AppleIIAsm-Collection/source/disk2_common/T.DELAYMS.SUB

*
*``````````````````````````````*
* DELAYMS :: DELAY X MILLISECS *
*-                            -*
* DELAYS FOR X NUMBER OF       *
* MILLISECONDS BY LOOPING      *
* THROUGH PRECISELY CALCULATED *
* NUMBER OF CYCLES.            *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: -XY-  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDY #250 ;1/4 SEC           *
*  JSR DELAYMS                 *
*-                            -*
* ENTRY                        *
*                              *
* Y = NUMBER OF MILLISECONDS   *
*-                            -*
* EXIT                         *
*                              *
* Y = COUNTER; TRASH           *
* X = COUNTER; TRASH           *
* A = UNCHANGED                *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
DELAYMS
*
MSCNT    EQU   $0CA       ; 202 TIMES THROUGH DELAY1
                          ; SPECIFIC TO 1.23 MHZ
                          ; SPEED OF APPLE II
:DELAY
         CPY   #0         ; 2 CYCLES
         BEQ   :EXIT      ; 2C (EXIT IF DEL=0)
         NOP              ; 2 CYCLES (MAKE OVERHEAD=25C)
*
** IF DELAY IS 1MS THEN GOTO LAST1
** THIS LOGIC IS DESIGNED TO BE
** 5 CYCLES THROUGH EITHER ATH
*
         CPY   #1         ; 2 CYCLES
         BNE   :DELAYA    ; 3C IF TAKEN, ELSE 2C
         JMP   :LAST1     ; 3C
*
** DELAY 1 MILLISENCOND TIMES (Y-1)
*
:DELAYA
         DEY              ; 2C (PREDEC Y)
:DELAY0
         LDX   #MSCNT     ; 2C
:DELAY1
         DEX              ; 2C
         BNE   :DELAY1    ; 3C
         NOP              ; 2C
         NOP              ; 2C
         DEY              ; 2C
         BNE   :DELAY0    ; 3C
:LAST1
*
** DELAY THE LAST TIME 25 CYCLES
** LESS TO TAKE THE CALL, RETURN,
** AND ROUTINE OVERHEAD INTO
** ACCOUNT.
*
         LDX   #MSCNT-3   ; 2C
:DELAY2
         DEX              ; 2C
         BNE   :DELAY2    ; 3C
:EXIT
         RTS              ; 6C
*