jsr printmsg ; print message
         
end:      jmp end ; loops forever after printing

printmsg:
         ldx #0         
more:       lda string1,x   ;loads the start of string1, but I want to point to the fixed message address msgbase   
         beq end
         sta data8251
         inx
         jsr wait  ; check 8251 status
         jmp more      
         rts

msgbase: word  msgbase ; want this to point to start of the string to be displayed

          LDA #LDA #string1 ; I get the error 'operand doesn't fit into 8-bits' edit: my code starts at $FE00
          STA MSGBASE         ;store high byte of message address at the message base address used by the subroutine

    ...
example:
    lda #>message
    ldy #<message
    jsr strout
    ...

    ...
strout:
    sty ptr
    sta ptr+1
    ldy #0
    beq lp2
lp:
    jsr cout
    iny
lp2:
    lda (ptr),y
    bne lp
    rts
    ...

    ...
message:
    dcb "Hello",0
    ...

;PROMPT routine: Send indexed text string to terminal. Index is contained in A reg.
; String buffer address is stored in variable PROMPTL/PROMPTH.
PROMPT          ASL     A               ;Multiply by two for msg table index
                TAX                     ;Xfer to X reg - index
                LDA     MSG_TABLE,X     ;Get low byte address
                LDY     MSG_TABLE+1,X   ;Get high byte address
;
;PROMPTR routine: takes message address in Y/A and prints via PROMPT2 routine
PROMPTR         STA     PROMPTL         ;Store low byte
                STY     PROMPTH         ;Store high byte
;
;PROMPT2 routine: prints message at address (PROMPTL) till null character found
PROMPT2         LDA     (PROMPTL)       ;Get string data
                BEQ     HINEXIT         ;If null character, exit (borrowed RTS)
                JSR     B_CHROUT        ;Send character to terminal
                INC     PROMPTL         ;Increment low byte index
                BNE     PROMPT2         ;Loop back for next character
                INC     PROMPTH         ;Increment high byte index
                BRA     PROMPT2         ;Loop back and continue printing
;
HINEXIT         RTS                     ;And return to caller

; strout:
;       Output a zero terminated string to the host. The string is in-line with
;       the code and is used like:
;
;       jsr strout
;       .asciiz "Hello, world"
;  or
;       jsr strout
;       .byte   "Hello, world",13,10,0
;
; Note: This uses self-modifying code, so has to run in RAM. The main reason
;       is to not use up any page 0 locations as it's hard to find something
;       guaranteed to be free for everything.
;
;       Destroys A
;********************************************************************************

strout:

; Pull return address off the stack to use to get the data from

        pla
        sta     strX+1
        pla
        sta     strX+2

strout1:
        inc     strX+1
        bne     strX
        inc     strX+2
strX:
        lda     $FFFF           ; This word modified.
        beq     stroutEnd
        jsr     osWrch
        bra     strout1

; Push return address back onto the stack

stroutEnd:
        lda     strX+2
        pha
        lda     strX+1
        pha
        rts

The parser understands a lo/hi-modifier to select low- or high-byte of a 16-bit word.
The character < is used to select the low-byte and > for the high-byte. It has to be the
first character before an expression.

    ...
example:
<edited by me>
lp2:
    lda (ptr),y
    bne lp
    rts
    ...

    ...
message:
    dcb "Hello",0
    ...

lda (ptr),y

ISR:    PHA          ; (Later you may need to push Y also, with PHY, if the
        PHX          ; ISR uses Y anywhere. NMOS will have to go through A.)

PRINT "Hello, World"

    ...
ptr     equ 2       ; zp locations 2 and 3 hold a 16-bit address
    ...

    ...
    ldy #4
    sty ptr
    lda #5
    sta ptr+1
    lda (ptr),y     ; effective address is $0508

.reserve 5

    .byte    $AD,0,0