[Thorham]

begin:

    movem.l     d0-d7/a0-a6,-(sp)               ; Backup the system registers
    jsr         closesys
;   jsr         mysys
;   jsr         restoresys

libfail:
    movem.l     (sp)+,d0-d7/a0-a6
    moveq.l     #0,d0                           ; No returncode to AmigaDOS
    rts

; First we backup the system and then take control

closesys:
    move.l      execbase,a6                     ; Address of execbase into a6
    lea         gfxlibraryname,a1               ; address of the gfx lib into a1
    moveq.l     #0,d0                           ; version of the library
    jsr         openlibrary(a6)                 ; Open the library, address is returned in d0
    move.l      d0,gfxbase                      ; store the address of the library
    move.l      d0,a6                           ; Graphics library base address into a6
    beq         libfail

Two things here:

1. When calling something like openlibrary, you have to compare the return value with 0. The system functions are not guaranteed to do that, so use a tst.l d0 before branching.

2. Look at how closesys is called, and then look at how the routine jumps back when openlibrary fails.