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34BC: THE 'USR STRING' FUNCTION (offset +19)
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 34BC CALL STK_FETCH Fetch the parameters of the string X$.
34BF DEC BC Decrease the length by 1 to test it.
34C0 LD A,B If the length was not 1, then jump to give error report A.
34C1 OR C
34C4 LD A,(DE) Fetch the single code of the string.
34C5 CALL ALPHA Does it denote a letter?
34C8 JR C,USR_RANGE If so, jump to gets its address.
34CA SUB $90 Reduce range for actual user-defined graphics to 0-20.
34CC JR C,REPORT_A Give report A if out of range.
34CE CP $15 Test the range again.
34D0 JR NC,REPORT_A Give report A if out of range.
34D2 INC A Make range of user-defined graphics 1 to 21, as for 'a' to 'u'.
USR_RANGE 34D3 DEC A Now make the range 0 to 20 in each case.
34D4 ADD A,A Multiply by 8 to get an offset for the address.
34D5 ADD A,A
34D6 ADD A,A
34D7 CP $A8 Test the range of the offset.
34D9 JR NC,REPORT_A Give report A if out of range.
34DB LD BC,($5C7B) Fetch the address of the first user-defined graphic (UDG) in BC.
34DF ADD A,C Add C to the offset.
34E0 LD C,A Store the result back in C.
34E1 JR NC,USR_STACK Jump if there is no carry.
34E3 INC B Increment B to complete the address.
USR_STACK 34E4 JP STACK_BC Jump to stack the address.
Report A - Invalid argument.
REPORT_A 34E7 RST $08 Call the error handling routine.
34E8 DEFB $09
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