C02/include/header.a02

; Program initialization code for C02 programs
; Template for System Specific Code

;System Specific ASCII Key Mappings
DELKEY  EQU $7F     ;Delete/Backspace Key ($08=Backspace, $7F=Delete)
ESCKEY  EQU $1B     ;Escape/Stop Key      ($03=Ctrl-C, $1B=Escape)
RTNKEY  EQU $0D     ;Return/Enter Key     ($0D=Carriage Return)
NULKEY  EQU $00     ;No Key was Pressed   ($00=Null)

;Zero Page Locations
SRCLO   EQU $00     ;Source String Pointer (stdio.asm)
SRCHI   EQU $01
DSTLO   EQU $02     ;Destination String Pointer (string.asm)
DSTHI   EQU $03

STKSAV  EQU $0D     ;Stack Pointer Storage
RDSEED  EQU $0E     ;Pseudo-RANDOM Seed
RANDOM  EQU $0F     ;Pseudo-RANDOM Number Storage
TEMP0   EQU $10     ;Temporary Storage
TEMP1   EQU $11
TEMP2   EQU $12
TEMP3   EQU $12

BLKSLO  EQU $20     ;Block Start Address
BLKSHI  EQU $21
BLKELO  EQU $22     ;Block End Address
BLKEHI  EQU $23
BLKLO   EQU $24     ;Block Pointer
BLKHI   EQU $25

STKSLO  EQU $28     ;Stack Start Address
STKSHI  EQU $29
STKELO  EQU $2A     ;Stack End Address
STKEHI  EQU $2B
STKLO   EQU $2C     ;Stack Pointer
STKHI   EQU $2D

        ORG $0200   ;Program Start Address

START:  NOP         ;System specific initialization code
        TXS         ;If an RTS is used to return to the Operating System,
        STX STKSAV  ;  the Stack Pointer should be preserved
        JMP MAIN    ;Execute Program

;Poll Character from Keyboard
PLKEY:  INC RDSEED  ;Cycle the Random Seed (if not provided by system)
        NOP         ;Code Read from Keyboad         
        RTS

;Read Character from Console
GETKEY;             ;Usually Drops into RDKEY, but may need to call RDKEY
                    ;  then clean/convert returned value (e.g. Apple-1) 

;Wait for Character from Console
RDKEY:  JSR PLKEY   ;Usually calls PLKEY 
        BEQ RDKEY   ;  until a non-zero is returned
        RTS         

;Delete Previous Character
DELCHR: RTS         ;Code to move Cursor to left and clear that position
                    ;May work differently on systems that don't support this

;Advance Cursor to Next line
NEWLIN: RTS         ;Code to move Cursor to beginning of next line
                    ;May emit Carriage Return, Line Feed, or both

;Print Character to Screen
PRCHR:  RTS         ;Code to write ASCII character to Screen

;Exit Program and Return to Operating System or Monitor
EXIT:   BRK         ;Usually BRK if returning to Monitor
        LDX STKSAV  ;If an RTS is used to return to the Operating System,
        TXS         ;  the Stack Pointer should be restored to original state 
        RTS         ;  in case program exits in the middle of a Function

;Note: The following two functions replicate calls available 
;in the Apple-1 monitor and are included for test purposes
;They will likely be removed before the final release

;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 Nybb
        SED         ;Set Decimal Flag for 
        CLC         ;  Addition Wizardry
        ADC #$90    ;Convert to $90-$99,$00-$05
        ADC #$40    ;Convert to $30-$39,$41-$46
        CLD         ;Clear Decimal Flag
        JMP PRCHR   ;Print Hex Digit and Return   

;Alternate Code for Systems with Interrupts that don't CLD
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