INT 21,3F - Read From File or Device Using Handle

AH = 3F

BX = file handle

CX = number of bytes to read

DS:DX = pointer to read buffer

on return:

AX = number of bytes read is CF not set

= error code if CF set (see ~DOS ERROR CODES~)

- read specified number of bytes from file into buffer DS:DX

- when AX is not equal to CX then a partial read occurred due

to end of file

- if AX is zero, no data was read, and EOF occurred before read