Commodore VIC-20 Programmer's Reference Manual page 226

LOAD THE SYSTEM RAM VECTORS
1) Clear the carry bit.
2) Set the .X and .Y registers to the address of the vector list in
RAM thai must be loaded
3) Call this routine.
EXAMPLE:
CHANGE THE INPUT ROUTINES TO NEW SYSTEM
LDX #<USER
LDY #>USER
SEC
JSR VHCTOR ;read old vectors
LDA #<MYINP ;change input
STA USER-5-10
LDA #>MYINP
STA USER +11
LDX #<USER
LDY #>USER
CLC
JSR VECTOR ;alter system
USER * = * + 26
ERROR CODES
The following is a list of error messages which can occur when
using the KERNAL routines. If an error occurs during a KERNAL
routine, the carry bit ot the accumulator is set and the number of the
error message is returned in the accumulator.
NUMBER
0
1
2
3
4
5
6
7
8
9
MEANING
Routine terminated by the STOP key
Too many open files
File already open
File not open
File not found
Device not present
File is not an input file
File is not an output file
File name is missing
Illegal device number
I
210