MIKBUG provides support routines. The listing file has the addresses of the routines.

INEEE is the input routine which gets a character from the current port.

OUTEEE is the output routine which sends a character to the current port.

Port loopback code:

; INEEE    EQU    $F1F3
; OUTEEE   EQU    $F20A

LOOP4VR    
    JSR    $F1F3
    JSR    $F20A    
    BRA    LOOP4VR

Code assembles to:

0000 LOOP4VR   
0000   bd F1 f3 JSR $F1F3   
0003   bd F2 0a JSR $C20A   
0006   20 f8    BRA LOOP4VR

E to change memory:

>E 0000
>0000 55 BD
>0001 55 F1
>0002 55 F3
>0003 55 BD
>0004 55 F2
>0005 55 0A
>0006 55 40
>0007 55 F8
>0008 55 <ENTER>

 V to dump memory:

>V
FROM ADDR 0000
0000 BD F1 F4 BD  F2 0A 40 F8  55 55 55 55  55 55 55 55
     ......@.UUUUUUUU

Type N to turn off echo.

J to jump to program:

>J TO ADDR 0000 

Type chars and they get echoed.

ITTEESSTTIINNGG  TTHHIISS  WWOORRKKZZ  LLOOLLOOLLOOLLOOOOLL