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93 lines
2.6 KiB
Plaintext
93 lines
2.6 KiB
Plaintext
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*``````````````````````````````*
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* DELAYMS :: DELAY X MILLISECS *
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*- -*
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* DELAYS FOR X NUMBER OF *
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* MILLISECONDS BY LOOPING *
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* THROUGH PRECISELY CALCULATED *
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* NUMBER OF CYCLES. *
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*- -*
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* CLOBBERS: *
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* *
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* FLAGS: ????---- REG: -XY- *
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*- -*
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* CYCLES: ??? *
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* SIZE: *
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*- -*
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* USAGE: *
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* *
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* LDY #250 ;1/4 SEC *
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* JSR DELAYMS *
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*- -*
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* ENTRY *
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* *
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* Y = NUMBER OF MILLISECONDS *
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*- -*
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* EXIT *
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* *
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* Y = COUNTER; TRASH *
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* X = COUNTER; TRASH *
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* A = UNCHANGED *
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*- -*
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* ADAPTED FROM LEVANTHAL AND *
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* WINTHROP'S /6502 ASSEMBLY *
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* LANGUAGE ROUTINES/. *
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* AS SUCH, THIS MAY NOT FALL *
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* UNDER THE APACHE 2.0 LICENSE *
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* AGREEMENT, SINCE THE BOOK *
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* WAS WRITTEN BEFORE THE *
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* LICENSE! *
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* *
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* THAT SAID, LEVENTHAL WROTE *
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* THAT THE PURPOSE OF THE BOOK *
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* WAS TO COPY THE SOURCE CODE, *
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* AS REINVENTING THE WHEEL IS *
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* TEDIOUS, AND HE PLACED NO *
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* RESTRICTIONS ON ITS USE. *
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*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
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*
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DELAYMS
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*
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MSCNT EQU $0CA ; 202 TIMES THROUGH DELAY1
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; SPECIFIC TO 1.23 MHZ
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; SPEED OF APPLE II
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:DELAY
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CPY #0 ; 2 CYCLES
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BEQ :EXIT ; 2C (EXIT IF DEL=0)
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NOP ; 2 CYCLES (MAKE OVERHEAD=25C)
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*
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** IF DELAY IS 1MS THEN GOTO LAST1
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** THIS LOGIC IS DESIGNED TO BE
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** 5 CYCLES THROUGH EITHER ATH
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*
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CPY #1 ; 2 CYCLES
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BNE :DELAYA ; 3C IF TAKEN, ELSE 2C
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JMP :LAST1 ; 3C
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*
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** DELAY 1 MILLISENCOND TIMES (Y-1)
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*
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:DELAYA
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DEY ; 2C (PREDEC Y)
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:DELAY0
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LDX #MSCNT ; 2C
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:DELAY1
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DEX ; 2C
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BNE :DELAY1 ; 3C
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NOP ; 2C
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NOP ; 2C
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DEY ; 2C
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BNE :DELAY0 ; 3C
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:LAST1
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*
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** DELAY THE LAST TIME 25 CYCLES
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** LESS TO TAKE THE CALL, RETURN,
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** AND ROUTINE OVERHEAD INTO
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** ACCOUNT.
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*
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LDX #MSCNT-3 ; 2C
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:DELAY2
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DEX ; 2C
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BNE :DELAY2 ; 3C
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:EXIT
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RTS ; 6C
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*
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