Spectrum ROM: Routine at 12A2

12A2: THE 'MAIN EXECUTION' LOOP

Used by the routine at MAIN_ADD. The main loop controls the 'editing mode', the execution of direct commands and the production of reports.
MAIN_EXEC	12A2	LD (IY+$31),$02	The lower part of the screen is to be two lines in size (see DF-SZ).
	12A6	CALL AUTO_LIST	Produce an automatic listing.
This entry point is used by the routine at NEW.
MAIN_1	12A9	CALL SET_MIN	All the areas from E-LINE onwards are given their minimum configurations.
MAIN_2	12AC	LD A,$00	Channel 'K' is opened before calling the EDITOR.
	12AE	CALL CHAN_OPEN	Channel 'K' is opened before calling the EDITOR.
	12B1	CALL EDITOR	The EDITOR is called to allow the user to build up a BASIC line.
	12B4	CALL LINE_SCAN	The current line is scanned for correct syntax.
	12B7	BIT 7,(IY+$00)	Jump forward if the syntax is correct (ERR-NR is +FF).
	12BB	JR NZ,MAIN_3	Jump forward if the syntax is correct (ERR-NR is +FF).
	12BD	BIT 4,(IY+$30)	Jump forward if other than channel 'K' is being used (bit 4 of FLAGS2 is set).
	12C1	JR Z,MAIN_4
	12C3	LD HL,($5C59)	Point to the start of the line with the error (E-LINE).
	12C6	CALL REMOVE_FP	Remove the floating-point forms from this line.
	12C9	LD (IY+$00),$FF	Reset ERR-NR and jump back to MAIN_2 leaving the listing unchanged.
	12CD	JR MAIN_2
The 'edit-line' has passed syntax and the three types of line that are possible have to be distinguished from each other.
MAIN_3	12CF	LD HL,($5C59)	Point to the start of the line (E-LINE).
	12D2	LD ($5C5D),HL	Set CH-ADD to the start also.
	12D5	CALL E_LINE_NO	Fetch any line number into BC.
	12D8	LD A,B	Is the line number a valid one?
	12D9	OR C	Is the line number a valid one?
	12DA	JP NZ,MAIN_ADD	Jump if it is so, and add the new line to the existing program.
	12DD	RST $18	Fetch the first character of the line and see if the line is 'carriage return only'.
	12DE	CP $0D
	12E0	JR Z,MAIN_EXEC	If it is then jump back.
The 'edit-line' must start with a direct BASIC command so this line becomes the first line to be interpreted.
	12E2	BIT 0,(IY+$30)	Clear the whole display unless the flag (bit 0 of FLAGS2) says it is unnecessary.
	12E6	CALL NZ,CL_ALL
	12E9	CALL CLS_LOWER	Clear the lower part anyway.
	12EC	LD A,$19	Set the appropriate value for the scroll counter (SCR-CT) by subtracting the second byte of S-POSN from +19).
	12EE	SUB (IY+$4F)
	12F1	LD ($5C8C),A
	12F4	SET 7,(IY+$01)	Signal 'line execution' (set bit 7 of FLAGS).
	12F8	LD (IY+$00),$FF	Ensure ERR-NR is correct.
	12FC	LD (IY+$0A),$01	Deal with the first statement in the line (set NSPPC to +01).
	1300	CALL LINE_RUN	Now the line is interpreted. Note: the address MAIN_4 goes on to the machine stack and is addressed by ERR-SP.
After the line has been interpreted and all the actions consequential to it have been completed a return is made to MAIN_4, so that a report can be made.
MAIN_4	1303	HALT	The maskable interrupt must be enabled.
	1304	RES 5,(IY+$01)	Signal 'ready for a new key' (reset bit 5 of FLAGS).
	1308	BIT 1,(IY+$30)	Empty the printer buffer if it has been used (bit 1 of FLAGS2 set).
	130C	CALL NZ,COPY_BUFF
	130F	LD A,($5C3A)	Fetch the error number (ERR-NR) and increment it.
	1312	INC A	Fetch the error number (ERR-NR) and increment it.
This entry point is used by the routine at REPORT_G.
MAIN_G	1313	PUSH AF	Save the new value.
	1314	LD HL,$0000	The system variables FLAGX, X-PTR-hi and DEFADD are all set to zero.
	1317	LD (IY+$37),H
	131A	LD (IY+$26),H
	131D	LD ($5C0B),HL
	1320	LD HL,$0001	Ensure that stream +00 points to channel 'K' (see STRMS).
	1323	LD ($5C16),HL	Ensure that stream +00 points to channel 'K' (see STRMS).
	1326	CALL SET_MIN	Clear all the work areas and the calculator stack.
	1329	RES 5,(IY+$37)	Signal 'editing mode' (reset bit 5 of FLAGX).
	132D	CALL CLS_LOWER	Clear the lower screen.
	1330	SET 5,(IY+$02)	Signal 'the lower screen will require clearing' (set bit 5 of TV-FLAG).
	1334	POP AF	Fetch the report value.
	1335	LD B,A	Make a copy in B.
	1336	CP $0A	Jump forward with report numbers '0 to 9'.
	1338	JR C,MAIN_5	Jump forward with report numbers '0 to 9'.
	133A	ADD A,$07	Add the ASCII letter offset value.
MAIN_5	133C	CALL OUT_CODE	Print the report code and follow it with a 'space'.
	133F	LD A," "
	1341	RST $10
	1342	LD A,B	Fetch the report value (used to identify the required report message).
	1343	LD DE,$1391	Print the message.
	1346	CALL PO_MSG	Print the message.
	1349	XOR A	Follow it by a 'comma' and a 'space'.
	134A	LD DE,$1536
	134D	CALL PO_MSG
	1350	LD BC,($5C45)	Now fetch the current line number (PPC) and print it as well.
	1354	CALL OUT_NUM_1
	1357	LD A,":"	Follow it by a ':'.
	1359	RST $10	Follow it by a ':'.
	135A	LD C,(IY+$0D)	Fetch the current statement number (SUBPPC) into the BC register pair and print it.
	135D	LD B,$00
	135F	CALL OUT_NUM_1
	1362	CALL CLEAR_SP	Clear the editing area.
	1365	LD A,($5C3A)	Fetch the error number (ERR-NR) again.
	1368	INC A	Increment it as usual.
	1369	JR Z,MAIN_9	If the program was completed successfully there cannot be any 'CONTinuing' so jump.
	136B	CP $09	If the program halted with 'STOP statement' or 'BREAK into program' CONTinuing will be from the next statement; otherwise SUBPPC is unchanged.
	136D	JR Z,MAIN_6
	136F	CP $15
	1371	JR NZ,MAIN_7
MAIN_6	1373	INC (IY+$0D)
MAIN_7	1376	LD BC,$0003	The system variables OLDPPC and OSPCC have now to be made to hold the CONTinuing line and statement numbers.
	1379	LD DE,$5C70
	137C	LD HL,$5C44	The values used will be those in PPC and SUBPPC unless NSPPC indicates that the 'break' occurred before a 'jump' (i.e. after a GO TO statement etc.).
	137F	BIT 7,(HL)
	1381	JR Z,MAIN_8
	1383	ADD HL,BC
MAIN_8	1384	LDDR
MAIN_9	1386	LD (IY+$0A),$FF	NSPPC is reset to indicate 'no jump'.
	138A	RES 3,(IY+$01)	'K mode' is selected (reset bit 3 of FLAGS).
	138E	JP MAIN_2	And finally the jump back is made but no program listing will appear until requested.