*******************************************************
   *                                                     *
   *        APPLE ][                                     *
   *                                                     *
   *  TERMINAL SIMULATION                                *
   *                                                     *
   *  ASYNCHR RS232 INPUT/OUTPUT ROUTINES                *
   *                                                     *
   *  COMPUTER BULLETIN 1981                             *
   *******************************************************
     
   ; DEFINES
     
 ORG $2000
    
   ; APPLE ZERO PAGE USE
     
CH * $0024 ; RELATIVE CURSOR POSITION
BASL * $0028 ; BASE ADDRESS SCREEN
BAUD * $00FB ; BAUD RATE CONSTANT
CHAR * $00FE ; DATAFIELD
     
   ; APPLE I/O ADRESSES
    
KB * $C000 ; KEYBOARD INPUT
KBSTR * $C010 ; KEYBOARD STROBE
MARK * $C058 ; GAME AN0 FLIP FLOP SET
SPACE * $C059 ; GAME AN0 FLIP FLOP RESET
PDLSW * $C061 ; GAME SW0 INPUT IN BIT 7
    
   ; APPLE MONITOR USE
    
BS * $FC10 ; NON DESTROYING BACKSPACE
VIDOUT * $FDF0 ; PRINT CHARACTER IN A ON SCREEN
    
    
   ; MAIN ROUTINE
   ;  TERMINAL SIMULATION ROUTINE
    
TERSIM LDY CH ; BEGIN  * TERMINAL SIMULATION *
 LDAIY BASL ;  REPEAT
 PHA  ;    SHOW CURSOR POSITION
 ANDIM $3F ;    IF KEY PRESSED THEN
 STAIY BASL ;      REMOVE CURSOR FROM SCREEN
TEST BIT KB ;      CHARACTER := KEYCODE
 BMI KEYIN ;      OUTPUT (CHARACTER)
 BIT PDLSW ;      SHOW (CHARACTER)
 BMI TEST ;     ENDIF
 PLA  ;    IF CHARACTER AT INPUT THEN
 STAIY BASL ;      REMOVE CURSOR FROM SCREEN
 JSR INPUT ;      INPUT (CHARACTER)
 JSR SHOW ;      SHOW (CHARACTER)
 JMP TERSIM ;     ENDIF
KEYIN STAIY BASL ;  UNTIL FOREVER
 LDA KB ; END * TERMINAL SIMULATION *
 BIT KBSTR ;
 JSR SHOW ;
 JSR OUTPUT ;
 JMP TERSIM ;
    
   ; SHOW CHARACTER ON APPLE SCREEN ROUTINE
    
SHOW CMPIM $8A ; BEGIN * SHOW CHARACTER ON SCREEN *
 BNE BACKSP ;
 LDYIM $00 ;   IF CHARACTER = LINEFEED THEN
 STY CH ;     COLUMNPOSITION := 0
 JMP VIDOUT ;     SEND (LINEFEED)
BACKSP CMPIM $88 ;    ENDIF
 BNE CARRET ;   IF CHARACTER = BACKSPACE THEN
 PHA  ;     * MAKE IT DESTROYING BACKSPACE *
 JSR BS ;     SEND (BACKSPACE)
 LDAIM $A0 ;     SEND (SPACE)
 JSR VIDOUT ;     SEND (BACKSPACE)
 PLA  ;    ENDIF
 JMP VIDOUT ;   IF CHARACTER = CARRIAGE RETURN THEN
CARRET CMPIM $8D ;     COLUMNPOSITION := 0
 BNE ESC ;    ENDIF
 LDAIM $00 ;   IF CHARACTER = ESCAPE THEN
 STA CH ;     * COMPLETE ESCAPE FROM TERMINAL SIMULATION *
 RTS  ;     EXIT
ESC CMPIM $9B ;    ENDIF
 BEQ EXIT ;   IF CHARACTER = NORMAL THEN
 JMP VIDOUT ;     SEND (CHARACTER)
EXIT PLA  ;    ENDIF
 PLA  ; END * SHOW CHARACTER ON SCREEN *
 RTS  ;
    
   ; INPUT CHARACTER FROM GAME SW0 ROUTINE
     
INPUT TXA  ; BEGIN * INPUT CHARACTER *
 PHA  ;   * READ CHARACTER FROM AN0 ASYNCHR *
 TYA  ;   * AN0 = BIT 7 $C061               *
 PHA  ;
 LDYIM $08 ;   BITCOUNT := 8
IN BIT PDLSW ;   WHILE NOT STARTBIT DO TEST FOR STARTBIT
 BMI IN ;   DELAY BITTIME
 JSR DELAY ;   DELAY HALF BITTIME
 JSR DEHALF ;   WHILE BITCOUNT > 0 DO
INP LDA PDLSW ;     READ BIT
 ANDIM $80 ;     SHIFT BIT IN CHARACTER
 LSR CHAR ;     BITCOUNT := BITCOUNT - 1
 ORA CHAR ;    ENDWHILE
 STA CHAR ;   DELAY BITTIME
 DEY  ;   SET MSB OF CHARACTER
 BNE INP ;   * FOR APPLE VIDEO ROUTINE *
 JSR DELAY ; END * INPUT CHARACTER
 PLA  ;
 TAY  ;
 PLA  ;
 TAX  ;
 ORAIM $80 ;
 RTS  ;
   
   ; OUTPUT CHARACTER TROUGH GAME AN0
    
OUTPUT PHA  ; BEGIN * OUTPUT CHARACTER *
 TYA  ;   * SEND CHARACTER ASYNCHR TROUGH AN0 *
 PHA  ;   * AN0 FLIPFLOP OUTPUT C058 C059     *
 TXA  ;   * 1 STARTBIT , 8 DATABITS , 2 STOP  *
 PHA  ;
 TSX  ;   CARRY := STARTBIT
 LDAAX $0103 ;   BITCOUNT := 11
 LDYIM $0B ;   WHILE BITCOUNT > 0 DO
 CLC  ;     OUTPUT := CARRY
OUT PHA  ;     DELAY BITTIME
 BCS MRKOUT ;     CARRY := STOPBIT
 LDA SPACE ;     SHIFT CHARACTER RIGHT
 BCC WAIT ;     LSB CHAR := CARRY
MRKOUT LDA MARK ;     CARRY := MSB CHAR
WAIT JSR DELAY ;    ENDWHILE
 PLA  ;
 SEC  ; END * OUTPUT CHARACTER *
 RORA  ;
 DEY  ;
 BNE OUT ;
 PLA  ;
 TAX  ;
 PLA  ;
 TAY  ;
 PLA  ;
 RTS  ;
     
   ; DELAY BITTIME ROUTINE
    
DELAY LDY BAUD ; BEGIN * DELAY BITTIME *
XD DEX  ;   DELAY (BAUD)*5 + 1 MICROSEC
 BNE XD ; END * DELAY BITTIME *
 RTS  ;
    
   ; DELAY HALF BIT TIME ROUTINE
    
DEHALF LDA BAUD ; BEGIN * DELAY HALF BITTIME *
 LSRA  ;   DELAY (BAUD/2)*5 + 6 MICROSEC
 TAX  ; END * DELAY HALF BITTIME *
XDH DEX  ;
 BNE XDH ;
 RTS  ;