DPRINTF is a kernel-debugging print-formatting package. You can include this function in the code to test your display driver.
To use the DPRINTF function, you must have a second device attached to COM1 or COM2. Due to difficulties passing variable-length argument lists through a call-gate transition, only one argument, a 0-terminated string, is passed to this routine. The string is sent to either COM1 or COM2 depending on how the variable, UR_DAT, is defined.
The only checking that this routine performs is to process XON/XOFF characters from the equipment attached to the debug port. This guarantees that the output does not overrun the receiving device. However, after receiving XOFF, this routine spins in a loop, waiting for XON.
The string to be output is ASCIIZ. It may contain literal characters.
A literal character is defined as any character that is not part of a format specification. Special non-printing characters are listed as follows: