;c02 Program Initialization Code for VIC-20 with 3K Expansion

;System Specific ASCII Key Mappings
DELKEY  EQU $7F   ;Delete/Backspace Key (Delete)
ESCKEY  EQU $03   ;Escape/Stop Key      (RUN/STOP)
RTNKEY  EQU $0D   ;Return/Enter Key     (RETURN)

;Zero Page Locations
RDSEED  EQU $A2   ;Software Jiffy Clock (Low Byte)
SRCLO   EQU $D1   ;Pointer to Address of Current Screen Line
SRCHI   EQU $D2   ;Pointer to Address of Current Screen Line
DSTLO   EQU $FB   ;Free Byte for User Programs
DSTHI   EQU $FC   ;Free Byte for User Programs
BLKLO   EQU $FD   ;Free Byte for User Programs
BLKHI   EQU $FE   ;Free Byte for User Programs

USER0   EQU $0310 ;Free Byte for User Programs
USER1   EQU $0311 ;Free Byte for User Programs
USER2   EQU $0312 ;Free Byte for User Programs
USER3   EQU $0313 ;Free Byte for User Programs
USER4   EQU $0334 ;Free Byte for User Programs
USER5   EQU $0335 ;Free Byte for User Programs
USER6   EQU $0336 ;Free Byte for User Programs
BLKSLO  EQU $0337 ;Free Byte for User Programs
BLKSHI  EQU $0338 ;Free Byte for User Programs
BLKELO  EQU $0339 ;Free Byte for User Programs
BLKEHI  EQU $033A ;Free Byte for User Programs
STKSAV  EQU $033B ;Free Byte for User Programs
TBFFR   EQU $033C ;Cassette I/O Buffer
TEMP0   EQU $03FC ;Free Byte for User Programs
TEMP1   EQU $03FD ;Free Byte for User Programs
TEMP2   EQU $03FE ;Free Byte for User Programs
TEMP3   EQU $03FF ;Free Byte for User Programs

;Video RAM and ROM 
VICSCN  EQU $1E00 ;Video Screen Memory Area (Unexpanded)
CHRROM  EQU $8000 ;Character Generator ROM
VICCLR  EQU $9600 ;Color RAM (Unexpanded)

;Machine Language Basic Stub
        ORG $0401              ;Start 
BASIC:  DC  $0C, $04           ; Pointer to Next Line (1036)
        DC  $00, $00           ; Line Number (0)
        DC  $9E                ; SYS 
        DC  $20                ; ' '
        DC  $31, $30, $33 ,$38 ; "1038" 
        DC  $00                ;End of Line Marker
        DC  $00, $00           ;End of Basic Program

START:  TSX         ;Get Stack Pointer
        STX STKSAV  ;and Save for Exit
        JMP main    ;Execute Program

EXIT:   LDX STKSAV  ;Retrieve Saved Stack Pointer
        TXS         ;and Restore It
        RTS         ;Return to BASIC

;Poll Keyboard for Character
PLKEY   EQU $FFE4  ;Aliased to Kernal GETIN Routine

;Get Character from Keyboard
GETKEY: EQU RDKEY

;Wait for Character from Keyboard
RDKEY:  JSR PLKEY   ;Poll Keyboard
        BEQ GETKEY  ;If No Key, Loop
        RTS

;Delete Previous Character
DELCHR: LDA #DELKEY ;Load Delete Character
        JMP PRCHR   :Print and Return

;Advance Character to Next line
NEWLIN: LDA #RTNKEY ;Load C/R into Accumulator

;Print Character to Console
PRCHR   EQU $FFD2  ;Aliased to Kernal CHROUT Routine

;Print Byte as Two-Digit Hex Number to Console
PRBYTE: PHA         ;Save Accumulater
        LSR         ;Shift Hi Nybble to Low Nybble
        LSR 
        LSR 
        LSR 
        JSR prhex   ;  and Print it
        PLA         ;Restore Accumulator 
                    ;  and fall into prhex

;Print Low Nybble as Hex Digit to Console
PRHEX:  AND #$0F    ;Strip High Nybble
        CMP #$0A    ;If Low Nybble >= 10
        BCC PRHEXC  ;  
        ADC #$06    ;  Convert ':' to 'A'...
PRHEXC: ADC #$30    ;Convert to ASCII Character
        JMP prchr   ;Print Hex Digit and Return