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Text File | 1990-02-12 | 36.0 KB | 781 lines

######## ################## ###### ###### ##### ##### #### #### ## ##### #### #### #### #### #### ##### ##### ## ## #### ## ## ## ### ## #### ## ## ## ##### ######## ## ## ## ##### ## ## ## ## ## ##### ## ## ######## ## ## ## ### ## ## #### ## ## ##### #### #### #### #### ##### #### #### #### #### #### ###### ##### ## ###### ###### ╓OLUME 1, ╔SSUE #4 ################## ╧CTOBER 5, 1992 ######## ----------------------------------------------------------------------------- ┼DITOR'S ╬OTES: BY ├RAIG ╘AYLOR (DUCK@PEMBVAX1.PEMBROKE.EDU) ═Y APOLOGIES ABOUT THIS ISSUE BEING POSTED LATER THAN WAS MENTIONED IN A PREVIEW POST ON COMP.SYS.CBM NEWSGROUP. ─UE TO SOME PROBLEMS WITH CODING, SCHOOL AND ═URPHY'S LAW THE ISSUE HAD TO BE DELAYED UNTIL NOW. ╔ HAVE ASKED THE SYSTEM ADMIN'S AT MY SITE CONCERNING A MAIL-SERVER BUT THEY SAID THEY DID NOT HAVE ENOUGH MAN-POWER (GO FIGURE) TO GET SOMEBODY TO RUN IT. ╔ WILL BE IMPLEMENTING A MAIL-SERVER SYSTEM IN MY ACCOUNT IN THE NEAR FUTURE FOR RETRIEVAL OF PROGRAMS AND BACK-ISSUES. ╔'LL POST DESCRIPTIONS OF HOW TO USE IT IT THE NEXT ISSUE OF ├= ╚ACKING AS WELL AS ON THE NEWSGROUP COMP.SYS.CBM WHEN ╔ FINISH WRITING IT. ╔N THIS ISSUE OF ├= ╚ACKING WE ALSO START ON AN AMBITIOUS TASK: ─EVELOPING A GAME FOR BOTH THE ├128 AND ├64 MODES THAT INCLUDES ALL OF THE FEATURES FOUND IN COMMERCIAL GAMES. ╘AKE A LOOK IN THE ╠EARNING ═╠ ├OLUMN FOR MORE INFORMATION. ┴LSO, ╘HE ARTICLE CONCERNING THE 1351 MOUSE HAS _AGAIN_ BEEN DELAYED DUE TO TIME CONSTRAINTS. ╥EST ASSURED THAT IT WILL BE IN THE NEXT ISSUE OF ├= ╚ACKING. ╔F YOU ARE INTERESTED IN HELPING WRITE FOR ├= ╚ACKING PLEASE FEEL FREE TO MAIL DUCK@PEMBVAX1.PEMBROKE.EDU (OR DUCK@HANDY.PEMBROKE.EDU). ╫E'RE ALWAYS LOOKING FOR NEW AUTHORS ON ALMOST ANY SUBJECT, SOFTWARE OR HARDWARE. ================================================================================ ┴LSO NOTE THAT THIS ISSUE AND PRIOR ONES ARE AVAILABLE VIA ANONYMOUS FTP FROM CCOSUN.CALTECH.EDU UNDER PUB/RKNOP/╚┴├╦╔╬╟.═┴╟. ================================================================================ ╬╧╘╔├┼: ╨ERMISSION IS GRANTED TO RE-DISTRIBUTE THIS "NET-MAGAZINE", IN WHOLE, FREELY FOR NON-PROFIT USE. ╚OWEVER, PLEASE CONTACT INDIVIDUAL AUTHORS FOR PERMISSION TO PUBLISH OR RE-DISTRIBUTE ARTICLES SEPERATELY. *** ┴╒╘╚╧╥╙ ╠╔╙╘┼─ ┬┼╠╧╫ ╥┼╘┴╔╬ ┴╠╠ ╥╔╟╚╘╙ ╘╧ ╘╚┼╔╥ ┴╥╘╔├╠┼╙ *** ================================================================================ ╔N THIS ISSUE: ╠EARNING ═╠ - ╨ART 4 ╔N THE NEXT ISSUE WE'LL EMBARK ON A PROJECT OF MAKING A SPACE INVADERS STYLE GAME FOR THE ├=64/128 FROM SCRATCH USING CUSTOM CHARACTERS, INTERRUPT-DRIVEN MUSIC, ANIMATION, USING THE JOYSTICK, MOUSE OR KEYBOARD. ╘HE ├64 AND ├128 VERSIONS WILL BE DEVELOPED CON-CURRENTLY, EACH PROGRAM TAKING ADVANTAGE OF THE MACHINE'S CAPABILITIES. ╘HIS IS THE FIRST IN A SERIES - WRITTEN BY ├RAIG ╘AYLOR. ╘HE ─EMO ├ORNER: ╞╠╔ - MORE COLOR TO THE SCREEN ┴LL OF US HAVE HEARD COMPLAINTS ABOUT THE COLOR CONSTRAINTS ON ├64. ╞╠╔ PICTURE CAN HAVE ALL OF THE 16 COLORS IN ONE CHARACTER POSITION. ╫HAT THEN IS THIS ╞╠╔ AND HOW IT IS DONE ? ╫RITTEN BY ╨ASI '┴LBERT' ╧JALA. ╥╙-232 ├ONVERTER ╘HIS ARTICLE DETAILS PLAN FOR A ╒SER PORT ╘╧ ╥╙232 CONNECTOR USING JUST ╧╬┼ ╔├ AND 4 CAPACITORS. ╘HE CIRCUIT IS INCLUDED, AND SUGGESTIONS ON ALTERNATIVE CHIPS AND PARTS ARE EXAMINED. ╫RITTEN BY ╫ARREN ╘USTIN ╔NTRODUCTION TO THE ╓╔├-╔╔ ╘HIS ARTICLE EXAMINES THE ╓╔├-╔╔ CHIP IN DETAIL AND PROVIDES AN EXPLANATION OF THE VARIOUS REGISTERS ASSOCIATED WITH THE CHIP. ╫RITTEN BY ╨ASI '┴LBERT' ╧JALA. ╠╔╘╘╠┼ ╥┼─ ╥┼┴─┼╥: ═╙-─╧╙ FILE READER FOR THE 128 AND 1571/81 DRIVES. ╘HIS ARTICLE PRESENTS A PROGRAM THAT READS ═╙-─╧╙ FILES AND THE ROOT DIRECTORY OF ═╙-─╧╙ DISKS. ╘HIS PROGRAM COPIES FILES FROM DISK TO DISK SO TWO DISK DRIVES ARE REQUIRED TO USE IT (OR A "VIRTUAL" DRIVE). ╘HIS SCHEME IMPOSES NO LIMIT ON THE MAXIMUM SIZE OF A FILE TO BE TRANSFERRED. ╘HE USER-INTERFACE CODE IS WRITTEN IN ┬┴╙╔├ AND PRESENTS A FULL-SCREEN FILE SELECTION MENU. ╘HE GRUNT-WORK CODE IS WRITTEN IN ASSEMBLY LANGUAGE AND OPERATES AT MAXIMUM VELOSITY. ├OMPLETE, EXPLAINED CODE LISITINGS ARE INCLUDED. ┬Y ├RAIG ┬RUCE. ============================================================================= ╠EARNING ═ACHINE ╠ANGUAGE - ╨ART 4 BY ├RAIG ╘AYLOR (DUCK@PEMBVAX1.PEMBROKE.EDU) +---------------------------+ ▄ ╙PACE ╔NVASION - ╨ART 1 ▄ ▄ ▄ ▄ ╨ROGRAMMING: ├RAIG ╘AYLOR ▄ ▄ ╟RAPHICS : ╨ASI ╧JALA ▄ ▄ ═USIC/╙OUND: ▄ ▄ ▄ +---------------------------+ ╔. ╔NTRODUCTION ------------ ╔N THIS AND FUTURE ╠EARNING ═ACHINE ╠ANGUAGE'S WE WILL DEVELOP A GAME CALLED ╙PACE ╔NVASION. ╘HE GAME WILL BE SIMILAIR TO ╙PACE ╔NDVADERS AND WILL RUN ON THE ├OMMODORE 64 OR THE ├OMMODORE 128 IN 80 COLUMNS. ╔T WILL FEATURE ALL THE "FEATURES" AND "PARTS" THAT ARE FOUND IN COMMERCIAL GAMES WITH INTERRUPT- DRIVEN MUSIC, CUSTOM CHARACTER DEFINITIONS, 100% MACHINE LANGUAGE, MULTI-LEVEL GAME PLAY, AND INPUT FROM THE KEYBOARD, JOYSTICK OR MOUSE. ╬OTE ▄ ╔ AM LOOKING FOR SOMEONE TO HELP AID MUSIC COMPOSITION THAT WILL -----+ BE INTRODUCED IN A LATER ISSUE. ╨ROGRAMMING OF THE 6502 IS HELPFUL BUT NOT A REQUIREMENT. ╨LEASE EMAIL ME AT DUCK@PEMBVAX1.PEMBROKE.EDU IF YOU ARE INTERESTED. ═ANY THANKS TO ╨ASI ╧JALA FOR HIS WORK WITH THE GRAPHICS IN THIS PROGRAM. ┴LSO PLEASE NOTE: ╘HIS ENTIRE PROGRAM HAS BEEN ASSEMBLED SUCESSFULLY WITH THE ┬UDDY-128 ASSEMBLER FOR BOTH THE ├=128 AND ├=64 VERSION. ─UE TO THE LENGTH OF THE SOURCE FILES (OVER 1,500 LINES) ╔'M NOT SURE IF ┬UDDY-64 WILL HANDLE IT. ╘HUS IF YOU GET ERRORS DURING ASSEMBLY, ALL ╔ CAN SAY IS: SORRY. ╔F THIS IS THE CASE THEN THE NEXT ISSUE WILL HANDLE DIVIDING THE PROGRAM AND DATA UP INTO SEGMENTS WHICH CAN THEN EACH BE LOADED SEPERATLY. ╔╔. ═ACHINE ╬OTES ------------- ╘HE ├OMMODORE 64 AND 128 PROGRAMS FOR ╙PACE ╔NVASION WILL DIFFER SLIGHTLY, MOSTLY IN THE FOLLOWING AREAS: - CUSTOM CHARACTER DEFINITION - MEMORY INITIALIZATION / SETUP - SOUND / MUSIC ┬ECAUSE THE ACTUAL GAME PLAY AND THE CHANGES NESSCESARY BETWEEN THE AREAS LISTED ABOVE, WE WILL USE THE ┬UDDY ┴SSEMBLER NOTATION FOR CONDITIONAL ASSEMBLY TO ALLOW THE DEVELOPMENT OF ONLY ONE FILE CONTAINING THE SOURCE CODE. ╔N ADDITION TO CONDITIONAL ASSEMBLY MOST OF THE ROUTINES WILL BE WRITTEN AS ONE WITH JUMPS TO SUBROUTINES CONTAINING THE ├64 OR 128 DIRECT CODE AS THE ALGORITHIMS ARE USUALLY THE SAME FOR EACH. ╔N ADDATION THERE WILL BE SEVERAL SOURCE FILES AND SOME MISCELLANEOUS INCLUDE FILES FOR GRAPHICS AND SOUND. ╞OR THOSE OF YOU WHO ARE OR WILL BE CONVERTING THE ASSEMBLY SOURCE OVER TO A DIFFERENT ASSEMBLER THE CONDITIONAL ASSEMBLY DIRECTIVES .IF (CONDITION) WILL ONLY BE TRUE IF THE CONDITION IS NON-ZERO. ╔E: IF THE SYMBOL COMPUTER IS DEFINED AS 128 THEN THE FOLLOWING EXAMPLE ILLUSTRATES IT: COMPUTER = 128 .IF COMPUTER-64 ; NON-ZERO ANSWER SO THEREFORE ; 128 CODE GOES HERE .ELSE ; 64 CODE GOES HERE .IFE ; END THE .IF CONDITION. ┴LSO NOTE THAT FOR MUCH OF THE PROGRAM WE WILL _NOT_ BE USING THE COMPUTER ROUTINES AND INSTEAD BE DEVELOPING OUR OWN. ╔N ADDITION THE PROGRAM WILL SHOW YOU HOW TO USE ╔╥╤ INTERRUPTS TO SIMPLIFY PROGRAMMING. ╫E WILL BE USING THEM TO PLAY MUSIC IN THE BACKGROUND ON THREE VOICES (SOUND EFFECTS WILL TEMPORARILY PRE-EMPT THE THIRD VOICE FROM PLAYING). ┴LSO ANIMATION OF CHARACTERS WILL BE DONE VIA THE ╔╥╤. ┴ LITTLE BACKGROUND ON INTERRUPTS FOR THOSE OF YOU WHO ARE A BIT HAZY ON WHAT THEY ARE OR HAVE NEVER SEEN THEM BEFORE (┴LSO TRY TAKING A LOOK AT ╥ASTERS: ╫HAT THEY ARE AND HOW TO USE THEM IN ├= ╚ACKING #3 - ╫HILE THIS DOES NOT NECESSARILY COVER WHAT WE ARE GOING TO BE USING INTERRUPTS FOR IT DOES DESCRIBE THEM QUITE WELL.) ┬ASICALLY THE COMPUTER GENERATES AN INTERRUPT EVERY 1/60TH A SECOND FROM A TIMER ON THE COMPUTER (USUALLY FROM THE ├╔┴ CHIP OR THE SCREEN FOR THOSE OF YOU WHO ARE CURIOUS). ╘HE COMPUTER WILL SAVE ALL THE REGISTERS, JUMP TO A SUBROUTINE - PERFORM THE INSTRUCTIONS THERE (USUALLY UPDATING TIME, SCANNING THE KEYBOARD ETC...) AND THEN RECALL ALL THE REGISTERS AND RETURN TO THE USER PROGRAM. ╘HIS IS AN INTERRUPT. ┴N ╔╥╤ INTERRUPT DESCRIBES AN INTERRUPT THAT WE CAN ALLOW TO BE "TURNED ON" AND "TURNED OFF" - IE: WE CAN TEMPORARILY DISABLE IT IF WE HAVE TO. ┴ ╬═╔ INTERRUPT DESCRIBES AN INTERRUPT WHICH WE CAN _NOT_ TEMPORARILY DISABLE -- WE WILL NOT BE USING ╬═╔ INTERRUPTS IN THIS PROGRAM. ╔╔╔. ╘HE ╨ROCESS ----------- ╨ART OF WHAT THIS SERIES OF ARTICLES IS FOCUSED AT IS THE DEVELOPMENT OF BEING ABLE TO ANALYZE PROGRAMMING TASKS AND BREAK THEM DOWN INTO SMALLER WORKABLE PROBLEMS. ╧NCE THESE PROBLEMS OR SUBROUTINES ARE COMPLETED YOUR ORIGINAL PROBLEM IS SOLVED. ╠ET'S TAKE THIS APPROACH TO ╙PACE ╔NVASION: ╨ROBLEM ╙TATEMENT: ┬UILD A ╙PACE ╔NVADER PROGRAM CALLED ╙PACE ╔NVASION. ----------------- ╒SUALLY, GIVEN A PROBLEM YOU HAVE TO RE-WORK THE PROBLEM STATEMENT TO ENCOMPASS ALL OF WHAT YOU WANT. ╠ET'S TRY AGAIN: ╨ROBLEM ╙TATEMENT: ─EVELOP A ╙PACE ╔NVADER PROGRAM CALLED ╙PACE ╔NVASION ----------------- UTILIZING THE 64 OR 128 SCREEN WITH INTERRUPT DRIVEN MUSIC / SOUND, AND ALLOWING INPUT FROM THE KEYBOARD, JOYSTICK OR MOUSE. ╚MMM... ╘HE PROBLEM STATEMENT LISTED ABOVE IS BETTER BUT IT HAS NO REAL ORDER; WE HAVE NO CLEAR IDEA OF WHERE TO START AND WHAT WE NEED TO DO. ╔T DOES HOWEVER TELL US THAT WE HAVE THE FOLLOWING SECTIONS: - 64 / 128 ╙CREEN ╚ANDLING - ═USIC / ╙OUND - ╔NPUT ╚ANDLING - ╟AME ─RIVER (IMPLIED) ╠ET'S THINK A BIT MORE ABOUT EACH OF THESE SECTIONS AND WHAT EACH WILL INVOLVE: 128 / 64 ╙CREEN ╚ANDLING: - ╨UTTING CHARACTERS ON SCREEN. ------------------------ - ╔NITIALIZING THE ╙CREEN / ╥EGISTERS. - ╙ETTING UP THE ├USTOM ├HARACTERS. - ╚ANDLING ANY ┴NIMATION. ═USIC / ╙OUND: - ╙ETTING UP THE ╙OUND ├HIP ╥EGISTERS. ------------- - ╨LAYING A NOTE READ FROM ═EMORY. - ┼XECUTING A ╙OUND ┼FFECT. ╔NPUT ╚ANDLING: - ─EVICE ╙ELECTION (KEYBOARD, MOUSE, JOYSTICK). -------------- - ╦EYBOARD ╙CANNING. - ═OUSE ╙CANNING. - ╩OYSTICK ╙CANNING. ╟AME ─RIVER: - ╘ITLE ╙CREEN. ----------- - ╔NITIALIZATION OF ═EMORY. - ╠EVEL ╙ETUP. - ═OVEMENT OF ┴LIENS. - ═OVEMENT OF ═ISSLES. - ═OVEMENT OF ╨LAYER. - ├OLLISION ├HECKING. - ├OLLISION ╚ANDLING. - ┼ND OF ╠EVEL. - ╙CORE ╒PDATING. - ┼ND - ╟AME HANDLING. - ╚IGH ╙CORE ╒PDATE. ╙HREW! ╠ONG LIST 'EH? - ╬OW YOU MAY HAVE THOUGHT OF SOME NOT LISTED ABOVE, AND WE MAY HAVE POSSIBLY OVERLOOKED SOME CRUCIAL ROUTINES -- THAT'S FINE -- THE ABOVE IS JUST INTENDED AS A BUILDING BLOCK - A PLACE TO START CODING FROM. ╔F WE THINK OF THESE AS SUBROUTINES WE CAN BUILD A SKELETON OUTLINE OF THE PROGRAM - YET WE NEED SOME ORDER IN HOW WE CALL THEM. ╧BVIOUSLY WE AREN'T GOING TO MOVE THE PLAYER UNTIL WE SCAN THE INPUT AND THAT REQUIRES PRIOR DEVICE SELECTION ETC... ╚MM... ╘AKING ORDER INTO ACCOUNT WE CAN RE-STATE THE PROBLEM AS: ╨ROBLEM ╙TATEMENT: ─EVELOP A GAME SIMILAIR TO ╙PACE ╔NVADERS CALLED ╙PACE ----------------- ╔NVASION BY INITIALIZING MEMORY, THE DISPLAY DEVICE, SETTING UP ├USTOM ├HARACTERS, SETTING UP THE ═USIC ╥EGISTERS AND DISPLAYING THE TITLE SCREEN. ╞ROM THERE, SELECT THE INPUT DEVICE AND AFTER THAT SETUP THE CURRENT LEVEL. ╬EXT, WHILE PLAYING MUSIC IN THE BACKGROUND AND SCANNING THE INPUT DEVICE, MOVE THE ALIENS, MISSLES AND PLAYER CHECKING FOR COLLISIONS AND TAKING APPROPRIATE ACTION AS REQUIRED (PLAYER DIES, SCORE INCREASES ETC OR WHAT-NOT). ┴FTER EACH LEVEL DISPLAY IF THE PLAYER IS DEAD, OR SET-UP FOR THE NEXT LEVEL AND REPEAT. ╫HEN THE GAME HAS ENDED UPDATE THE HIGH SCORE IF NECESSARY. ╘RY SAYING THAT FIVE TIMES REAL FAST! :-) ┬UT THAT PROBLEM STATEMENT IS A WHOLE LOT BETTER THAN THE ONE WE HAD AT THE BEGINNING WHICH SIMPLY SAID TO DEVELOP A GAME. ╔╓. ╬OT ┴LL ┴T ╧NE ╘IME - ╫HAT ╫E'RE ─OING ╘HIS ╘IME ------------------------------------------------ ╬OW THIS PROGRAM IS TOO COMPLEX, (AS SEEN BY THE PROBLEM STATEMENT ABOVE) TO HAVE IN ONE ARTICLE SO THIS ISSUE WE'LL CONCENTRATE ON THE BASIC MAIN LOOP AND THE INITIALIZATION OF THE ├USTOM ├HARACTERS AND THE TITLE SCREEN. ╧RIGINALLY, ╔ WAS PLANNING ON UPDATING AND LISTING THE REVISED CODE IN EACH ISSUE. ╚OWEVER, DUE TO SPACE LIMITATIONS AND THE ENORMITY OF THE PROGRAM CURRENTLY (1,500+ LINES!!) IT WILL BE PLACED FOR ANONYMOUS FTP AT CCOSUN.CALTECH.EDU UNDER THE DIRECTORY: PUB/RKNOP/╚┴├╦╔╬╟.═┴╟. ╓. ╘HE ═AIN ╠OOP ------------- ╫HAT IS A MAIN LOOP? ┬ASICALLY IT'S WHERE EVERYTHING GETS DONE. ╔T CALLS OTHER SUBROUTINES AND KEEPS REPEATING UNTIL CERTAIN CRITERIA ARE MET - USUALLY WHEN THE PLAYER REQUESTS TO EXIT THE GAME. ╚OWEVER, INSIDE YOU'LL FIND INNER LOOPS FOR LEVEL PLAY ETC. ╧UR MAIN LOOP FOR THIS PROGRAM WILL BE: ------------------------------------------------------------------------------- ;; * ═AIN ╠OOP - ╘HIS SHOULD BE THE LAST SECTION IN THE SOURCE CODE. ; ; ═AIN ╠OOP ; MAIN'LOOP = * JSR MEMORY'SETUP ; ╙ET-╒P MEMORY. JSR DISPLAY'SETUP ; " " DISPLAY. JSR CHAR'SETUP ; " " CUSTOM CHARACTER DISPLAY. JSR MUSIC'SETUP ; " " MUSIC CHIP. JSR TITLE'SCREEN ; ─ISPLAY THE TITLE SCREEN. JSR SELECT'INPUT ; ╙ELECT ╔NPUT ─EVICE. LEVEL'LOOP = * JSR PLAY'MUSIC ; ╙TART THE MUSIC PLAYING. JSR SETUP'LEVEL ; ╙ETUP THE CURRENT LEVEL. - JSR ALIEN'MOVE ; ═OVE ALIENS JSR MISSLE'MOVE ; " MISSLES JSR PLAYER'MOVE ; " PLAYER JSR CHECK'COLLISION ; ├HECK FOR COLLISIONS LDX COLLISION'FLAG ; ├HECK COLLISION FLAG. BEQ - DEX ; ─ECREASE .╪ BY 1 SO IF ╪ WAS 1 THEN BEQ PLAYER'DIE ; IT'S NOW 0 SO WE KNOW PLAYER DIED. DEX ; ─ECREASE .╪ AGAIN SO IF ╪ WAS 2 THEN BEQ ALIEN'DIE'SOUND ; IT'S NOW 0 SO WE MAKE ALIEN DEATH. JSR END'LEVEL ; ╔F WE GOT HERE - THAN END OF LEVEL. JSR WAIT'NEXT ; ╫AIT FOR NEXT KEYPRESS. JSR INCREASE'LEVEL ; INCREASE LEVEL #. SEC ; ┴ND GO BACK.... BCS LEVEL'LOOP ALIEN'DIE'SOUND = * JSR MAKE'ALIEN'SOUND ; MAKE ALIEN SOUND. SEC ; SET CARRY BCS - ; AND JUMP BACK. PLAYER'DIE JSR SHOW'PLAYER'DIE ; ╙HOW IT ON-SCREEN. LDA LIVES ; ├HECK # OF LIVES. BEQ END'OF'GAME ; ╔F 0 THE END-OF-GAME. BNE LEVEL'LOOP ; GO BACK AND RE-START LEVEL. BRK ; ╔F WE GET HERE - THAN AN ERROR. END'OF'GAME JSR END'GAME'SCREEN ; ╙HOW END-OF-GAME SCREEN. JSR HIGH'SCORE'UPDATE ; ╒PDATE THE HIGH SCORE IF NEED-BE. JSR WAIT'NEXT ; ╫AIT FOR NEXT-GAME SELECTION. LDA QUIT BEQ + JSR SETUP'LEVEL'1 ; ╙ET-╒P FIRST LEVEL. SEC BCS LEVEL'LOOP ; AND START PLAYING IT. + JMP QUIT'GAME ; ; ┼ND OF ═AIN ╠OOP ; ------------------------------------------------------------------------------- ╙OME OF THE ROUTINES LISTED ABOVE WE WILL LATER REPLACE WITH ACTUAL CODE. ╔T'S MUCH EASIER TO SEE: INC LEVEL THAN TO SEE A JSR INCREASE'LEVEL AND TRY TO HUNT DOWN THE CODE. ╔'VE INCLUDED THEM IN FOR NOW SO THAT WE CAN HAVE A BETTER IDEA OF WHAT IS GOING ON. ╔N THE FILE: INVASION.SRC MOST OF THE STATEMENTS ABOVE ARE COMMENTED OUT. ╧NCE WE WRITE THE ROUTINES WE'LL UN-COMMENT THEM. ╞OR NOW, THIS SERVES TO STILL REMIND US OF THE ROUTINES WE NEED TO WRITE. ┴LSO THERE ARE A COUPLE OF PROGRAMMING TRICKS THAT ╔ USED IN THE MAIN LOOP THAT PROBABLY NEED SOME CLARIFYING. ╫HEN HANDLING THE COLLISIONS THE .╪ REGISTER IS LOADED WITH THE RESULT OF THE COLLISION CHECKING - $00 = NO COLLISIONS, $01 = PLAYER DIED, $02 = ALIEN DIED, $03 = END OF LEVEL. ┴NYTIME A LOAD TO A REGISTER IS DONE THE FLAGS ARE AUTOMATICALLY SET AS IF YOU HAD COMPARED IT TO 0 - HENCE WE CAN LDX THE COLLISION FLAG AND IMMEDIATELY BRANCH IF EQUAL TO ZERO FOR NO COLLISIONS. ╔N ADDITION TO THE LOAD ANYTIME THE .╪ OR .┘ REGISTERS ARE INCREMENTED OR DECREMENTED AN IMPLICIT COMPARISON TO ZERO IS PERFORMED. ╙O IF THE .╪ REGISTER IS 1 PREVIOUSLY, WE DECREMENT IT THEN IT WILL BE ZERO AND OUR ┬┼╤ INSTRUCTION WILL BRANCH. ╔F IT'S TWO THEN IT WILL BE ONE AND WE CAN CONTINUE LIKE THIS. [╬╧╘┼: ╘ECHNICALLY IT'S NOT A REAL COMPARISON TO ZERO BUT CALLING IT A COMPARISON TO ZERO SERVERS OUR PURPOSE HERE. ╘HE ONLY SIGNIFICANT DIFFERENCE WOULD BE IN THE EFFECT OF THE CARRY FLAG WHICH IS INSIGNIFICANT IN OUR CODE SEGMENT HERE.] ┴LSO IN SEVERAL LOCATIONS ARE THE TWO INSTRUCTIONS: SEC BCS [LABEL] ╫HAT THESE ARE DOING ARE SIMPLY PROGRAMMING STYLE - THEY COULD BE SUBSTITUTED WITH ╩═╨ [LABEL] - HOWEVER THEY OFFER ADVANTAGES OVER ╩═╨. ╘HEY TAKE UP THE A LARGER AMOUNT OF EXECUTION TIME, HOWEVER THEY ARE RELOCATABLE SO ANY MUCKING AROUND / MOVING SECTIONS OF CODE DURING DEBUGGING WILL BE LESS LIKELY TO CRASH. ╒SING OTHER FLAGS ARE ALSO VALID -- THE USE OF WHICH FLAG (╔ PREFER THE CARRY FLAG) IS USUALLY DEPENDENT ON THE PROGRAMMER. ╟EOS DEFINES A SIMILAIR MACRO CALLED ┬╥┴ (BRANCH ALWAYS) WHICH IS EQUIVLENT TO: CLV BVC [LABEL] ╬OTE THAT THE ABOVE IS JUST PROGRAMMING STYLE, HELD OVER FROM MY PROGRAMMING IN ASSEMBLY DAYS. ╘HE USE OF ╩═╨ IS PROBABLY PREFERABLE IN TERMS OF EXECUTION AND ALSO IN BEING ABLE TO BRANCH MORE THAN 127 BYTES AWAY (THE BRANCH INSTRUCTIONS ONLY HAVE A RANGE OF +128/-127). ╓╔. ├USTOM ├HARACTERS ----------------- ╙INCE WE'RE WRITING FOR EACH OF THE SEPERATE MODES (64 MODE, 128 MODE) WE HAVE TO TAKE A LOOK AT THE DIFFERENCES BETWEEN THE ╓╔├ CHIP (64 MODE) AND THE 8563 CHIP IN THE 128. ╘HE ╓IC-├HIP ------------ ╘HE CHARACTER SETS IN THE ╓╔├ CHIP ARE DEFINED AS IN THE EXAMPLE BELOW OF THE CHARACTER CODE $00 "@" (ALL REFERENCES ARE TO SCREEN "POKE" CODES - NOT PRINT CODES). .BYT #%00111100 ╘RY HOLDING THE PAGE (OR MOVING AWAY FROM THE .BYT #%01100110 SCREEN) AND TAKING A LOOK AT THE PATTERNS THE 1'S .BYT #%01101110 AND 0'S MAKE. ┼ACH CHARACTER IS THUS DEFINED AS .BYT #%01101110 EIGHT BYTES WHO'S BIT PATTERNS DEFINE IT. ╚AVING A .BYT #%01100000 TOTAL OF 256 CHARACTERS AVAILABLE MAKES IT .BYT #%01100010 NECCESARY TO SET ASIDE A TOTAL OF 2,048 BYTES. .BYT #%00111100 .BYT #%00000000 ╬OW, INSTEAD OF DESIGNING ALL 256 CHARACTER SETS WE'LL JUST TAKE ADVANTAGE OF THE FACT THAT THE LETTERS AND NUMBERS WE WANT WILL ALREADY BE THERE -- WE'LL JUST COPY THEM FROM THE ╥╧═ SET INTO ╥┴═, MODIFY SOME OF THE OTHER CHARACTERS TO REFLECT WHAT WE WANT AND THEN TELL THE ╓╔├ CHIP TO LOOK AT ╥┴═ TO GET THE CHARACTER SET DEFINITIONS. ╘HERE ARE SOME PROBLEMS WITH COPYING THE 'SYSTEM' CHARACTERS, HOWEVER. ╘HE ├OMMODORE 64 USUALLY MASKS OUT THE CHARACTER SET AND TYPICALLY IT IS ONLY AVAILABLE TO THE ╓╔├ CHIP SO THAT MORE SPACE CAN BE PRESENT FOR USER PROGRAMS AND SUCH. ╔T ALSO TAKES UP THE SECTION OF MEMORY THAT THE ╔/╧ BLOCK IN $D000-$DFFF DOES SO THAT SWITCHING IT IN WHILE INTERRUPTS ARE ENABLED IS SURE TO RESULT IN A CRASH. ╫E'RE ALSO GOING TO BE DOING A FEW THINGS THAT YOU MAY NOT EXPECT -- INSTEAD OF COPYING ALL 256 CHARACTERS - WE'RE GONNA _JUST_ COPY THE FIRST 128. ╘HIS WILL GIVE US ALL OF THE NORMAL CHARACTERS AS THE LAST 128 ARE THE REVERSE- VIDEO COUNTERPARTS TO THE FIRST 128 CHARACTERS. ╫E'RE DOING THIS TO CONSERVE SPACE AND BECAUSE WE REALLY DON'T NEED THAT MANY CHARACTERS DEFINED. ┴LSO LOCATION $01 CONTAINS WHAT $D000-$DFFF HOLDS AND WE WILL HAVE TO MODIFY BIT 2 TO SWITCH THE CHARACTER ╥╧═ IN. ╚ENCE, THE FOLLOWING PROGRAM CODE IS USED TO COPY THE CHARACTER SET: ------------------------------------------------------------------------------- COPY'CHARS = * ; MUST BE RUN W/ INTERRUPTS DISABLED LDA $01 ; REGISTER 1 = THE CONTROL TO SWITCH IN THE CHAR. ; ROM. PHA ; SAVE IT AS WE'LL LATER NEED TO STA' IT BACK. AND #%11111011 ; ┬IT 2 CONTROLS IT - CLEAR IT TO SWITCH IT IN. STA $01 ; AND MAKE IT SO WE CAN READ IT IN. LDA #>$3000 ; MOVE CHARS TO $3000 STA DEST+1 LDA #>$D800 ; FROM $D800 (START OF CHAR SET) (LOWER-CASE) STA SRC+1 LDY #$00 ; LO-BYTES OF BOTH SRC, DEST = $00. STY SRC STY DEST LDX #$10 ; COPY 2K OF DATA. - LDA (SRC),Y ; COPY BYTE. STA (DEST),Y INY BNE - ; CONTINUE UNTIL .┘ = 0. INC SRC+1 ; INCREASE SOURCE & DEST BY 256 INC DEST+1 DEX ; DECREASE .╪ COUNT. BNE - ; IF NON-ZERO THEN CONTINUE COPYING, ELSE PLA ; RESTORE VALUE OF $01 STA $01 ; AND PUT BACK. LDA $D018 ; SET ╓╔├-CHIP ADDRESS. AND #$F1 ; TO SHOW CHAR SET. ORA #$0C STA $D018 ; AND FINALLY TELL ╓╔├ WHERE THE CHAR SET IS... RTS ; AND RETURN. ------------------------------------------------------------------------------- ╬OTE THAT WE STILL NEED TO CHANGE THE ACTUAL CHARACTERS WE'RE GONNA BE USING. ╘HAT WILL BE HANDLED IN THE SECTION AFTER NEXT: ├HANGING THE ├HARACTERS AS THERE IS A GREAT DEAL OF SIMILARITY BETWEEN THE 128 AND 64 IMPLEMENTATIONS. ╘HE 8563 ├HIP ------------- ╘HE 8563 80-├OLUMN CHIP USUALLY HAS 16K OR 64K ╥AM ATTATCHED TO THE CHIP WHICH THE ├╨╒ DOES NOT HAVE DIRECT CONTROL OVER. ╔T HAS TO DIRECT THE 8563 TO STORE AND RETRIEVE VALUES TO THAT MEMORY. ╫HAT MAKES CONTROL OVER THAT MEMORY ALL THE MORE DIFFICULT IS THE FACT THAT THE 8563 ONLY HAS TWO LINES OR ADDRESSES THAT THE ├╨╒ CAN CONTROL. ╘HE 8563 HAS A CHARACTER SET IN MUCH THE SAME WAY THE ╓╔├ CHIP DOES, SAVE ONE EXCEPTION - EACH CHARACTER SET CAN HAVE UP TO 16 LINES. ╬ORMALLY, THE LAST EIGHT LINES ARE FILLED WITH $00 AND ARE NOT SHOWN. (╨ROVISIONS CAN BE MADE TO HAVE 8X16 CHARACTERS BUT IT IS NOT NEEDED FOR THIS GAME AND THUS, WILL NOT BE SHOWN - ╞OR MORE INFORMATION ╙EE ├= ╚ACKING ╔SSUE #2: 8563: ┴N ╔N-─EPTH ╠OOK.) ╘HUS THE ALGORITHIM IS SIMILAIR TO THE ├=64 BUT 8 ZERO-BYTES WILL NEED TO BE WRITTEN AT THE END OF EVERY EIGHT BYTES READ. ╚OWEVER, THE 8563 DOES MAKE THINGS EASIER FOR US! - ╫HEN THE COMPUTER IS FIRST TURNED ON A COPY OF THE ├HARACTER ╙ET FROM ╥╧═ IS COPIED INTO THE 8563. ╘HE 8563 HAS NO ╥╧═ ├HARACTER ╙ET ASSOCIATED WITH IT AND THUS WE ARE ABLE TO JUST SIMPLY MODIFY THE CHARACTER SET THAT IS IN THE 8563 MEMORY INSTEAD OF COPYING IT OVER. ┬ECAUSE OF THIS NO ROUTINE WILL BE PRESENTED TO COPY A CHARACTER SET INTO THE 8563 MEMORY, RATHER THE DISCUSSION OF COPYING INDIVIDUALLY DEFINED CHARACTERS WILL TAKE PLACE IN THE NEXT SECTION. ╘HE ├=128 ALSO MAKES LIFE EVEN EASIER FOR US AT THE END WHEN WE WILL EXIT THE PROGRAM, MODIFYING THE CHARACTER SET BACK TO THE "STANDARD" ├OMMODORE CHARACTER SET BY A ROUTINE IN THE ╦┼╥╬┴╠ THAT WILL COPY THE CHARACTERS BACK. ╫E'LL TAKE A LOOK AT IT CLOSER WHEN WE WRITE THE EXIT ROUTINE. ┴LSO NOTE THAT SINCE THE 8563 CHIP SUPPORTS THE 80 COLUMN SCREEN WE WILL BE DEFINING TWO CHARACTERS THAT CAN BE PLACED SIDE BY SIDE FOR EACH ALIEN SO THAT THE PLAYING FIELD WILL BE SIMILAIR TO THE ├64 VERSION. ╚OWEVER, FOR THE TITLE SCREEN WE WILL BE SWITCHING THE 8563 INTO A "40 COLUMN" MODE TO MAKE PROGRAMMING EASIER, IN ADDITION TO EXPANDING THE CHARACTER BIT-MAPPED LOGO. ├HANGING THE ├HARACTERS ----------------------- ┴ LOT OF THE TIMES YOU'LL FIND YOURSELF RE-USING SUBROUTINES AND CODE THAT YOU HAVE PREVIOUSLY CREATED, GRADUALLY, OVER A PERIOD OF TIME BUILDING UP A LIBRARY OF ROUTINES. ╫HEN THINKING THROUGH THE PURPOSE AND INTENT OF THIS ROUTINE ╔ THOUGHT ABOUT POSSIBLY BUILDING IT SO IT WOULD READ A TABLE AND CHANGE THE CHARACTER SET BASED ON THAT TABLE. ╘HE 64/128 CHARACTER SETS WOULD BE THE SAME - THIS ROUTINE WOULD AUTOMATICALLY GENERATE THE EIGHT ADDITIONAL BYTES NEEDED BY THE 8563 IF NEED-BE AND IT WOULD CALL THE APPROPRIATE STORAGE ROUTINE - STORE TO EITHER THE 8563 OR THE COMPUTER MEMORY. ╬OW YOU MAY BE ASKING WHY WOULD YOU WANT TO STORE TO THE COMPUTER MEMORY IN 128 MODE? ╫HY NOT JUST HAVE TWO SEPERATE VERSIONS? - ┘ES - THAT COULD BE POSSIBLE BUT ╔'M IMPLEMENTING IT THIS WAY BECAUSE IN THE FUTURE ╔ MAY SEE A NEED TO DEFINE CUSTOM CHARACTERS IN 128 MODE FOR THE 40 COLUMN SCREEN. ╘HIS WAY ╔ CAN JUST EXTRACT THE ROUTINE, POP IT INTO MY PROGRAM AND ╔'VE GOT THAT SECTION OF THE CODE COMPLETE. ╘HIS IS WHAT ╔ WAS THINKING OF FOR THE DATA TABLE: .BYT 1 = 8563, 0 = COMP. MEMORY. .WORD ADDRESS ; ADDRESS BASE OF CHAR-SET IN COMPUTER OR 8563 MEMORY. .BYTE CHAR # ; (TO START) .BYTE # OF CHARS TO DEFINE .BYTE # OF CHARACTERS TO DEFINE .BYTE DATA,DATA,....,DATA8 ; CHARACTER DATA. .BYTE DATA,DATA,....,DATA8 ; CHARACTER DATA. ETC.... . . . ┼NTRANCE INTO THE ROUTINE WILL CONSIST OF .┴┘ HOLDING THE LOCATION OF THE TABLE. ╫E WILL KEEP THE ADDRESS OF THE TABLE AND KEEP INCREMENTING IT AS WE GO ALONG IN Z-PAGE LOCATIONS. ------------------------------------------------------------------------------- INSTALL'CHAR = * STA ZP1 ; SAVE .AY IN TABLE ADDRESS STY ZP1+1 LDY #$00 ; READ COMPUTER MODE. JSR GET'BYTE STA MODE JSR GET'BYTE ; GET ADDRESS BASE. STA ADR JSR GET'BYTE STA ADR+1 JSR GET'BYTE ; GET NUMBER OF CHARACTERS TO COPY. STA NUMB JSR GET'BYTE ; GET NEXT CHARACTER #. STA WRK ; SAVE IN TEMP. LOCATION. LDA #$00 STA WRK+1 ASL WRK ; SHIFT LEFT X3 TIMES = *8 ROL WRK+1 ASL WRK ROL WRK+1 ASL WRK ROL WRK+1 LDA MODE ; IF FOR 8563 THEN MULTIPLY 1 MORE TIME. BEQ + ASL WRK ROL WRK+1 + LDA ADR ; ADD CHARACTER ADDRESS IN. CLC ADC WRK STA WRK LDA ADR+1 ADC WRK+1 STA WRK+1 ; ADDRESS NOW CALCULATED JSR SETADRS ; SET ADDRESS IN PROPER CHIP LOOP'INSTALL LDX #$08 ; COPY 8 BYTES. - JSR GET'BYTE JSR WRITEBYTE ; WRITE OUT BYTE. DEX BNE - LDA MODE ; IF 128 THEN FILL OUT 8 MORE $00 BYTES. BEQ + LDA #$00 LDX #$08 - JSR WRITEBYTE DEX BNE - + DEC NUMB BNE LOOP'INSTALL RTS ------------------------------------------------------------------------------- ╫HAT? ╫E HAVE THRE SUBROUTINES : WRITEBYTE, SETADRS, AND GET'BYTE THAT WE HAVEN'T EXAMINED YET. ╘HESE ARE GOING TO BE THE ROUTINES THAT ARE DEPENDANT ON THE COMPUTER TYPE. ┴LSO, WRITEBYTE WILL REQUIRE THAT .╪┘ NOT BE DISTURBED; SETADRS REQUIRES THAT .┘ NOT BE DISTURBED HENCE THE FOLLOWING: ------------------------------------------------------------------------------- SETADRS TYA ; SAVE .YX PHA TXA PHA LDA MODE ; CHECK COMPUTER TYPE. BEQ + ; IF ├=64, THEN JUMP AHEAD. LDX #18 ; ╓─├ REGISTER - CURRENT MEMORY ADDRESS HI LDA WRK+1 ; GET ADDRESS HI JSR WR'VDC LDX #19 ; ╓─├ REGISTER - CURRENT MEMORY ADDRESS LO LDA WRK ; GET ADDRESS LO JSR WR'VDC + PLA ; RESTORE .╪┘ TAX PLA TAY RTS ; AND RETURN. ------------------------------------------------------------------------------- ╬OTE THAT WE REALLY DON'T NEED A SETADRS FOR THE ├=64 -- WE CAN JUST INDEX OFF (WRK) IN THE WRITEBYTE ROUTINE WHICH FOLLOWS: ------------------------------------------------------------------------------- WRITEBYTE STA TEMP ; SAVE AS WE NEED IT LATER. TXA ; ╙AVE .╪┘ PHA TYA PHA LDA MODE ; NOW CHECK COMPUTER TYPE. BEQ + ; IF C64 JUMP AHEAD LDA TEMP ; RECALL TEMP. JSR WR'VRAM SEC BCS ++ ; JUMP AHEAD + LDY #$00 ; ├64 / Y-INDEX = $00 LDA TEMP ; GET VALUE STA (WRK),Y ; STORE INC WRK ; NOW INCREASE ADDRESS BNE + INC WRK+1 + PLA ; NOW RETURN AFTER RECALLING .╪┘ TAY PLA TAX RTS ; AND RETURN. ------------------------------------------------------------------------------- ╬OTE THAT THE FOLLOWING ROUTINE IS FAIRLY SHORT BUT IT IS CALLED NUMEROUS TIMES WITHIN THE ROUTINES THAT USE DATA TABLES SUCH AS INSTALL'CHAR, WRITE'TXT AND WRITE'COL. ------------------------------------------------------------------------------- GET'BYTE = * LDA (ZP1),Y INY BNE + ; IF ZERO THEN INCREASE ZP1 HI INC ZP1+1 + RTS ------------------------------------------------------------------------------- ╬OT BAD 'EH? ┴ QUICK NOTE: ╘HE INSTRUCTIONS: ╨╠┴, ╘┴┘, ╨╠┴, ╘┴╪, ╨╚┴, ETC.. ARE ROUTINES THAT ╨USH OR ╨ULL (PHA,PLA) THE .┴ ONTO THE STACK. ╘HE ╘┴┘, ╘┴╪, ╘╪┴, ╘┘┴ ARE INSTRUCTIONS THAT TRANSFER A REGISTER TO ANOTHER (IE: THE ╘┴┘ TRANSFERS THE ┴ REGISTER TO .┘, ╘╪┴ TRANSFERS .╪ TO .┴ ETC...) ┬Y USING THE COMBINATION OF THESE WITH THE STACK WE CAN SAVE THE REGISTERS AND LATER RE-CALL THEM SO THAT THEY ARE THE SAME WHEN WE ENTERED THE ROUTINE. ╘HE STACK IS USUALLY A "MYSTERY" ITEM TO NEW PROGRAMMERS OF THE 6502 SERIES. ┬ASICALLY IT'S JUST LIKE ANY OTHER STACKS IN THE REAL WORLD - THE LAST ITEM THROWN (╔'M NON-PRACTICING PERFECTIONIST SO ╔ THROW STUFF.. ;-) ) OR PUSHED ON THE STACK WILL THE FIRST ITEM REMOVED OR PULLED FROM THE STACK. ╞OR EXAMPLE ╔'VE GOT A STACK OF BOOKS SITTING NEAR ME : ═APPING THE ├OMMODORE 128 128 ╔NTERNALS AND ╔'M HOLDING ═APPING THE ├OMMODORE 64 IN MY HANDS. ╔F ╔ PUSH (OR TOSS) THE BOOK ONTO THE STACK (AND HOPEFULLY HIT THE STACK INSTEAD OF THE FLOOR) ╔'LL HAVE THE FOLLOWING STACK: ═APPING THE ├OMMODORE 64 ═APPING THE ├OMMODORE 128 128 ╔NTERNALS AND IT SHOULD BE EASY TO SEE THAT IF ╔ "PULL" THE NEXT BOOK OFF THE STACK THAT ╔'LL GET THE ═APPING THE ├OMMODORE 64 BOOK. ╘HE NEXT BOOK TO BE "PULL"ED AFTER THAT WOULD BE THE ═APPING THE ├OMMODORE 128 BOOK. ╘HIS IDEA CAN BE APPLIED TO THE 6502 STACK -- ╔T WILL KEEP STORING VALUES (UP TO 256) WHEN YOU "PUSH" THEM ON (VIA THE ╨╚┴ INSTRUCTION) AND WILL RETRIEVE THE LAST VALUE STORED WHEN YOU "PULL" THEM OFF (VIA THE ╨╠┴ INSTRUCTION). ┴NOTHER ╨╠┴ INSTRUCTION WOULD RETURN THE NEXT VALUE THAT HAD BEEN STORED. ╘HE ├HARACTER ┬ITMAPS --------------------- ╨ASI ╧JALA IS TO BE CREDITED WITH ALL THE GRAPHICS AND MANY THANKS GO OUT TO HIM. ╘HE GAME LOGO IS MADE UP OF 120 CUSTOM DEFINED CHARACTERS THAT WILL BE PRINTED IN THE FOLLOWING MANNER (ON THE 128 SCREEN THEY WILL BE CENTERED). (IN REVERSE VIDEO)... ┴┬├─┼╞╟╚ . . . [UP TO 40 CHARACTERS] ╔╩╦╠═╬╧╨ ╤╥╙╘╒╓╫╪ AND EVERYTHING WILL LINE UP. ╙O THAT IT WILL LOOK LIKE A "MINI-BITMAP". ╫E COULD HAVE USED BITMAP MODE AND MADE A VERY NICE LOOKING TITLE SCREEN BUT THAT WOULD HAVE INVOLVED SWITCHING AND ALLOCATING MEMORY FOR THE BITMAP, ETC . . . ╧N BOTH THE 8563 AND THE ╓╔├ THAT INVOLVES A BIT MORE WORK AND SO CUSTOM CHARACTERS WILL BE USED FOR THE TITLE SCREEN. ╘HE REGULAR LETTER AND NUMERIC CHARACTERS WILL BE AVAILABLE SO THAT WE CAN DISPLAY CREDITS AND GAME INSTRUCTIONS BELOW THE LOGO. ╬OW - IN THE PROGRAM LISTING WE COULD LIST THEM AS BINARY #'S AND THAT WOULD MAKE EDITING THEM VERY EASY BUT WE'RE GONNA USE THEIR DECIMAL REPRESENTATION IN THE PROGRAM LISTING. ╘HE CHARACTERS ARE DEFINED SIMILAIR TO THE LOGO EXCEPT THEY ARE TREATED AS SINGLE CHARACTERS. ╔N THE 128 VERSION DUE TO THE 80 COLUMN SCREEN WE ARE GOING TO USE TWO CHARACTERS SIDE BY SIDE TO SIMULATE ONE ALIEN SO THAT THE PLAYING FIELD WILL BE SIMILAIR TO THE ├64 VERSION. ╔N ADDATION, DURING THE MAIN LOOP WE WILL MODIFY THE CHARACTER SETS TO SUPPORT ANIMATION OF THE ALIENS. ╔N THE DATA LISTING THERE IS A REFERENCE TO "FRAMES" - FOR EACH OF THE ALIENS THERE ARE 8 DIFFERANT FRAMES. ╧H! - ╘HERE WILL BE MORE CHARACTERS DEFINED IN THE FUTURE. ╥IGHT NOW ╔'M MAINLY INTERESTED IN GETTING SOME BASE CHARACTERS DOWN SO YOU CAN SEE HOW CUSTOM CHARACTERS ARE IMPLEMENTED. ╫HEN WE START SETTING UP DIFFERENT LEVELS AND SUCH WE'LL ADD MORE CHARACTERS THEN. ├URRENTLY THE CUSTOM CHARACTERS ARE NOT USED - ONLY THE CHARACTERS FOR THE LOGO. ╞OR THOSE OF YOU WHO ARE CURIOUS TRY INSTALLING THE CHARACTERS VIA INSTALL'CHAR AND TAKING A LOOK AT THE ALIENS. ╓╔╔. ╘ITLE ╙CREEN ------------ ╘HE TITLE SCREEN IS USUALLY A LEAD-IN TO THE ACTUAL GAME AND IT'S AIM IS TO TELL THE PLAYER HOW TO PLAY THE GAME, ANY AVAILABLE OPTIONS AND P'HAPS PRESENT A NICE GRAPHIC OR TWO TO "WOW" THE USER INTO PLAYING. ╔N ADDATION, THE MAIN MUSICAL THEME CAN BE INTRODUCED HERE TO UNIFY THE GAME-PLAYING. ╘HE DISCUSSION BELOW DOES NOT TAKE INTO ACCOUNT COLOR BUT REST-ASSURED WE WILL BE USING VARYING COLORS IN THE TITLE SCREEN. ╘HE FORMAT FOR THE COLOR DATA WILL BE ALMOST IDENTICAL TO THE TITLE SCREEN FORMAT EXCEPT IT WILL BE STRUCTURED VIA THE FOLLOWING: .WORD ADDRESS .BYTE NUM_OF_CHARS TO PUT COLOR ($00= END OF DATA) .BYTE COLOR_VALUE ╘HE ROUTINE (COLOR'TEXT) CAN BE FOUND IN THE SOURCE LISTINGS AT THE END OF THIS ARTICLE. ┬ECAUSE OF THE SIMILARITY BETWEEN IT AND WRITE'TEXT IT IS NOT DISCUSSED IN THIS ARTICLE.