INT 0x21 - 0x40 - Write To File or Device Using Handle

Request/Call:

reg	value	descr
AH	0x40
BX		File Handle
CX		number of bytes to write, a zero value truncates/extends the file to the current file position
DS:DX	pointer to data buffer to write

CF cleared : success

reg	descr
AX	number of written bytes

CF Set: error

reg	descr
AX	error code (see (see DOS Error Code))

if AX is not equal to CX on return, a partial write occurred
this function can be used to truncate a file to the current file position by writing zero bytes