INT 21h AH=57h: get or set a file's date and time — 8086 assembly guide

Get or set the date and time a file was last written, in DOS's packed 16-bit format. Works through an open handle.

DOS keeps one timestamp per file, the date and time of the last write, in two 16-bit words of the directory entry. INT 21h AH=57h reads them (AL = 0) or replaces them (AL = 1) through an open handle; unlike INT 21h AH=43h it does not take a name.

The packed format

Time (CX) bits	Hold
15-11	hours, 0 to 23
10-5	minutes, 0 to 59
4-0	seconds ÷ 2, 0 to 29

Date (DX) bits	Hold
15-9	year − 1980, 0 to 127 (1980 to 2107)
8-5	month, 1 to 12
4-0	day, 1 to 31

So the time word is hours × 2048 + minutes × 32 + seconds ÷ 2, and the date word (year − 1980) × 512 + month × 32 + day. Seconds have a resolution of two: an odd second cannot be stored. The same two words appear in the record that find first / next fill in (INT 21h AH=4Eh: time at offset 16h, date at 18h). Unpacking is shifts and masks: SHR by 9 for the year, by 5 and AND 0Fh for the month, AND 1Fh for the day.

prints two lines: "packed: time 63C0 date 1ECF" and "1995-06-15 12:30:00"

MOV     AH, 3Ch       ; DOS: create TEST.TXT
XOR     CX, CX        ; CX = 0: normal attributes
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK         ; stop if CF = 1
MOV     BX, AX        ; BX = the handle
MOV     AX, 5701h     ; DOS: set the date and time
MOV     CX, 63C0h     ; 12:30:00 = 12 x 2048 + 30 x 32 + 0
MOV     DX, 1ECFh     ; 1995-06-15 = 15 x 512 + 6 x 32 + 15
INT     21h
CALL    CHECK
MOV     AH, 3Eh       ; close: the stamp is written now
INT     21h
MOV     AX, 3D00h     ; open the file again
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK
MOV     BX, AX
MOV     AX, 5700h     ; DOS: get the date and time
INT     21h
CALL    CHECK
MOV     SI, DX        ; SI = date
MOV     DI, CX        ; DI = time
MOV     AH, 3Eh       ; close the handle
INT     21h
MOV     AH, 41h       ; delete TEST.TXT
MOV     DX, OFFSET FNAME
INT     21h
MOV     DX, OFFSET M1
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AX, DI
CALL    PHEX
MOV     DX, OFFSET M2
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AX, SI
CALL    PHEX
MOV     DX, OFFSET CRLF
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AX, SI        ; year: bits 15-9, + 1980
MOV     CL, 9
SHR     AX, CL
ADD     AX, 1980
CALL    PDEC
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print DL
INT     21h
MOV     AX, SI        ; month: bits 8-5
MOV     CL, 5
SHR     AX, CL
AND     AX, 0Fh
CALL    P2
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print DL
INT     21h
MOV     AX, SI        ; day: bits 4-0
AND     AX, 1Fh
CALL    P2
MOV     DL, " "
MOV     AH, 02h       ; DOS: print DL
INT     21h
MOV     AX, DI        ; hours: bits 15-11
MOV     CL, 11
SHR     AX, CL
CALL    P2
MOV     DL, ":"
MOV     AH, 02h       ; DOS: print DL
INT     21h
MOV     AX, DI        ; minutes: bits 10-5
MOV     CL, 5
SHR     AX, CL
AND     AX, 3Fh
CALL    P2
MOV     DL, ":"
MOV     AH, 02h       ; DOS: print DL
INT     21h
MOV     AX, DI        ; seconds: bits 4-0, times 2
AND     AX, 1Fh
SHL     AX, 1
CALL    P2
MOV     DX, OFFSET CRLF
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
 
FNAME   DB      "TEST.TXT", 0
M1      DB      "packed: time $"
M2      DB      " date $"
CRLF    DB      13, 10, "$"
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 $"
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
PHEX    PROC    NEAR  ; print AX as four hex digits
PUSH    AX
MOV     AL, AH
CALL    PHEXB
POP     AX
CALL    PHEXB
RET
PHEX    ENDP
PDEC    PROC    NEAR  ; print AX in decimal
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BX, 10
XOR     CX, CX        ; CX = digits pushed
PDEC1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
OR      AX, AX
JNZ     PDEC1
PDEC2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    PDEC2
POP     DX
POP     CX
POP     BX
RET
PDEC    ENDP
P2      PROC    NEAR  ; print AX (0..99) as two digits
MOV     DL, 10
DIV     DL            ; AL = tens, AH = ones
ADD     AX, 3030h     ; both to characters
PUSH    AX
MOV     DL, AL
MOV     AH, 02h
INT     21h
POP     AX
MOV     DL, AH
MOV     AH, 02h
INT     21h
RET
P2      ENDP

When a set stamp sticks

DOS keeps the new stamp with the open file and writes it to the directory when the file is closed. A write through the handle after the set replaces it with the current date and time again, so set the stamp last, after all writes. Archivers and copy programs use this to give files their original dates.

In DOSBox-X, on a host folder, the stamp becomes the host file's modification time when the file is closed, and a get always reads the host file. A value just set therefore shows up only after closing and reopening, which is what the example does.