The disk transfer area is a buffer DOS writes results into, at an address the program chooses. The name comes from CP/M: the FCB services (14h, 15h, 21h, 22h) read and write their records through it. Today its main user is the directory search, INT 21h AH=4Eh and 4Fh, which put each entry they find in the DTA.

1Ah sets the DTA to DS:DX. DOS only stores the far address; it does not check or clear the buffer. 2Fh returns the current address in ES:BX, which means it changes ES: in a .COM, PUSH CS / POP ES puts it back.

The default DTA

When a program starts, its DTA is PSP:0080h, the 128 bytes that hold the command tail. A find first with that DTA overwrites the program's arguments, so either read the tail first, or give the search a DTA of its own: 43 bytes are enough for 4Eh and 4Fh, 128 for FCB records.

prints: DTA at 0080 in our PSP / DTA now at our buffer
MOV     AH, 2Fh       ; DOS: get the DTA in ES:BX
INT     21h
MOV     OLDOFS1, BX
MOV     OLDSEG1, ES
MOV     DX, OFFSET ATM1
MOV     AH, 09h
INT     21h
MOV     AX, OLDOFS1
CALL    PHEX          ; 0080
MOV     DX, OFFSET INPSP1
MOV     AX, CS
CMP     AX, OLDSEG1   ; in a .COM, CS is the PSP
JE      SAY1
MOV     DX, OFFSET OTHER1
SAY1:
MOV     AH, 09h
INT     21h
MOV     AH, 1Ah       ; DOS: the DTA is DS:DX from now on
MOV     DX, OFFSET DTA1
INT     21h
MOV     AH, 2Fh       ; ask again
INT     21h
MOV     DX, OFFSET NOWM1
CMP     BX, OFFSET DTA1
JE      SAY2
MOV     DX, OFFSET OTHER1
SAY2:
MOV     AH, 09h
INT     21h
PUSH    CS            ; 2Fh changed ES
POP     ES
MOV     AH, 1Ah       ; and the DTA back where it was
MOV     DX, 80h
INT     21h
 
OLDOFS1 DW      0
OLDSEG1 DW      0
ATM1    DB      "DTA at $"
INPSP1  DB      " in our PSP", 13, 10, "$"
OTHER1  DB      " somewhere else", 13, 10, "$"
NOWM1   DB      "DTA now at our buffer", 13, 10, "$"
DTA1    DB      128 DUP(0)
PHEX    PROC    NEAR  ; print AX as four hex digits (changes AX BX CX DX)
MOV     BX, AX
MOV     CX, 4
PHEX1:
PUSH    CX
MOV     CL, 4
ROL     BX, CL        ; the top digit to the bottom
POP     CX
MOV     DL, BL
AND     DL, 0Fh
ADD     DL, "0"
CMP     DL, "9"
JBE     PHEX2
ADD     DL, 7         ; "A" to "F"
PHEX2:
MOV     AH, 02h
INT     21h
LOOP    PHEX1
RET
PHEX    ENDP

Whose DTA

DOS keeps a single DTA address, the running program's. EXEC sets it to the child's PSP:0080h, and a parent should not count on finding its own again after INT 21h AH=4Bh: set it again. A resident program or an interrupt handler that searches for files must save the DTA with 2Fh, set its own with 1Ah, and restore it before returning, or it destroys the search state of whatever program it interrupted.