INT 21h AH=43h: get or set file attributes — 8086 assembly guide

Read or change a file's attribute bits: read-only, hidden, system, archive. By name; the file need not be open.

Each directory entry has an attribute byte. INT 21h AH=43h with AL = 0 returns it in CX; with AL = 1 it writes CX into it. The file is found by name, like 41h and 56h, and does not need to be open.

Bit	Value	Meaning
0	01h	read-only: DOS refuses to write, truncate or delete the file (05h)
1	02h	hidden: DIR does not list it, find first skips it unless asked
2	04h	system: like hidden, for DOS's own files
3	08h	volume label: the entry is the disk's name, not a file
4	10h	directory
5	20h	archive: DOS sets it whenever the file is written; backup programs clear it

Set can change bits 0, 1, 2 and 5. The directory and volume-label bits say what the entry is, and trying to set them is error 05h. Set replaces the whole byte, so to change one bit, get the attributes first, change the bit with OR or AND, and set the result.

prints two lines: "new file: 20" and "after setting read-only: 21"

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     AH, 3Eh       ; close it
INT     21h
MOV     AX, 4300h     ; DOS: get the attributes of TEST.TXT
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK
MOV     DX, OFFSET M1
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AL, CL        ; CX = the attributes
CALL    PHEXB
MOV     DX, OFFSET CRLF
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AX, 4301h     ; set: read-only + archive
MOV     CX, 21h
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK
MOV     AX, 4300h     ; get them again
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK
MOV     DX, OFFSET M2
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AL, CL
CALL    PHEXB
MOV     DX, OFFSET CRLF
MOV     AH, 09h       ; DOS: print the string at DS:DX
INT     21h
MOV     AX, 4301h     ; back to 20h, or the delete fails with 05h
MOV     CX, 20h
MOV     DX, OFFSET FNAME
INT     21h
CALL    CHECK
MOV     AH, 41h       ; delete it
MOV     DX, OFFSET FNAME
INT     21h
 
FNAME   DB      "TEST.TXT", 0
M1      DB      "new file: $"
M2      DB      "after setting read-only: $"
CRLF    DB      13, 10, "$"
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

A new file comes with the archive bit set: it has changed since the last backup, which was never. Get works on directories too, returning 10h plus any other bits, which makes it a quick test whether a name is a directory.

On a host folder DOSBox-X maps read-only onto the host file's write permission and, on hosts other than Windows, keeps hidden and system in a small file of its own. How strictly read-only is enforced then depends on the host: a DOSBox-X running with administrator rights may still be able to write the file.