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155D: THE 'MAIN-ADD' SUBROUTINE
Used by the routine at MAIN_EXEC.
This subroutine allows for a new BASIC line to be added to the existing BASIC program in the program area. If a line has both an old and a new version then the old one is 'reclaimed'. A new line that consists of only a line number does not go into the program area.
Input
BC BASIC line number
Output
MAIN_ADD 155D LD ($5C49),BC Make the new line number the 'current line' (E-PPC).
1561 LD HL,($5C5D) Fetch CH-ADD and save the address in DE.
1564 EX DE,HL
1565 LD HL,$1555 Push the address of REPORT_G on to the machine stack. ERR-SP will now point to REPORT_G.
1568 PUSH HL
1569 LD HL,($5C61) Fetch WORKSP.
156C SCF Find the length of the line from after the line number to the 'carriage return' character inclusively.
156D SBC HL,DE
156F PUSH HL Save the length.
1570 LD H,B Move the line number to the HL register pair.
1571 LD L,C
1572 CALL LINE_ADDR Is there an existing line with this number?
1575 JR NZ,MAIN_ADD1 Jump if there was not.
1577 CALL NEXT_ONE Find the length of the 'old' line and reclaim it.
157A CALL RECLAIM_2
MAIN_ADD1 157D POP BC Fetch the length of the 'new' line and jump forward if it is only a 'line number and a carriage return'.
157E LD A,C
157F DEC A
1580 OR B
1581 JR Z,MAIN_ADD2
1583 PUSH BC Save the length.
1584 INC BC Four extra locations will be needed, i.e. two for the number and two for the length.
1585 INC BC
1586 INC BC
1587 INC BC
1588 DEC HL Make HL point to the location before the 'destination'.
1589 LD DE,($5C53) Save the current value of PROG to avoid corruption when adding a first line.
158D PUSH DE
158E CALL MAKE_ROOM Space for the new line is created.
1591 POP HL The old value of PROG is fetched and restored.
1592 LD ($5C53),HL
1595 POP BC A copy of the line length (without parameters) is taken.
1596 PUSH BC
1597 INC DE Make DE point to the end location of the new area and HL to the 'carriage return' character of the new line in the editing area (WORKSP-2).
1598 LD HL,($5C61)
159B DEC HL
159C DEC HL
159D LDDR Now copy over the line.
159F LD HL,($5C49) Fetch the line's number (E_PPC).
15A2 EX DE,HL Destination into HL and number into DE.
15A3 POP BC Fetch the new line's length.
15A4 LD (HL),B The high length byte.
15A5 DEC HL
15A6 LD (HL),C The low length byte.
15A7 DEC HL
15A8 LD (HL),E The low line number byte.
15A9 DEC HL
15AA LD (HL),D The high line number byte.
MAIN_ADD2 15AB POP AF Drop the address of REPORT_G.
15AC JP MAIN_EXEC Jump back and this time do produce an automatic listing.
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