INT 21h AH=39h: make a directory — 8086 assembly guide

Create a directory, the MD command. The name is ASCIIZ; every directory above it must already exist.

INT 21h AH=39h creates one directory, what MD (MKDIR) does at the prompt. The name at DS:DX ends with a 0 byte, not with "$": all the DOS services that take a path use ASCIIZ strings, and only the console's 09h uses "$". The path can be relative to the current directory (TESTDIR, SUB\NEW) or absolute (\TOOLS\NEW, C:\TOOLS\NEW), up to 64 characters, and each part is an 8.3 name.

Only the last part of the path is created. Every directory before it must exist, or the call fails with error 3.

AX (CF = 1)	Why
3	path not found: a directory on the way does not exist, or the name is not valid
5	access denied: a file or directory of that name exists, the parent directory is full, or the disk is write-protected

A FAT disk's root directory has a fixed number of entries (112 on a 360 KB floppy, 512 on a hard disk), so making one more directory there fails with 5 even when the disk has room. Subdirectories grow as needed. The example makes TESTDIR, tries again, and removes it:

prints: mkdir TESTDIR: ok / mkdir TESTDIR again: error 5 / rmdir TESTDIR: ok

MOV     DX, OFFSET MKM1
MOV     AH, 09h
INT     21h
MOV     AH, 39h       ; DOS: make the directory named at DS:DX
MOV     DX, OFFSET DIR1
INT     21h
CALL    RESULT        ; ok
MOV     DX, OFFSET AGAINM1
MOV     AH, 09h
INT     21h
MOV     AH, 39h
MOV     DX, OFFSET DIR1
INT     21h
CALL    RESULT        ; error 5: it exists now
MOV     DX, OFFSET RMM1
MOV     AH, 09h
INT     21h
MOV     AH, 3Ah       ; DOS: remove it again
MOV     DX, OFFSET DIR1
INT     21h
CALL    RESULT
 
DIR1    DB      "TESTDIR", 0
MKM1    DB      "mkdir TESTDIR: $"
AGAINM1 DB      "mkdir TESTDIR again: $"
RMM1    DB      "rmdir TESTDIR: $"
RESULT  PROC    NEAR  ; print "ok", or "error" and AX when CF = 1
PUSHF                 ; keeps AX and CF for the caller
PUSH    AX
MOV     DX, OFFSET OKMSG
JNC     RESULT1
MOV     DX, OFFSET ERRMSG
MOV     AH, 09h
INT     21h
POP     AX
PUSH    AX
CALL    PDEC          ; the error code
MOV     DX, OFFSET CRLF
RESULT1:
MOV     AH, 09h
INT     21h
POP     AX
POPF
RET
RESULT  ENDP
OKMSG   DB      "ok", 13, 10, "$"
ERRMSG  DB      "error $"
PDEC    PROC    NEAR  ; print AX in decimal (changes AX BX CX DX)
MOV     BX, 10
XOR     CX, CX        ; digits on the stack
PDEC1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
OR      AX, AX
JNZ     PDEC1
PDEC2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    PDEC2
RET
PDEC    ENDP
CRLF    DB      13, 10, "$"

Making a whole path

To create several levels, make each in turn and treat error 5 on the way as "already there":

MOV     AH, 39h
MOV     DX, OFFSET LEVEL1  ; "OUT", 0
INT     21h                ; error 5 is fine: it exists
MOV     AH, 39h
MOV     DX, OFFSET LEVEL2  ; "OUT\LOGS", 0
INT     21h

In DOSBox-X a drive mounted from a host folder gets a real folder on the host. The name is stored as DOS gives it, in capitals; with long file names enabled, function 7139h of INT 21h makes directories with long names.