╔ HAVE SEEN SO LITTLE INSTRUCTIONAL INFORMATION CONCERNING RELATIVE FILES AND ├┬═'S ─╧╙ V2.6 THAT ╔ AM BEGINNING TO WORRY THAT FUTURE VERSIONS OF THE 1541 MAY ABANDON THEM AS INCOMPATIBLE WITH ├64 USERS! ╘HIS FEAR BECOMES MORE PRONOUNCED WHEN ╔ NOTICE THAT SEVERAL COMMERCIAL DATABASE PROGRAMS WRITTEN FOR THE ├64 VIRTUALLY THROW OUT THE ─╧╙ AND REPLICATE FEATURES OF THE RELATIVE FILE HANDLER WITHIN THE ├64, ACCESSING THE DISK WITH RANDOM-ACCESS TECHNIQUES AT THE EXPENSE OF POWER, SPEED, AND CAPACITY. ╫HAT HAS APPEARED IN PRINT OVER THE LAST FEW YEARS SUGGESTS THAT 64/1541 PROGRAMMERS HAVE NOT BEEN PREPARED TO TAKE FULL ADVANTAGE OF THEIR POWER: IS IT THE SAD STATE OF AVAILABLE INSTRUCTIONS, OR THE BUG THAT OCCASIONALLY--AND PREVENTABLY--CRASHES A RELATIVE DATABASE?
╨ERSONALLY, ╔ SUSPECT THAT SOMEONE WHO SETS OUT TO PROGRAM A COMPUTER IS NOT LIKELY TO BE INTIMIDATED BY A SLIGHTLY MORE COMPLEX COMMAND STRUCTURE THAN "╔╬╨╒╘#7,╔$" AND WOULD USE RELATIVE FILES WHEN THEY ARE THE APPROPRIATE FILETYPE. ╙O HERE IS A THOROUGH DISCUSSION OF RELATIVE FILES--WHAT THEY ARE, HOW THEY ARE CREATED, HOW TO HANDLE AND DEVELOP THEM, AND THE BUG: WHAT IT DOES AND HOW TO AVOID IT. ═Y DISSECTION OF THE BUG WAS PRESENTED IN ╘HE ╘RANSACTOR, ╓OLUME 7, ╔SSUE 04, AND WILL NOT BE REPEATED HERE: IN THE EVENT THAT YOU DISCOVER SOME OTHER OMISSION, SEND IT TO ME WITH A DISK IN A REUSEABLE MAILER WITH RETURN POSTAGE AND ╔'LL RETURN THE DISK WITH MY FAVORITE ╨─ UTILITIES AND A SIGNED LETTER OF THANKS SUITABLE FOR FRAMING.
╥┴╬─╧═ ┴├├├┼╙╙ ╞╔╠┼╙
┴ RELATIVE FILE IS ESSENTIALLY A SERIES OF RECORDS ALL OF THE SAME LENGTH. ╘HIS "FIXED-LENGTH" FORMAT IS ACTUALLY AN INDUSTRY-WIDE STANDARD, DATING BACK TO THE DAYS OF THE 80-COLUMN PUNCH-CARD, WHEN MACHINES LESS POWERFUL THAN THE 64 FILLED UP ENTIRE ROOMS: MAINFRAMES ACCESSED DATA IN 128-BYTE "FILE BLOCKS," AND MOST DISK OPERATING SYSTEMS DESIGNED FOR MICROCOMPUTERS JUST FOLLOWED IN ┬IG ┬LUE ┬ROTHER'S FOOTSTEPS. ├╨/═, ═╙-─╧╙ AND MOST DISK-BASED INFORMATION SYSTEMS ARE SET UP FOR RETRIEVAL OF FIXED-LENGTH FILE BLOCKS, WHICH ARE THEN DIVIDED WITHIN THE COMPUTER TO DELIVER THE DESIRED BYTES: A LARGE CHUNK OF ╥┴═ IS USED JUST TO KEEP TRACK OF IT ALL, AND A HUGE CHUNK OF THE DISK IS USED FOR THE SAME PURPOSE.
├╨/═, FOR EXAMPLE, ALLOCATES DISK SPACE IN 1╦ OR 2╦ CHUNKS, USUALLY USES AT LEAST THREE FULL TRACKS FOR DIRECTORY AND SYSTEM PURPOSES, AND THEN STARTS EATING AWAY COMPUTER SPACE FOR A DISK ACCESS SYSTEM (THE "BIOS") TO READ THE DIRECTORIES AND FETCH THE FILE BLOCKS. ╘HE MINIMUM ├╨/═ ACCESS UNIT ("FILE BLOCK") IS USUALLY 128 BYTES OR AN ENTIRE SECTOR. ═╙/─╧╙ IS EVEN WORSE--THE SMALLEST ACCESS UNIT IS 512 BYTES, AND SO MUCH OF THE ├╨╒'S ╥┴═ IS TAKEN UP BY THE DISK OPERATING SYSTEM THAT DISK SPACE IS USED FOR TASKS USUALLY PERFORMED IN ╥┴═. ╘HE "VIRTUAL MEMORY" (MEMORY OUTSIDE THE DIRECT REACH OF THE ├╨╒, I.E., ON DISK) ACCESSED BY ═╙/─╧╙ IS ACTUALLY A SYMPTOM OF THE PROBLEM: THE COMPUTER'S MEMORY IS EATEN UP BY THE DISK OPERATING SYSTEM! ╙O THESE FIXED LENGTH FORMAT SYSTEMS HAVE A WELL-DESERVED REPUTATION FOR BEING WASTEFUL IN MEMORY USE, BOTH ON THE DISK AND IN THE COMPUTER.
╧BVIOUSLY, THEY MUST HAVE SOME OTHER ADVANTAGES: ├OMMODORE INCREASES THOSE ADVANTAGES BY ELIMINATING THE DISADVANTAGES OTHER SYSTEMS ENDURE. ╘HE 1541 IS ACTUALLY A COMPUTER WITH 2╦ OF ╥┴═ AND A 16╦ ╥╧═ OPERATING SYSTEM: EXTREMELY RELIABLE AND EFFICIENT FILES MANAGEMENT ROUTINES MAKE UP OVER 75% OF THE OPERATING SYSTEM, AND NO ├64 ╥┴═ IS CONSUMED FOR A DISK ACCESS SYSTEM. ╘HE 1541 USES ONLY ONE TRACK FOR DIRECTORY AND TWO BYTES PER SECTOR FOR FORWARD TRACK AND SECTOR POINTERS; NOR IS THE 1541 LIMITED TO ONLY ONE OR TWO SIZES OF FILE BLOCK, BUT CAN ACCESS ONE BYTE OR ONE SECTOR WITH EQUAL EASE.
═OST DRAMATICALLY, THE 1541 USES ONLY SIX SECTORS TO TRACK A FIXED-LENGTH FILE THAT FILLS UP THE REST OF THE DISK, MAINTAINED BY ─╧╙ "ON THE SIDE" RATHER THAN REQUIRING A FULL TRACK PLUS COMPUTER MEMORY: A TOTAL OF 4.4% OF THE DISK AND NONE OF THE ├╨╒'S ╥┴═. ╫ITH A LITTLE INGENUITY, THIS CAN BE REDUCED TO 1.9% OF THE DISK AND NO ├╨╒ ╥┴═--BUT IN THIS GUIDE, WE'LL STICK TO THE NORMAL ─╧╙. ╔N ANY CASE, ALL OF THE MEMORY USE INEFFICIENCIES OF FIXED-LENGTH FILES ARE RESOLVED WITH ├OMMODORE RELATIVE FILES--WHICH WOULD BE THE INDUSTRY STANDARD IF NOT FOR THE REAL WORLD, WHICH LOVES ITS NOTION THAT A MICROCOMPUTER SHOULD ACT LIKE A MINIATURE MAINFRAME.
╘╚┼ ╥┼╠┴╘╔╓┼ ╞╔╠┼
┴ ├OMMODORE RELATIVE FILE IS ACTUALLY THREE FILES MADE UP OF FIXED-LENGTH RECORDS, TWO OF WHICH ARE MAINTAINED AND USED ONLY BY THE ─╧╙. ╘HE FIRST FILE, THE RECORDS FILE ACCESSED BY THE PROGRAMMER, IS SUPPORTED BY THE ─╧╙ WITH TWO MORE FILES KEPT IN "SIDE SECTORS" THAT ARE INVISIBLE TO THE ├64 AND OF NO CONCERN FOR THE PROGRAMMER. ╘HE SIDE SECTOR FILE LISTS, IN ORDER, THE SECTORS HOLDING THE RECORDS: SINCE THE RECORDS ARE THE SAME SIZE, ─╧╙ CAN CALCULATE THE LOCATION OF ANY RECORD RELATIVE TO THE BEGINNING OF THE FILE AND FIND IT ON THE DISK IN A FRACTION OF A SECOND, USING THE LIST OF SECTORS IN THIS FILE. ┴NY BYTE OR SERIES OF BYTES WITHIN ANY PARTICULAR RECORD (SAY THE SECOND TEN CHARACTERS) CAN BE READ WITH THIS SAME SPEED.
╘HE THIRD FILE IS A LIST OF SIDE SECTORS, WHICH APPEARS AT THE BEGINNING OF EACH SIDE SECTOR. ╙INCE ─╧╙ HAS ONE OF THE SIDE SECTORS IN AN INTERNAL BUFFER WHENEVER A RELATIVE FILE IS OPEN, THIS LOOK-UP TABLE ALLOWS ─╧╙ TO DIRECTLY FETCH THE SIDE SECTOR CONTAINING THE LOCATION OF A DESIRED RECORD, WITHOUT HAVING TO TRACK THROUGH THE FORWARD POINTERS OF THE PRECEDING SIDE SECTORS. ╔N OTHER WORDS, THIS 12-BYTE FILE (ALWAYS IN 1541 ╥┴═) LISTS THE SIDE SECTORS, WHICH LIST THE RECORD SECTORS; AND ONE OF THE SIDE SECTORS, LISTING 120 RECORD SECTORS, IS ALREADY IN 1541 ╥┴═: THE PROGRAMMER MERELY CALLS FOR OR WRITES TO ALL OR SOME OF A PARTICULAR RECORD AND THE ─╧╙ HANDLES THE CALL.
╫ITHIN THE ACTUAL RECORDS, RELATIVE FILE DATA IS PLACED IN ALLOCATED "FIELDS"--EACH A PROGRAMMER-SPECIFIED SERIES OF BYTES WITHIN THE RELATIVE RECORD. ┼ACH DATA ITEM IS ALWAYS LOCATED IN THE LEFTMOST PORTION OF ITS FIELD IN EVERY RECORD--THUS EACH DATA ITEM CAN BE ACCESSED INDEPENDENTLY OF THE OTHER DATA FIELDS IN THE RECORD. ╘HIS SEEMINGLY RIGID STRUCTURE TRANSFORMS THE SURFACE OF EACH DISK INTO NEAR-VIRTUAL MEMORY: IT IS THIS USER-DESIGNED RECORD STRUCTURE, COMBINED WITH THE INTELLIGENT ─╧╙, THAT MAKES THE RELATIVE FILE THE MOST FLEXIBLE AND POWERFUL FORM OF DATA STORAGE.
┴ RELATIVE FILE DISK CAN HAVE UP TO 65,535 DISTINCT RECORDS (OF TWO CHARACTERS EACH), OR FEWER RECORDS WITH LENGTHS UP TO 254 CHARACTERS, CONTAINING OVER 163╦ OF DATA: RETRIEVAL OF ANY RECORD TO THE ├64 SCREEN FROM ┬┴╙╔├ TAKES AN AVERAGE 1.2 SECONDS (1.13 SECONDS WITH MACHINE LANGUAGE), AND RETRIEVAL OF ANY CHARACTER IN ANY RECORD TAKES LESS THAN A SECOND. ═EMORY-RESIDENT INDICES INTO THE RELATIVE FILE ENABLE RAPID SEARCHES ON ANY FIELDS SELECTED BY THE PROGRAMMER: ANY INDEXED DATA IN EVEN A 163╦ FILE CAN BE FOUND IN 30 SECONDS OR LESS.
╧N-LINE ACCESS TO INFORMATION STORED ON SEVERAL DISKS IS LIMITED ONLY BY THE CAPACITY OF ├64 MEMORY: A SEARCH THROUGH A DATABASE RESIDING ON FOUR DISKS IN FOUR DRIVES (ABOUT 2/3 OF A MEGABYTE) TAKES ONLY SLIGHTLY LONGER THAN SEARCHES OF A SINGLE DISK, BUT TWO BYTES OF ├64 ╥┴═ ARE REQUIRED FOR EACH INDEX ENTRY. ╞OR PERSPECTIVE, THIS MEANS THAT A 6╦ ┬┴╙╔├ PROGRAM FOR THE ├64 WOULD BE LIMITED TO ABOUT 16,000 LISTINGS INDEXED ON A SINGLE FIELD, AND MACHINE LANGUAGE COULD INCREASE THIS ONLY TO ABOUT 25,000: OR A FOUR-DISK MAILING LIST OF 8000 LISTINGS INDEXED BY NAME, CITY AND ┌╔╨ CODE. ╞OR ADDITIONAL PERSPECTIVE, THE ╓┴╪ MAINFRAMES AT THE LOCAL LIBRARY TAKE LONGER TO FIND A SINGLE BOOK TITLE THAN ╘HE ═AIL╥OOM (WHICH WE WILL EXAMINE) TAKES TO FIND A SERIES OF LISTINGS FOR MAILING LABELS: THE REAL DIFFERENCE MADE BY THE ABILITY TO FETCH A SINGLE BYTE RATHER THAN A FILE BLOCK.
╙INCE RELATIVE FILES ARE DESIGNED TO BE THE CORE OF A COMPREHENSIVE DYNAMIC DATABASE SYSTEM, WE WILL EXAMINE THAT SYSTEM AND THE DETAILS OF ITS WORKINGS, AS IMPLEMENTED IN MY ═AIL╥OOM V1.0. ╘HE ═AIL╥OOM MANAGES DYNAMIC ONE-DISK MAILING LISTS, EACH CONSISTING OF 2000 LISTINGS INDEXED BY NAME, CITY, AND ┌╔╨ CODE, RETRIEVABLE IN 25 SECONDS OR LESS: ITS SUBROUTINES ILLUSTRATE THE GUIDE WITH A WORKING APPLICATION.
├╥┼┴╘╔╬╟ ╘╚┼ ╞╔╠┼
╘HE COMPOSITION OF A RELATIVE RECORD IS DETERMINED BY THE PROGRAMMER--IT CAN CONSIST ENTIRELY OF DATA BYTES, WITHOUT ANY COMMAS OR CARRIAGE RETURNS TO SEPARATE THE FIELDS OR TERMINATE THE RECORD. ┬┴╙╔├'S ╔╬╨╒╘#, HOWEVER, CAN RETRIEVE A MAXIMUM OF 88 CHARACTERS; AND FOR LONGER RECORDS ┬┴╙╔├ WILL REQUIRE ONE (FOR UP TO 176 DATA BYTES) OR TWO (FOR UP TO 252 DATA BYTES) SEPARATOR BYTES WITHIN THE RECORD. ╘HE ╦┼╥╬┴╠ ROUTINE FOR SERIAL INPUT SUBSTITUTES A CARRIAGE RETURN FOR THE ─╧╙' ┼╧╔ (┼ND ╧F ╔NFORMATION), SO NONE IS NEEDED AT THE END OF THE RECORD.
┴ RELATIVE RECORD MUST BE PLANNED BEFORE THE RELATIVE FILE IS OPENED. ╘HIS MEANS ONLY THAT FIELDS MUST BE ALLOCATED FOR EACH ITEM OF DATA. ╘HE ═AIL╥OOM ALLOCATES 28 CHARACTERS FOR A NAME, 25 FOR AN ADDRESS, 16 FOR A CITY, TWO FOR A ╙TATE, FIVE FOR A ┌╔╨ CODE AND ONE FOR MAILING LIST CONTROLS, FOR A TOTAL OF 77 DATA BYTES IN SIX FIELDS; NO SEPARATORS ARE NEEDED WITHIN THIS SIZE RECORD. 2000 SUCH RECORDS RESIDE ON A SINGLE DISK, WITH ROOM TO SPARE FOR FILE STATISTICS AND THREE COMPLETE INDICES INTO THE RELATIVE DATA. ┼VEN FROM ┬┴╙╔├, ├64 MEMORY EASILY HOLDS ALPHABETICAL INDICES BY NAME AND CITY, AND A THIRD NUMERIC INDEX BY ┌╔╨ CODE, FOR RAPID SEARCH AND RETRIEVAL OF MATCHES ON ANY OR ALL OF THESE THREE FIELDS.
╧NCE THE RECORD IS PLANNED, THE FILE IS OPENED WITH A SYNTAX USED ONLY TO CREATE IT:
╧╨┼╬ ╞╬,─╬,╙┴,"0:╞╔╠┼╬┴═┼,╠,"+├╚╥$(╥╠)
WHERE ╠ (THINK "╠╔╙╘┼─") IS THE FILE TYPE AND ╥╠ IS THE LENGTH OF THE RECORD INCLUDING ANY SEPARATOR BYTES.
┴ FILE CANNOT BE CREATED WITH A NAME CONTAINING AN ASTERISK (*), A COLON (:), OR A QUESTION MARK (?) SINCE THESE ARE "WILD CARDS" TO THE ─╧╙; AND IT HAS BEEN REPORTED THAT ├╚╥$(╥╠) CANNOT EQUAL THESE THREE CHARACTERS, SO THAT RECORD LENGTHS 42, 58 AND 63 CANNOT BE USED. ╘HIS IS NOT THE CASE: THE WILD CARD ERROR #33, ╙┘╬╘┴╪ ┼╥╥╧╥, IS ENCOUNTERED ONLY WHEN THE "0:" IS OMITTED FROM THE ╧╨┼╬ COMMAND, AND THESE RECORD LENGTHS ARE ALLOWED BY THE ─╧╙ WHEN THE "0:" SYNTAX IS USED.
╫HEN AN ╧╨┼╬ COMMAND IS RECEIVED, ─╧╙ WILL IMMEDIATELY ALLOCATE TWO SECTORS--ONE FOR THE FIRST RECORDS, AND ONE SIDE SECTOR TO KEEP A LIST OF RECORD SECTORS. ╬OW WE MUST WRITE TO THE RELATIVE FILE, SO THAT THE LIST WILL GET STARTED.
╘╚┼ ╨╧╙╔╘╔╧╬ ├╧══┴╬─
╘HE ACTUAL WRITE TO A RELATIVE FILE USES THE SAME ╨╥╔╬╘# COMMAND AS ANY OTHER WRITE OPERATION. ╫ITH RELATIVE FILES, HOWEVER, THE WRITE GOES TO A SPECIFIC RECORD WITHIN THE FILE: ─╧╙ HAS TO BE POSITIONED TO THE RECORD, AND TO THE SPOT WITHIN THAT RECORD, WHERE YOU WANT TO BEGIN WRITING. ╘HIS IS DONE WITH THE "POSITION" COMMMAND, SENT ON THE ─╧╙ COMMAND CHANNEL. ╘HIS CHANNEL IS OPENED WITH THE SYNTAX:
╧╨┼╬ ╞╬,─╬,15 (A DIFFERENT ╞╬ FROM THE RELATIVE FILE)
AND THE "POSITION" COMMAND IS SENT TO THIS FILE, NOT TO THE RELATIVE FILE. ╘HE ACTUAL INFORMATION TO BE WRITTEN TO THAT RECORD IS THEN SENT TO THE RELATIVE FILE. ╘HE POSITION COMMAND IS SENT WITH THE SYNTAX:
╨╥╔╬╘# ╞╬, "╨" ├╚╥$(╙┴) ├╚╥$(╠╧)├╚╥$(╚%) ├╚╥$(╨);
WHERE "╨" IS THE ACTUAL "POSITION" INSTRUCTION, FOLLOWED BY THREE PARAMETERS AND A SEMICOLON (";") TO SUPPRESS THE SENDING OF A CARRIAGE RETURN AFTER THE COMMAND STRING.
├╚╥$(╙┴) SENDS ─╧╙ THE SECONDARY ADDRESS (╙┴) OF THE RELATIVE FILE ╧╨┼╬ COMMAND, WHICH IS USED BY ─╧╙ TO ASSIGN INTERNAL CHANNELS AND BUFFERS FOR THE RELATIVE FILE OPERATIONS: THIS VALUE IS SOMETIMES ╧╥'D WITH 96 ($60) TO FORM THE BYTE SENT TO THE ─╧╙. ├╚╥$(64+╙┴) WILL ALSO WORK, WHICH ALLOWS THE USE OF THE EASIER SYNTAX
╨╥╔╬╘# ╞╬, "╨┬" ├╚╥$(╠╧)├╚╥$(╚%) ├╚╥$(╨);
FOR A SECONDARY ADDRESS OF 2, SINCE "┬" IS ├╚╥$(64+2).
├╚╥$(╠╧) AND ├╚╥$(╚%) ARE ONE PARAMETER, THE RECORD NUMBER: ╠╧ IS THE LOW BYTE OF THE RECORD NUMBER IN LOW-BYTE/HIGH-BYTE FORMAT, "╚%" IS THE HIGH BYTE. ╘HE "╚%" AND "╠╧" OF A VALUE "╬" ARE MOST EASILY TAKEN BY
╚%=╬/256:╠╧=╬-╚%*256
USING "╚%" INSTEAD OF A FLOATING POINT VARIABLE, WHICH AVOIDS ONE CALL TO THE ┬┴╙╔├ INTERPRETER; "╚%" IS A VALID VARIABLE WITHIN THE ├╚╥$() COMMAND.
├╚╥$(╨) IS THE EXACT POSITION WITHIN THE RELATIVE RECORD WHERE THE WRITE IS TO BEGIN, AND IS AN OPTIONAL PARAMETER. ╚OWEVER, UNLESS YOU SUPPRESS THE CARRIAGE RETURN THAT FOLLOWS THE COMMAND STRING, THE PARAMETER ├╚╥$(╨) MUST BE INCLUDED: OTHERWISE, ─╧╙ WILL READ THE ├╚╥$(13) CARRIAGE RETURN AS THE PARAMETER AND POSITION TO THE THIRTEENTH CHARACTER OF THE RECORD.
╫HEN THE POSITION COMMAND IS SENT, ─╧╙ WILL RETRIEVE THE RECORD SECTOR YOU HAVE ADDRESSED INTO ITS ╥┴═ BUFFERS AND POSITION THE RELATIVE FILE CHANNEL TO THE SELECTED POSITION IN THE RECORD.
╘╚┼ ┬╒╟
╘HE "POSITION" COMMAND IS 100% RELIABLE FOR READING THE FILE; FOR WRITING, IT IS 99.9% RELIABLE. ├OMMODORE'S LATEST 1541 ╒SER'S ╟UIDE ALMOST ACKNOWLEDGES THE BUG, CALLING IT A "SUBTLE ERROR (BUG) COMMON TO ALL ├OMMODORE DISK DRIVES" AND ITS BITE A "REMOTE POSSIBILITY." ╔N ACTUALITY, ITS BITE IS A 100% CERTAINTY UNDER CERTAIN CONDITIONS THAT OCCUR WITH INCREASED FREQUENCY IN SMALLER RELATIVE FILES.
╫HEN A WRITE IS PERFORMED THAT RUNS FROM ONE SECTOR (┴) TO THE NEXT (┬), AND A SECOND WRITE IS PERFORMED ENTIRELY IN SECTOR (┬), THEN A THIRD WRITE DIRECTED TO THE FOLLOWING SECTOR (├) WILL GO TO SECTOR (┴). ╒SE OF THE FOLLOWING POSITION ROUTINE ELIMINATES THIS BUG:
╠INE 70 IS THE WRITE ENTRY POINT: IF THE LAST CALL SET THE JEOPARDY FLAG ╩, THEN IT SETS THE JEOPARDY COUNT ╩├ ACTIVE FOR TWO CALLS AND IDENTIFIES ╩┴ AND ╩┬ AS THE JEOPARDIZED RECORD SECTORS.
╠INE 90 CALCULATES THE END OF THE CURRENT RECORD WITHIN THE SECTOR AND, IF A SPLIT RECORD, THE ACCESS POSITION; AND SETS THE JEOPARDY FLAG ╩ WHEN THE ACCESS SPANS TWO SECTORS, TO BE DETECTED IN ╠INE 50. ╘HIS IS THE READ ENTRY POINT SINCE READS DO NOT NEED PROTECTION BUT DO NEED TO SET THE JEOPARDY FLAG.
╠INE 110 BEGINS THE ACTUAL POSITION ROUTINE, CALCULATING THE HI AND LO BYTES OF THE RECORD NUMBER. ╔F THE JEOPARDY FLAG IS SET, IT CHECKS THE CURRENT ACCESS, POSITIONING ONCE AND SETTING THE WAIT FLAG FOR JEOPARDY SECTORS. ╘HIS IS THE ENTRY TO RETRIEVE A SINGLE CHARACTER, WHICH CANNOT SPAN TWO SECTORS.
╠INE 120 SENDS THE POSITION COMMAND AND, IF THE WAIT FLAG IS SET, WAITS 30 JIFFIES BEFORE RETURNING.
╫ITH ╙HILOH'S ╥AID IN PLACE, THE POSITION COMMAND IS 100% RELIABLE.
╫╥╔╘╔╬╟ ╘╧ ╘╚┼ ╞╔╠┼
╫RITING A RECORD REQUIRES ATTENTION TO THE WAY IT WILL BE ACCCESSED, JUST AS IN SEQUENTIAL FILES. ╞OR RELATIVE FILES, WE HAVE TO MAKE SURE THAT THE OUTPUT STRING HAS THE VARIOUS FIELDS PLACED PROPERLY WITHIN THE STRING. ╞OR THIS WE NEED A "PADDING" STRING, MADE UP OF SPACES, EQUAL IN LENGTH TO THE LONGEST FIELD. ╘HIS IS EASILY SET UP DURING THE INITIALIZATION USING A LOOP,
╞╧╥ ╘=1 ╘╧ ╠┼╬╟╘╚:┬$=┬$+" ":╬┼╪╘
AND USED WITH EVERY INPUT FIELD: ╬$=╠┼╞╘$(╬$+┬$,29):┴$=╠┼╞╘$(┴$+┬$,25); ETC.
AND THEN ADDING THEM ALL TOGETHER, TOGETHER WITH ANY INTERNAL SEPARATOR,
╧$=╬$+┴$+├$+╙$+┌$
INTO A SINGLE STRING TO WRITE TO THE RECORD. ╞ORTUNATELY, THIS DOES NOT CONSUME AS MUCH MEMORY AS IT APPEARS TO, SINCE THE PROCESSING TAKES PLACE USING THE TEMPORARY STRING DESCRIPTOR STACK AND ONLY THE FINAL RESULTS OF A CONCATENATION ARE FINALLY ALLOCATED FROM FREE MEMORY. ╞OLLOWING A POSITION COMMAND, A SECOND COMMAND IS DIRECTED TO THE RELATIVE FILE TO WRITE TO IT, IN THE SYNTAX
╨╥╔╬╘#2,╧$;
WHERE ╧$ CONTAINS ALL THE DATA THAT MUST BE WRITTEN, UP TO THE END OF THE RECORD. ╘HE FINAL SEMICOLON (";") SUPPRESSES THE SENDING OF A FOLLOWING CARRIAGE RETURN.
╙EVERAL ADDITIONAL THINGS NEED TO BE CONSIDERED WHEN WRITING TO A RELATIVE FILE.
╫HEN YOU ATTEMPT TO ACCESS A RECORD THAT HAS NOT YET BEEN CREATED BY THE ─╧╙, THE POSITION COMMAND WILL PRODUCE AN ERROR #50, ╥┼├╧╥─ ╬╧╘ ╨╥┼╙┼╬╘. ┴LTHOUGH THIS ERROR CAN BE SAFELY IGNOREDWHEN WRITING TO THE FILE, IT IS BETTER FOR SEVERAL REASONS TO AVOID IT IN THE FIRST PLACE. ╘HIS IS DONE BY FORCING THE CREATION OF AS MANY EMPTY RECORDS AS YOU EXPECT TO NEED IN THE FILE IMMEDIATELY UPON CREATING THE FILE. ╫HEN YOU "POSITION" THE ─╧╙ TO THE HIGHEST RECORD IN THE INTENDED FILE, AND THEN WRITE TO THAT RECORD, THE ATTEMPT TO WRITE FORCES ─╧╙ TO CREATE THE ENTIRE FILE UP TO AND INCLUDING THE LAST RECORD. ╧THERWISE, THE FILE IS CREATED PIECEMEAL AS ADDITIONAL SECTORS ARE NEEDED.
╞OR COMPARISON, IT TAKES 5.5 MINUTES TO WRITE A ├╚╥$(0) TO JUST THE LAST RECORD OF A NEW DISK-SIZED FILE OF 1671 100-CHARACTER RECORDS, THEN 19.5 MINUTES MORE TO WRITE A FULL 100-CHARACTER STRING TO ALL 1671 RECORDS. ╫RITING A 100-CHARACTER STRING TO EACH RECORD IN TURN, EXPANDING THE FILE AS NEEDED UNTIL THE DISK IS FULL, REQUIRES OVER 51 MINUTES: OVER TWICE AS LONG.
├REATING THE FILE IN A SINGLE LARGE BLOCK ALSO MAKES IT COMPACT ON THE SURFACE OF THE DISK RATHER THAN IN SCATTERED PIECES: THIS CAN REDUCE HEADSTEPPING WHEN READING FROM THE FILE. ╙INCE HEADSTEPPING IS A SUBSTANTIAL PORTION OF ACCESSING THE FILE RANDOMLY (FORCING THE USE OF FAST-STEPPING MODE REDUCES ACCESS TIME A FULL 10%), THIS COMPACTNESS CAN BE AN IMPORTANT FACTOR.
╔N THE EVENT THAT YOU ATTEMPT TO CREATE A FILE THAT IS TOO LARGE FOR THE AVAILABLE SPACE ON THE DISK, THE POSITION COMMAND WILL RETURN AN ERROR #52, ╞╔╠┼ ╘╧╧ ╠┴╥╟┼. ╔T IS USEFUL TO KNOW THIS ┬┼╞╧╥┼ YOU HAVE WRITTEN SEVERAL HUNDRED RECORDS TO THE FILE, AND FORCING THE CREATION OF THE ENTIRE FILE SPACE WHEN OPENING IT CAN AVOID THIS CONDITION ALSO.
╘HE SECOND THING IS THAT WHENEVER YOU WRITE TO A RELATIVE RECORD, ─╧╙ WILL WRITE THE CHARACTERS AND THEN WRITE ZEROES UNTIL IT REACHES THE END OF THE RECORD. ╘HUS ALTHOUGH YOU CAN READ ONLY PART OF A RECORD, YOU CANNOT WRITE ONLY PART OF A RECORD: YOU WILL ZERO OUT WHATEVER FOLLOWS IT. ╙UPPRESSION OF A FINAL CARRIAGE RETURN DOES NOT PREVENT THIS, SINCE ─╧╙ TAKES OVER WHEN AN ╒╬╠╔╙╘┼╬ IS RECEIVED AND ZEROES THE REST OF THE RECORD. ╘HIS IS ALSO THE REASON THAT THE OUTPUT STRING IS CONCATENATED AND SENT WITH A SINGLE ╨╥╔╬╘# STATEMENT.
╘HIRD, WHEN WRITING A RECORD, THE OUTPUT STRING SHOULD NOT BE LONGER THAN THE RECORD SIZE, OR ─╧╙ WILL REPORT AN ERROR #51, ╧╓┼╥╞╠╧╫ ╔╬ ╥┼├╧╥─. ╘HE RESULT OF THIS ERROR WILL BE THAT THE RECORD YOU WROTE BECOMES TRUNCATED, UNLESS THE PROGRAM DETECTS AND CORRECTS IT. ╙INCE THE OUTPUT STRING SHOULD REACH THE LAST POSITION IN THE RECORD, THE FINAL CARRIAGE RETURN MUST BE SUPPRESSED WITH A SEMICOLON TO AVOID THIS ERROR.
├╠╧╙╔╬╟ ╘╚┼ ╞╔╠┼
╫HEN YOU HAVE FINISHED WRITING TO A RELATIVE FILE, THE FILE SHOULD BE CLOSED. ╘HIS WILL INSURE THAT ANY CHANGED RECORDS STILL IN THE BUFFERS WILL BE WRITTEN TO THE DISK, AND THAT ANY NEW SECTORS ALLOCATED FOR RECORDS WILL BE ADDED TO THE SIDE SECTOR LISTINGS. ╘HE EASIEST WAY TO CLOSE THE FILE IS TO CLOSE THE COMMAND CHANNEL, WHICH WILL CAUSE ─╧╙ TO CLOSE ALL OPEN FILES IN THE DRIVE; IT ALSO FREES THE INTERNAL CHANNELS OF THE DRIVE, POSSIBLY SAVING AN ERROR #70, ╬╧ ├╚┴╬╬┼╠ ON A NEW ATTEMPT TO ACCCESS THE DRIVE.
┬Y ITSELF, FAILING TO CLOSE THE FILE PROPERLY WILL NOT USUALLY POISON THE FILE OR THE ┬┴═; HOWEVER, SUBSEQUENT DISK ACCESS CAN DESTROY THE ┬┴═, IF ─╧╙ STEALS ITS BUFFER FOR OTHER USE BECAUSE THE THREE RELATIVE FILE BUFFERS ARE STILL ALLOCATED. ╞OR ╘HE ═AIL╥OOM'S 2000 RECORDS PLUS THREE INDICES, WHICH IS A TYPICAL FULL-SCALE APPLICATION, THE INDICES NEED TO BE SCRATCHED AND RE-WRITTEN TO THE DISK AFTER ANY REVISION: FAILURE TO CLOSE THE RELATIVE FILE PRIOR TO THESE OPERATIONS COULD BE DISASTROUS.
╥┼-╧╨┼╬╔╬╟ ╘╚┼ ╞╔╠┼
╧NCE THE FILE IS CREATED, YOU CAN OPEN IT WITH THE SYNTAX
╧╨┼╬2,8,2,"0:╞╔╠┼╬┴═┼"
WITHOUT THE ",╠,"+├╚╥$(╥╠): ─╧╙ KNOWS IT IS OPENING A RELATIVE FILE. ╥EAD AND WRITE ACCESS TO THE FILE IS THE SAME AS IF THE CREATING SYNTAX HAD BEEN USED.
╥┼┴─╔╬╟ ╞╥╧═ ╘╚┼ ╞╔╠┼
╘O READ FROM A RELATIVE RECORD, USE THE SAME "POSITION" COMMAND DESCRIBED ABOVE AND THEN ╟┼╘# OR ╔╬╨╒╘# THE CHARACTERS OR STRING THAT YOU WANT.
┼XCEPT FOR SEARCHES CONDUCTED USING ╟┼╘#, THE FASTEST WAY TO READ RELATIVE RECORDS IS BY USING THE LONGEST INPUT STRINGS POSSIBLE, PREFERABLY THE ENTIRE RECORD AT A SINGLE ACCESS. ╧NCE INSIDE THE ├64, THE INPUT STRING(S) CAN BE DIVIDED INTO THEIR FIELD COMPONENTS USING ╠┼╞╘$, ═╔─$ AND ╥╔╟╚╘$ COMMANDS. ╔N A RECORD LONGER THAN 88 CHARACTERS, ANY INTERNAL SEPARATORS CAN EASILY BE PLACED AT THE END OF A DATA FIELD, SO CONCATENATION CAN BE EASILY AVOIDED WHEN RETRIEVING RECORDS.
┬Y FAR THE MOST POWERFUL ASPECT OF RELATIVE FILES ARISES FROM THE FACT THAT DATA IS CONSISTENTLY SITUATED IN THE SAME PLACE IN EVERY RECORD. ╘HIS MEANS THAT THE FILE CAN BE SEARCHED FOR A PARTICULAR ITEM--A SPECIFIC CITY, FOR INSTANCE--BY SIMPLY POSITIONING TO ITS FIELD AND READING WHAT'S THERE.
╧N-LINE RETRIEVAL POWER COMES IN WHEN WE START INDEXING RECORDS ON ONE OR MORE FIELDS CONTAINING DATA LIKELY TO BE THE SUBJECT OF A SEARCH. ╞OR ╘HE ═AIL╥OOM, THE NAME, CITY, AND ┌╔╨ CODE ARE INDEXED: ├64 MEMORY EASILY HOLDS ALL THREE, SINCE EACH INDEX IS SIMPLY A LIST OF NUMBERS. ╥ECORD NUMBERS SELDOM EXCEED INTEGER LIMITS (32767), AND THREE 2000-ENTRY INDICES OCCUPY LESS THAN 12╦ INSIDE THE 64, HELD IN AN INTEGER ARRAY. ╓1.0 OF ╘HE ═AIL╥OOM USES ┬┴╙╔├ POINTERS TO SAVE OUT AND RELOAD 48 BLOCKS OF ARRAYS AS A PROGRAM FILE TO SAVE TIME AT SETUP AND SHUTDOWN.
╞IXED FIELDS AND RETRIEVAL OF INDEXED RECORDS IS EXACTLY ANALAGOUS TO LOOKING THROUGH A 3X5 CARD DECK OR A ╥OLODEX THAT HAS A NAME AT THE UPPER LEFT, A PRODUCT OR SERVICE AT THE UPPER CENTER, AND A PHONE NUMBER AT THE UPPER RIGHT: ONCE YOU FIND THE ITEM YOU'RE LOOKING FOR, THEN YOU LOOK AT THE REST OF THE CARD. ╒SING MULTIPLE INDICES IS LIKE HAVING TWO OR THREE DECKS OF CARDS ALL WITH THE SAME INFORMATION BUT FILED IN DIFFERENT ORDERS--SOME OFFICES DO EXACTLY THAT. ╙EARCHING INVOLVES GOING TO THE DECK ARRANGED BY THE SUBJECT OF YOUR SEARCH--WITH RELATIVE FILES IT IS THE SAME, EXCEPT THAT WE HAVE LISTS OF RECORD NUMBERS IN MEMORY AND USE THEM TO SEARCH THE ONE FILE ON THE DISK.
╔NDEXING IS THE MOST INTIMIDATING ASPECT OF MASTERING RELATIVE FILES. ╔T IS INSTRUCTIVE TO NOTE THAT ╘HE ═AIL╥OOM IS THE ONLY AVAILABLE DATABASE HANDLER FOR THE ├64 THAT USES MULTIPLY-INDEXED RELATIVE FILES: THE SUBROUTINES ARE BELOW. ╔ AM INDEBTED TO ─R. ╟ERALD ╬EUFELD (╔NSIDE ├OMMODORE ─╧╙, ─ATAMOST, 1984) FOR THE PRINCIPLE OF THE BINARY CHOP SEARCH ALGORITHM, WHICH ALLOWS ╘HE ═AIL╥OOM TO FIND MATCHING NAMES, CITIES AND ┌╔╨ CODES (I.E., A COMPLETE SEARCH ON ALL THREE INDEXED FIELDS) IN ABOUT 30-45 SECONDS FROM A 2000-ENTRY MAILING LIST.
╘HERE ARE FIVE SUBROUTINES:
210 ╞ETCH ONE BYTE FOR COMPARISON
240 ╞ETCH A FIELD FOR COMPARISON
320 ├OMPARE A FIELD
430 ┬INARY CHOP AND COMPARE A BYTE
510 ╙EARCH CONTROL
TO LOOK FOR THE VALUES, IF ANY, OF THE FOLLOWING ARRAY POINTERS:
╠═ (LOW MATCH): THE LOWEST MATCHING FIELD
╚═ (HIGH MATCH): THE HIGHEST MATCHING FIELD
╚╥ (HIGHER RECORD): THE NEXT HIGHER NON-MATCHING RECORD
USING
╠╥ (LOWER RECORD): A LOWER NON-MATCHING RECORD
═$ (MATCH STRING): THE FIELD TO BE FOUND, WITH FIRST CHARACTER ═1$
╔$ (INPUT STRING): FETCHED FROM THE DISK
┴╨ (ARRAY POINTER): POINTS TO THE RECORD NUMBER BEING EXAMINED
╞ (FIELD): IDENTIFIES THE FIELD (NAME, CITY OR ┌╔╨) BEING SEARCHED
┴%(╞,┴╨) IS THE ARRAY ELEMENT HOLDING THE CURRENT RECORD NUMBER
┬%(╞) HOLDS THE POSITION VALUE FOR THE START OF THE FIELD ╞
═╞ (MATCH FLAG): RETURNS AS ┴╨ WHEN A MATCH IS FOUND, 0 IF NO MATCH
╥╞ (RETURN FLAG): TO ESCAPE EARLY FOR TWO SPECIAL SITUATIONS.
╚ (HIGH), THE NUMBER OF RECORDS INDEXED, IS EXTERNAL, AS ARE ═$ AND ╞, THE PARAMETERS PASSED BY THE CALLER. ╔N ADDITION, THERE ARE 8 SPECIAL CASES, 6 OF WHICH ARE LIMITATIONS OF THE BINARY CHOP, WHICH CANNOT "FIND" THE HIGHEST ARRAY ELEMENT OR REACH BELOW THE LOWEST ELEMENT:
═$ IS LOWER THAN THE LOWEST FIELD ╞ IN THE FILE
═$ IS EQUAL TO THE LOWEST FIELD
═$ IS EQUAL TO THE LOWEST ┴╬─ THE HIGHEST FIELDS
═$ IS EQUAL TO THE HIGHEST FIELD
═$ IS GREATER THAN THE HIGHEST FIELD
╘HERE ARE NO RECORDS IN THE FILE
╘HERE IS ONE RECORD IN THE FILE
╘HERE ARE TWO RECORDS IN THE FILE
╫HEN THERE ARE NO RECORDS IN THE FILE, THEN THE FETCH IS GOING TO RETURN EITHER THE ├╚╥$(255) THAT ─╧╙ WRITES TO NEW RELATIVE FILES OR A ├╚╥$(0) RESULTING IN A ╙╘╥╔╬╟ ╘╧╧ ╠╧╬╟ ┼╥╥╧╥. ╘HE ═AIL╥OOM DOESN'T BOTHER, BUT DETECTS THIS CONDITION PRIOR TO THE SEARCH AND OMITS IT FOR THE FIRST RECORD. ╫HEN THERE ARE ONE OR TWO RECORDS IN THE FILE, THE BINARY CHOP CAN ONLY SEE THE FIRST RECORD AND CAN FALL INTO AN ENDLESS LOOP. ╘HE CONTROL ROUTINE MUST HANDLE THESE CASES EITHER BEFORE OR DURING THE BINARY SEARCH.
╘HE SUBROUTINES (┼NTRY IS AT 510-550, DEPENDING ON WHICH FIELD IS BEING SEARCHED):
═ IS A BIT-MAPPED VARIABLE REFLECTING THE RESULTS OF THE SEARCH: IFMAND4 THEN NAME WAS MATCHED; IFMAND2 THEN CITY; IF MAND1 THEN ┌╔╨. ┼ACH FIELD SEARCH RETURNS EITHER A NEGATIVE RESULT OR A RANGE OF ARRAY POINTERS WHICH MATCH THE SEARCH STRING, SUCH THAT ┴%(═╠,╞) THROUGH ┴%(═╚,╞) ARE MATCHES. ╘HE PLACE IN THE ARRAY WHERE THE NEW ENTRY IS TO BE INSERTED IS RETURNED IN ┴%(╔╙,╞). (╘HE ═AIL╥OOM USES HIGH ARRAY ELEMENTS--2001 THROUGH 2004--FOR UTILITY PURPOSES: ═╠=2001, ═╚=2002, ╔╙=2003, AND THE THREE ┴%(2004,╞) ARE USED FOR THE NUMBER OF RECORDS, THE NUMBER OF DE-ALLOCATED RECORDS (BELOW), AND THE NUMBER OF DEFINED LISTS RESPECTIVELY.)
╬EGATIVE SEARCHES OF 2000 RECORDS FOR ONE FIELD REQUIRE A MAXIMUM OF 13 ACCESSES, REQUIRING 9-12 SECONDS; POSITIVE SEARCHES WILL REQUIRE FROM 14 TO 23 TOTAL ACCESSES, DEPENDING ON WHETHER THE FIRST MATCH IS FOUND LATE (FEWER ACCESSES) OR EARLY. ╔N ANY CASE, THE TIME REQUIRED IS FROM 9-20 SECONDS FOR A DISK-SIZED FILE PER INDEXED FIELD. ╘HE ═AIL╥OOM USES THREE, AND CONSISTENTLY FINDS MATCHING ENTRIES IN LESS THAN 45 SECONDS.
┼─╔╘╔╬╟, ╙╘┴╘╔╙╘╔├╙ ┴╬─ ╞╔╠┼ ═┴╬┴╟┼═┼╬╘
┼DITING A RELATIVE RECORD IS A MATTER OF READING IT INTO THE ├64, PERFORMING WHATEVER EDITING IS REQUIRED, AND WRITING IT BACK TO THE SAME PLACE. ╔N THE EVENT THAT AN INDEXED FIELD HAS BEEN EDITED, THEN THE INDICES WILL NEED TO BE UPDATED--THE OLD INDEX ENTRY REMOVED, THE ARRAY CLOSED UP TO FILL THE GAP, THE NEW INDEX ENTRY PROPERLY LOCATED AND ADDED. ╘HIS IS SIMPLE ARRAY MANIPULATION, WHICH IS BEYOND OUR SCOPE HERE; FINDING THE SPOT HAS ALREADY BEEN COVERED.
─ELETION IS EASIER BUT ADDS A NEW ASPECT: TO DELETE FROM INDICES IS A MATTER OF CLOSING THE ARRAYS OVER THE DELETED ELEMENT; BUT WHEN YOU DELETE A RECORD FROM A RELATIVE FILE, IT DOES NOT "CLOSE UP" AND FILL THE GAP. ╘HIS MEANS THAT AFTER A RELATIVE FILE HAS BEEN REVISED SEVERAL TIMES, THERE MAY BE "DEAD RECORDS" WITHIN THE BLOCK OF RECORDS THAT COULD BE USED FOR NEW ENTRIES. ╘HERE ARE SEVERAL WAYS TO MANAGE THIS, SO THAT THESE "DEAD RECORDS" CAN BE USED; ALL OF THEM INVOLVE MAINTAINING A LIST OF DE-ALLOCATED RECORD NUMBERS THAT CAN BE ACCESSED WHEN A NEW RECORD IS TO BE WRITTEN TO THE FILE.
╘HE ═AIL╥OOM USES THE RELATIVE FILE ITSELF--RECORDS 2001 AND 2002 ARE ALLOCATED FOR HOLDING TWO-BYTE RECORD NUMBERS, AND EACH OF THESE RECORDS IS DIVIDED INTO 38 FIELDS OF TWO BYTES EACH. ┴ SEPARATE ALGORITHM IS USED TO SET THE POSITION (╨) VALUE WHEN READING FROM OR WRITING TO THESE RECORDS, AND SINCE THEY ARE "LAST IN, FIRST OUT" STACKS, ACCESSED OR WRITTEN BYTE BY BYTE, THEY MAY ACTUALLY STILL CONTAIN NUMBERS FOR RECORDS WHICH HAVE SUBSEQUENTLY BEEN RE-ALLOCATED: ┴%(2004,1) HOLDS A COUNT OF OPEN RECORDS, WHICH IS USED AS A POINTER INTO THE RECORD, AND ONLY THE AVAILABLE RECORDS ARE RETRIEVED. ╫HEN A RECORD IS DELETED, THEN THE WRITE ZEROES ANY SUBSEQUENT LEFTOVER RECORD NUMBERS.
╘HIS ILLUSTRATES TWO PRINCIPLES: FIRST, THAT THE MAIN STRUCTURE OF A RELATIVE RECORD--ITS DIVISION INTO FIELDS--IS ENTIRELY DETERMINED BY THE PROGRAMMER, AND NEED NOT BE CONSISTENT THROUGHOUT THE RELATIVE FILE, BUT ONY IN THOSE PORTIONS THAT ARE TO BE USED IN THAT FORMAT. ╠ATER VERSIONS OF ╘HE ═AIL╥OOOM USE RELATIVE RECORDS 2003 THROUGH 2008 FOR MAILING LIST DEFINITIONS, FLAGGED BY THE LAST BYTE OF THE LISTINGS.
╙ECOND, THE INDEX ARRAY ITSELF CAN BE USED FOR PURPOSES OTHER THAN MERE INDEXING--┴%(2004,0) HOLDS THE NUMBER OF RECORDS INDEXED; ┴%(2004,2) IN LATER VERSIONS HOLDS THE NUMBER OF MAILING LISTS DEFINED; ETC. ┴%(2001,╞) AND ┴%(2002,╞) ARE USED TO HOLD THE ╠═ AND ╚═ FOUND BY THE SEARCH ROUTINE, FOR USE IN THE LOOPS THAT FULFILL USER SEARCHES OF THE LISTINGS; AS MENTIONED ABOVE; AND SO ON.
╔T IS EVEN POSSIBLE TO USE RELATIVE RECORDS TO HOLD MACHINE LANGUAGE ROUTINES TO BE CALLED INTO ├64 MEMORY AS NEEDED...
