passport/src/id/trace.a

;-------------------------------
; Trace
; common code for all boot tracers
; in:  A = lo byte of callback
;      X = hi byte of callback
; never returns (exits via callback or UseUniversal)
;-------------------------------
!zone {
Trace
         pha
         txa
         pha
         lda   #$C6
         ldx   #$16
         jsr   CopyMemory1
         pla
         ldx   #0
         ldy   $16F8
         cpy   #$4C
         beq   +
         ldx   #3
         ldy   $16FB
         cpy   #$4C
         bne   ++
+        sta   $16FA,x               ; hi byte of callback
         pla
         sta   $16F9,x               ; lo byte of callback
         jmp   $1600

++       pla
         jsr   PrintByID             ; unknown disk controller
         !byte s_canttrace
         ; /!\ execution falls through here to UseUniversal
}