INT 21h AH=56h: rename or move a file — 8086 assembly guide

Rename a file, or move it to another directory on the same drive. Old name at DS:DX, new name at ES:DI.

INT 21h AH=56h changes a file's name. The old name is at DS:DX and the new one at ES:DI, both ASCIIZ. In a .COM program ES = DS from the start; in an .EXE, ES points to the PSP until you load it, a classic reason for this call to fail with a garbage name.

If the new name has a different directory, the file moves there: its directory entry is taken out of one directory and put into the other, and the data is not copied, so a move is instant whatever the size. Both names must be on the same drive; a different drive is error 11h, not the same device. To move a file to another drive, copy it and delete the original.

AX	Meaning
02h	the old file does not exist
03h	a directory in either name does not exist
05h	access denied: something already has the new name, or the target directory is full
11h	the names are on different drives

prints three lines: "renamed OLD.TXT to NEW.TXT", "open OLD.TXT: error 02" and "rename NEW.TXT to OLD.TXT: error 05"

MOV     AH, 3Ch          ; create OLD.TXT
XOR     CX, CX
MOV     DX, OFFSET OLDN
INT     21h
CALL    CHECK
MOV     BX, AX
MOV     AH, 3Eh          ; and close it
INT     21h
MOV     AH, 56h          ; DOS: rename
MOV     DX, OFFSET OLDN  ; DS:DX = the old name
MOV     DI, OFFSET NEWN  ; ES:DI = the new name (ES = DS here)
INT     21h
CALL    CHECK
MOV     DX, OFFSET M1
MOV     AH, 09h          ; DOS: print the string at DS:DX
INT     21h
MOV     DX, OFFSET M2
MOV     AH, 09h          ; DOS: print the string at DS:DX
INT     21h
MOV     AX, 3D00h        ; is the old name still there?
MOV     DX, OFFSET OLDN
INT     21h
CALL    REPORT           ; error 02
MOV     AH, 3Ch          ; make a new OLD.TXT
XOR     CX, CX
MOV     DX, OFFSET OLDN
INT     21h
CALL    CHECK
MOV     BX, AX
MOV     AH, 3Eh
INT     21h
MOV     DX, OFFSET M3
MOV     AH, 09h          ; DOS: print the string at DS:DX
INT     21h
MOV     AH, 56h          ; rename onto a name that is taken
MOV     DX, OFFSET NEWN
MOV     DI, OFFSET OLDN
INT     21h
CALL    REPORT           ; error 05
MOV     AH, 41h          ; delete both files
MOV     DX, OFFSET OLDN
INT     21h
MOV     AH, 41h
MOV     DX, OFFSET NEWN
INT     21h
 
OLDN    DB      "OLD.TXT", 0
NEWN    DB      "NEW.TXT", 0
M1      DB      "renamed OLD.TXT to NEW.TXT", 13, 10, "$"
M2      DB      "open OLD.TXT: $"
M3      DB      "rename NEW.TXT to OLD.TXT: $"
CRLF    DB      13, 10, "$"
CHECK   PROC    NEAR     ; CF = 1: print the error code in AL and exit
JNC     CHECK1
PUSH    AX
MOV     DX, OFFSET FAILMSG
MOV     AH, 09h
INT     21h
POP     AX
CALL    PHEXB
MOV     AX, 4C01h        ; exit with code 1
INT     21h
CHECK1:
RET
CHECK   ENDP
FAILMSG DB      "failed: error $"
REPORT  PROC    NEAR     ; CF = 0: "ok"; CF = 1: "error" and AL
JC      REPORT1
MOV     DX, OFFSET OKMSG
MOV     AH, 09h
INT     21h
RET
REPORT1:
PUSH    AX
MOV     DX, OFFSET ERRMSG
MOV     AH, 09h
INT     21h
POP     AX
CALL    PHEXB            ; the error code
MOV     DX, OFFSET CRLF
MOV     AH, 09h
INT     21h
RET
REPORT  ENDP
OKMSG   DB      "ok", 13, 10, "$"
ERRMSG  DB      "error $"
PHEXB   PROC    NEAR     ; print AL as two hex digits
PUSH    CX
PUSH    DX
MOV     DH, AL           ; keep the byte
MOV     CL, 4
SHR     AL, CL           ; the high digit first
MOV     CX, 2
PHEXB1:
AND     AL, 0Fh
ADD     AL, "0"
CMP     AL, "9"
JBE     PHEXB2
ADD     AL, 7            ; 10..15 -> "A".."F"
PHEXB2:
MOV     DL, AL
MOV     AH, 02h          ; DOS: print DL
INT     21h
MOV     AL, DH           ; then the low digit
LOOP    PHEXB1
POP     DX
POP     CX
RET
PHEXB   ENDP

Pitfalls

An existing file with the new name is never replaced: delete it first with INT 21h AH=41h if that is what you want. Wildcards are not allowed; rename a group of files one by one, found with INT 21h AH=4Eh. Renaming a file that is open is asking for trouble on MS-DOS: close it first.