INT 21h 4E01h vs COM/EXE: Determining the Maximum Command‑Line Length

Answer

The absolute maximum length of a command‑line string that a COM or EXE program can receive is 127 characters. This value is stored in the length byte at PSP offset 0x80; the characters themselves begin at offset 0x81 and are terminated by a carriage‑return (0x0D), not a null byte.

Confirmed facts (official MS‑DOS kernels 1.x‑6.22)

1. The PSP reserves one byte at offset 0x80 for the command‑line length.
2. All official MS‑DOS kernels limit this byte to a maximum of 0x7F (127).
3. The limit applies equally to COM and EXE programs loaded by DOS.
4. The length byte counts only the characters; the terminating CR is not counted.
5. INT 21h AH=4Eh (Find First File) does not return the PSP address; to obtain the PSP use AH=62h (DOS 3+) or AH=51h (all versions).

Likely explanation / variations

• Very early DOS releases (1.x) use the same 0x7F ceiling; documentation is sparse but behavior matches later versions.
• Some DOS extenders advertise an enlarged PSP (e.g., allowing a length byte of 0xFF) but this is non‑standard and can cause native programs to misinterpret the command tail.
• The 127‑character ceiling limits the total byte count of the command tail; a program may still split that single string into many arguments, but the sum cannot exceed 127 bytes.

Verification steps (minimal, safe)

; Sample COM program (NASM syntax) that prints the command‑line length
org 0x100          ; COM programs start at 0x100
mov ah, 0x62       ; Get PSP segment
int 0x21
mov es, bx         ; ES = PSP segment
mov al, [es:0x80]  ; Load length byte (0‑0x7F)
; Convert AL to hex and print (omitted for brevity)
mov ah, 0x4C       ; Exit to DOS
int 0x21

1. Assemble the code into a COM file, e.g., cmdlen.com.
2. Run it under an authentic MS‑DOS 6.22 image (DOSBox or real hardware).
3. Invoke it with progressively longer command lines, e.g.:
   cmdlen.com A
cmdlen.com AA … up to 127 characters.
4. Observe that the printed length byte equals the actual character count until 0x7F is reached.
5. Supply a line of 128 characters; the length byte will remain 0x7F and the tail will be truncated after 127 characters, followed by the CR.
6. If a DOS extender is present, repeat the test and note whether the length byte exceeds 0x7F; report any side‑effects on native programs.

Missing diagnostic detail

If you are testing with a DOS extender that claims to modify the PSP, please specify the extender name and version so that the impact on compatibility can be assessed.