable objects -- that can be designed and displayed anywhere without disrupting the screen. Sprites are easily used for things like pointer and video game characters because they can freely move on top of on-screen characters. The whole screen can be nudged, pixel by pixel, in any direction, enabling smooth scrolling effects -- especially when combined with split screen capabilities. Sound is remarkable for a one-chip device, including three synthesized voices. The synthesizer's envelope has Attack, Decay, Sustain, and Release parameters, and include various filters and resonance settings. Waveforms include noise, sawtooth, triangle, and adjustable pulse. And, with extreme cleverness, 4-bit recorded sound can be recorded and played by the computer. [ROBIN'S BETA NOTE:] "Extreme cleverness" is a bit of an exaggeration! Playing 4-bit digis is actually about the easiest thing you can do with the SID (just one poke, repeatedly!) as long as you can handle CIA coding to make an IRQ or NMI happen over and over again at a steady pace. [DAVE'S BETA RESPONSE:] Right, Robin! the CIA is the Complex Interface Adaptor, which does all the timing for the C-64 (IRQ and NMI are the two forms of forced timing). So the "cleverness" is to get the timing smooth. So while the C-64 has certain limitations, it was crafted in such a way that truly capable programmers can create most anything computational. I have seen real-time three-dimensional displays (like DOOM, only very low resolution), the MGM Lion roar, and hundreds of other truly amazing sound and video effects. But most important, anything one can do on any computer can be at least MODELED on the C-64. The model may be rough, but the concepts, skills, and personal satisfaction in accomplishing effects are the same as with a big computer. The day may come when you will want to tackle C, C++, C#, Java, Java Script, Perl, or Visual Basic on a PC. EVERYTHING conceptual and logical learned on the C-64 will apply to any other computer or language. The C-64 is where one starts -- as did thousands of today's software design professionals. And hundreds of hobbyist programmers still find more than enough challenge to sit up all hours of the night hunching over their C-64s, fixing just one more thing! ON TO BASIC 2.0 When you turn on the C-64 (or launch VICE), the screen displays some title information, then presents the word READY. READY? Ready for what? Ready for anything you want to do! Type: PRINT "HELLO" and press <RETURN>. (On VICE, the double-quotes are <Shift-2>, and <RETURN> is the <ENTER> key.) The computer immediately complies -- printing to the screen: HELLO Type: ? 5 + 7 * 10 (VICE: the plus is the <-> key (next to the <0> key), the asterisk is the <CLOSE BRACKET> key.) (Remember to press the <RETURN> which tells the C-64 to go ahead and do it.) 75 The question mark is short for PRINT. In the first example, you printed a STRING, a group of characters in order. You marked off the beginning and end of the string with double-quotes. In the second example, you printed numeric values, multiplied and added according to mathematic rules (multiply and divide are performed first, followed by addition and subtraction). Both of these are examples of Immediate Mode. The C-64 immediately responded to your commands when the <RETURN> was pressed. Now Type: 10 ? "HELLO" 20 ? 5 + 7 * 10 (pressing <RETURN> at the end of each line). Nothing happened -- at least not obviously. But inside the C-64 a lot has taken place. Type: LIST and the lines appear again. You have written these lines in Program Mode. The difference between Immediate Mode and Program Mode is very simple: If a NUMBER comes first in the line, the line is put in Program Memory. You can look at the program with LIST. If a command comes first, the line is in Immediate Mode and processed immediately. To run your program, Type: RUN Wonderful! You have written your first program! The number you use at the beginning of a Program line will determine where that line occurs in the program. Type: 15 ? "WORLD" and LIST 10 PRINT "HELLO" 15 PRINT "WORLD" 20 PRINT 5 + 7 * 10 Since every program line must have a number and will be ordered by its number, it is a good idea to use 10's as you start writing your program. Then, if you want to tuck something in between two existing lines, you can -- just like you just did. VARIABLES A variable is a "box" that contains something. Each variable has a name, comprised of one or two letters or a letter and a number. Longer variable names are OK (unless they contain a BASIC command word), but the computer will not "see" more than the first two characters. Let's wipe out your first program. Type: NEW and LIST It's gone. Now Type: 10 A1$ = "HELLO" 20 A2$ = "WORLD" 30 A1 = 5 40 A2 = 7 50 A3 = 10 100 ? A1$ + A2$ 110 ? A1 + A2 * A3 120 END We have two types of data -- string and numeric. So we have two types of variables -- string and numeric. A string variable has a dollar sign after the one or two characters. A numeric variable doesn't. String variables cannot be part of a math formula -- but a plus sign will string two or more strings together. A numeric variable cannot be strung onto a string -- but they can be part of a math formula. The C-64 has the greatest programming interface ever put on an 8-bit computer. If you want to change a program line, all you have to do is * put the line on the screen, * move your cursor up to it, * type your changes onto the line, * and press <RETURN>. To list one line, include the line number after the LIST command: LIST 100 Other list possibilities include: LIST -100 list up to line 100 LIST 100- list line 100 and after LIST 30-50 list from 30 to 50 As a list is scrolling, you can slow it down by pressing the <CTRL> key (<TAB> for VICE), or stop it by pressing <STOP> (<ESC> for VICE). List line 100: 100 PRINT A1$ + A2$ and change it to 100 PRINT A1$ + ", " + A2$ Press <RETURN> then RUN the program. Play around with this program for a while! Try all sorts of things. You can print strings and numerics together on the same line: 100 PRINT A1$ + ", " + A2$; A1 Use a semicolon to separate stuff you print. Use a comma to tab items to columns. Try this: 90 T = A1 + A2 * A3 95 ? T; and run the program. Or do this: 80 A1$ = A1$ + A1$ 85 A2$ = A2$ + A1$ Try every combination of variables and print you can think of. It is YOUR program. And there is nothing you can do from the keyboard that will hurt a C-64! NOTHING! ABOUT ERRORS You have surely seen some errors by now. Two things about errors: 1. The program stops short at any error, and 2. The C-64 always says READY. after an error. When you get an error, just list the line and try to figure out what is wrong. SYNTAX means that something you typed is incomprehensible to the stupid machine. Fix it! Then try again. This is just the beginning. Everything you really need to become a C-64 programmer will be revealed in Part II!