Return codes from validity checking routines

Your validity checking routines must return a code in general register 15 to the Parse Service Routine. These codes inform the Parse Service Routine of the results of the validity check and determine the action that parse should take. Table 1 shows the return codes, their meaning, and the action taken by the Parse Service Routine.

Table 1. Return codes from a validity checking routine
Return code dec(Hex)	Meaning	Action taken by parse
0(0)	The operand is valid.	No additional processing is performed on this operand by the Parse Service Routine.
4(4)	The operand is not valid.	The Parse Service Routine writes an error message to the terminal and prompts for a valid operand.
8(8)	The operand is not valid.	The validity checking routine has issued an error message; parse prompts for a valid operand.
12(C)	The operand is not valid; syntax checking cannot continue.	The Parse Service Routine stops all further syntax checking, sets a return code of 20, and returns to the calling routine.

If the Parse Service Routine receives a return code of 4 or 8, it processes new data entered in response to the prompt as though it were the original data, and passes control again to the validity checking routine. This cycle continues until a valid operand is obtained.

Prior to issuing a return code of 12, your validity checking routine should issue a message indicating that it has requested that parse terminate.