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1795: THE 'LIST and LLIST' COMMAND ROUTINES
The routines in this part of the 16K program are used to produce listings of the current BASIC program. Each line has to have its line number evaluated, its tokens expanded and the appropriate cursors positioned.
The entry point AUTO_LIST is used by both MAIN_EXEC and ED_UP to produce a single page of the listing.
Input
Output
AUTO_LIST 1795 LD ($5C3F),SP The stack pointer is saved at LIST-SP allowing the machine stack to be reset when the listing is finished (see PO_SCR).
1799 LD (IY+$02),$10 Signal 'automatic listing in the main screen' (set bit 4 of TV-FLAG and reset all other bits).
179D CALL CL_ALL Clear this part of the screen.
17A0 SET 0,(IY+$02) Switch to the editing area (set bit 0 of TV-FLAG).
17A4 LD B,(IY+$31) Now clear the the lower part of the screen as well (see DF-SZ).
17A7 CALL CL_LINE
17AA RES 0,(IY+$02) Then switch back (reset bit 0 of TV-FLAG).
17AE SET 0,(IY+$30) Signal 'screen is clear' (set bit 0 of FLAGS2).
17B2 LD HL,($5C49) Now fetch the the 'current' line number (E-PPC) and the 'automatic' line number (S-TOP).
17B5 LD DE,($5C6C)
17B9 AND A If the 'current' number is less than the 'automatic' number then jump forward to update the 'automatic' number.
17BA SBC HL,DE
17BC ADD HL,DE
17BD JR C,AUTO_L_2
The 'automatic' number has now to be altered to give a listing with the 'current' line appearing near the bottom of the screen.
17BF PUSH DE Save the 'automatic' number.
17C0 CALL LINE_ADDR Find the address of the start of the 'current' line and produce an address roughly a 'screen before it' (negated).
17C3 LD DE,$02C0
17C6 EX DE,HL
17C7 SBC HL,DE
17C9 EX (SP),HL Save the 'result' on the machine stack whilst the 'automatic' line address is also found (in HL).
17CA CALL LINE_ADDR
17CD POP BC The 'result' goes to the BC register pair.
A loop is now entered. The 'automatic' line number is increased on each pass until it is likely that the 'current' line will show on a listing.
AUTO_L_1 17CE PUSH BC Save the 'result'.
17CF CALL NEXT_ONE Find the address of the start of the line after the present 'automatic' line (in DE).
17D2 POP BC Restore the 'result'.
17D3 ADD HL,BC Perform the computation and jump forward if finished.
17D4 JR C,AUTO_L_3
17D6 EX DE,HL Move the next line's address to the HL register pair and collect its line number.
17D7 LD D,(HL)
17D8 INC HL
17D9 LD E,(HL)
17DA DEC HL
17DB LD ($5C6C),DE Now S-TOP can be updated and the test repeated with the new line.
17DF JR AUTO_L_1
Now the 'automatic' listing can be made.
AUTO_L_2 17E1 LD ($5C6C),HL When E-PPC is less than S-TOP.
AUTO_L_3 17E4 LD HL,($5C6C) Fetch the top line's number (S-TOP) and hence its address.
17E7 CALL LINE_ADDR
17EA JR Z,AUTO_L_4 If the line cannot be found use DE instead.
17EC EX DE,HL
AUTO_L_4 17ED CALL LIST_ALL The listing is produced.
17F0 RES 4,(IY+$02) The return will be to here unless scrolling was needed to show the current line; reset bit 4 of TV-FLAG before returning.
17F4 RET
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