| Routines

13500: THE 'USR STRING' FUNCTION (offset 25)

The address of this routine is found in the table of addresses. It is called indirectly via fp_calc_2. This subroutine handles the function USR X$, where X$ is a string. The subroutine returns in BC the address of the bit pattern for the user-defined graphic corresponding to X$. It reports error A if X$ is not a single letter between 'a' and 'u' or a user-defined graphic.
usr | 13500 | CALL STK_FETCH | Fetch the parameters of the string X$.
| 13503 | DEC BC | Decrease the length by 1 to test it.
| 13504 | LD A,B | If the length was not 1, then jump to give error report A.
| 13505 | OR C
| 13506 | JR NZ,REPORT_A
| 13508 | LD A,(DE) | Fetch the single code of the string.
| 13509 | CALL ALPHA | Does it denote a letter?
| 13512 | JR C,USR_RANGE | If so, jump to gets its address.
| 13514 | SUB 144 | Reduce range for actual user-defined graphics to 0-20.
| 13516 | JR C,REPORT_A | Give report A if out of range.
| 13518 | CP 21 | Test the range again.
| 13520 | JR NC,REPORT_A | Give report A if out of range.
| 13522 | INC A | Make range of user-defined graphics 1 to 21, as for 'a' to 'u'.
USR_RANGE | 13523 | DEC A | Now make the range 0 to 20 in each case.
| 13524 | ADD A,A | Multiply by 8 to get an offset for the address.
| 13525 | ADD A,A
| 13526 | ADD A,A
| 13527 | CP 168 | Test the range of the offset.
| 13529 | JR NC,REPORT_A | Give report A if out of range.
| 13531 | LD BC,(23675) | Fetch the address of the first user-defined graphic (UDG) in BC.
| 13535 | ADD A,C | Add C to the offset.
| 13536 | LD C,A | Store the result back in C.
| 13537 | JR NC,USR_STACK | Jump if there is no carry.
| 13539 | INC B | Increment B to complete the address.
USR_STACK | 13540 | JP STACK_BC | Jump to stack the address.
Report A - Invalid argument.
REPORT_A | 13543 | RST 8 | Call the error handling routine.
| 13544 | DEFB 9