ld bc,1024                ;maximum (for +3DOS) is actually
                                          ;64x13+13 = 845
                ld (hl),0
                ldir                      ;make sure at least first entry is
                                          ;zeroed
                ld b,64                   ;the number of entries in the
                                          ;buffer
                ld c,1                    ;include system files in the cata
                                          ;log
                ld de,catbuff             ;the location to be filled with the
                                          ;disk catalog
                ld hl,stardstar           ;the file name ("*.*")
                call dos_catalog          ;call the DOS entry
                push af                   ;save flags and possible error num
                                          ;ber returned by DOS
                pop hl
                ld (dosret),hl            ;put it where it can be seen from
                                          ; NextBASIC
                ld c,b                    ;move number of files in catalog to
                                          ;low byte of BC
                ld b,0                    ;this will be returned in NextBASIC
                                          ;by the USR function
                                          ;
                                          ;If the above worked, then BC holds
                                          ;number of files in catalog, the
                                          ;"catbuff"
                                          ;will be filled with the alpha-
                                          ;numerically sorted catalog and the
                                          ;carry flag but
                                          ;in "dosret" will be set. This will
                                          ;be peeked from NextBASIC to check
                                          ;if all went well.
                                          ;
                                          ;Having made the call to DOS, it is
                                          ;now necessary to undo the ROM and
                                          ;RAM switch and put BASIC's stack
                                          ;back to where it was on entry.
                                          ;The following will achieve this.
                di                        ;about to ROM/RAM switch so be
                                          ;careful
                push bc                   ;save number of files
                ld   bc,port1             ;I/O address of horizontal ROM/RAM
                                          ;switch
                ld   a,(bankm)            ;get current switch state
                set  4,a                  ;move left to right (ROM 2 to ROM
                                          ;3)
                and  F8h                  ;also want RAM page 0
                ld   (bankm),a            ;update the system variable (very
                     ;important)
                out  (c),a                ;make the switch
                pop  bc                   ;get back the saved number of files
                                          ;in catalog
                ld   sp,(staksto)         ;put NextBASIC's stack back
                ret                       ;return to NextBASIC, value in BC
                                          ;is returned to USR
stardstar:
                defb "*.*",FFh            ;the file name, must be terminated
                     ;with FFh
dosret:
                defw 0                    ;a variable to be peeked from BASIC
                                          ;to see if it worked

As some of you may not have an assembler available, the following is a NextBASIC program that pokes the above code into memory, calls it, and then uses the value returned by the USR function and the contents of dosret to print a very simple catalog of the disk: