6502 Counting Loop Example

Here is a solution for part of the SPO600 64-bit Assembly Language Lab but implemented in 6502 Assembly language for use with the 6502 Emulator:

; ROM routines
define      SCINIT      $ff81
define      CHROUT      $ffd2

START:
    LDX #$00    ; Loop index (0-9)

GO:
    TXA     ; Put loop index into A
    CLC
    ADC #$30    ; Add $30 (ASCII '0')
    STA MSG_DIGIT   ; Store into the string

    LDY #$00    ; Character number to print
DO:
    LDA MSG,Y   ; Get a character
    BEQ DONE    ; Done if it's NULL
    JSR CHROUT  ; Print character
    INY     ; Increment char number
    JMP DO      ; Process next character

DONE:
    INX     ; Increment loop index
    CPX #10     ; Is it 10?
    BNE GO      ; If not, branch to GO
    BRK     ; Stop

MSG:
    DCB "L","o","o","p",$20
MSG_DIGIT:
    DCB "#",$0D,$00

; Note that MSG_DIGIT is a position in
; the middle of the message - not a 
; separate message.