cc65/libsrc/cbm/getdevice.s

;
; Oliver Schmidt, 2012-09-04
;
; unsigned char getfirstdevice (void);
; unsigned char __fastcall__ getnextdevice (unsigned char device);
;

        .export         _getfirstdevice
        .export         _getnextdevice
        .import         isdisk
        .import         opencmdchannel
        .import         closecmdchannel
        .importzp       ST
        .importzp       tmp2

;------------------------------------------------------------------------------
; _getfirstdevice

_getfirstdevice:
        lda     #$FF
        ; Fall through

;------------------------------------------------------------------------------
; _getnextdevice

_getnextdevice:
        tax
next:   inx
        cpx     #$FF
        beq     done

; [open|close]cmdchannel already call isdisk internally but they
; interpret a non-disk as a no-op while we need to interpret it
; as an error here

        jsr     isdisk
        bcs     next

; [open|close]cmdchannel don't call into the Kernal at all if they
; only [in|de]crement the reference count of the shared cmdchannel
; so we need to explicitly initialize ST here

        lda     #$00
        sta     ST

        stx     tmp2
        jsr     opencmdchannel
        ldx     tmp2
        jsr     closecmdchannel
        ldx     tmp2

; As we had to reference ST above anyway we can as well do so
; here too (instead of calling READST)

        lda     ST

; Either the Kernal calls above were successfull or there was
; already a cmdchannel to the device open - which is a pretty
; good indication of its existence ;-)

        bmi     next

done:   txa
        ldx     #$00
        rts