; Initialization
    *= $200     ; Start at $200 -> 512

io_clrw = $E000
io_putc = $E001
io_putn = $E002
io_puth = $E003
io_getc = $E004
io_chol = $E005
io_cvel = $E006

    ; Start Calculator System
    STA io_clrw ; Clean Screen
    LDY #$0

InputLoop: ; <--------------------------------------------------+
    JSR Input       ;               |
    ;JSR Output     ;               |
    PHA         ;               |
    JSR IsNumber        ;               |
    JSR Outhex      ;               |
    PLA         ;               |
    STA InputString, Y  ;               |
    INY         ;               |
                ;               |
                ; if Input == KEY_ENTER     |
    CMP #$0D        ;   jmp InputLoopFail ---+  |
    BEQ InputLoopCalc   ; else               |  |
    JMP InputLoop       ;   jmp InputLoop   -----|--+
InputLoopCalc:          ; <--------------------------+
    JSR Error
    JMP Exit        ; -------+
InputLoopError:         ;    |
    JSR Error       ;    |
    JMP Exit        ; -------+
                ;    |
                ;    v
                ;        |
Exit:               ; <------+
    BRK ; End of Program

    ; Subroutines   (Standard)

Output  ; input: ascii char per Accumulator
    STA io_putc
    RTS

Outhex  ; input: ascii char per Acumulator
    STA io_puth
    RTS

Input   ; output: ascii char per Accumulator
.Input_wait:
    LDA io_getc
    BEQ .Input_wait
    RTS
Error   ;
    LDA #'E'
    STA io_putc
    LDA #'r'
    STA io_putc
    STA io_putc
    PHA
    LDA #'o'
    STA io_putc
    PLA
    STA io_putc
    RTS

IsNumber
    CMP #$39
    BCC .IsNumber_Next
    JMP .IsNumber_Not
.IsNumber_Next:
    CMP #$30
    BCS .IsNumber_True
    JMP .IsNumber_Not
.IsNumber_True:
    LDA #$1 ; True
    JMP .IsNumber_Exit
.IsNumber_Not:
    LDA #$0 ; False
    JMP .IsNumber_Exit
.IsNumber_Exit:
    RTS

PrintString
    RTS



    ; Subroutines   (Calc)

    ; Global Variables
InputString:    .DB $00

    ; Global Constants
KeyEnter:   .BYTE $0D