From QB64 Wiki
ERDEV is an integer function that returns an error code from the last device to create an error.
- Currently NOT supported in QB64!
- The code is bit-encoded containing DOS error information, the first 8-bit (first byte) contain the DOS error code, while the second bit contains device specific information (bits 15, 14, 13, (12-4 always zero), 3, 2, 1 in that order of the device attribute word).
- ERDEV is set by the critical error handler (interrupt 24h) when DOS encounters a error that prevents it from continuing.