mov bx, Handle ;handle of file or device
mov cx, MaxBytes ;maximum number of bytes to write
mov dx, seg Buffer
mov ds, dx
mov dx, offset Buffer ;ds:dx points to buffer containing data
mov ah, 40h ;Write File or Device
int 21h
jc error_handler ;carry set means error
mov ActualBytes, ax ;number of bytes written
Write File or Device (Function 40h) writes up to the specified number of bytes of data from a buffer to a file or device.
Handle
Identifies the file or device that is to receive the data.
MaxBytes
Specifies the maximum number of bytes to write.
Buffer
Points to a buffer that contains the data to write.
If the function is successful, the carry flag is clear and the AX register contains the number of bytes written to the file or device. Otherwise, the carry flag is set and the AX register contains an error value, which may be one of the following values:
| Value | Name |
| 0005h | ERROR_ACCESS_DENIED |
| 0006h | ERROR_INVALID_HANDLE |
Handle can be a handle for a standard device or a handle created by using such a function as Open File with Handle (Function 3Dh).
When MS-DOS writes to a file, it writes data starting at the current location of the file pointer. When this function returns, the file pointer is positioned at the byte immediately after the last byte written to the file.
Writing 0 bytes to the file truncates the file at the current position of the file pointer.
If the number of bytes written is fewer than the number requested, the destination file or disk is full. Note that the carry flag is not set in this situation.
Function 3Ch Create File with Handle
Function 3Dh Open File with Handle
Function 3Fh Read File or Device
Function 42h Move File Pointer
Function 5Ah Create Temporary File
Function 5Bh Create New File
Function 6Ch Extended Open/Create