Spectrum ROM: Routine at 26C9

Used by the routine at S_ALPHNUM.

When a variable name has been identified a call is made to LOOK_VARS which looks through those variables that already exist in the variables area (or in the program area at DEF FN statements for a user-defined function FN). If an appropriate numeric value is found then it is copied to the calculator stack using STACK_NUM. However a string or string array entry has to have the appropriate parameters passed to the calculator stack by STK_VAR (or in the case of a user-defined function, by STK_F_ARG as called from LOOK_VARS).

Used by the routine at S_ALPHNUM. When a variable name has been identified a call is made to LOOK_VARS which looks through those variables that already exist in the variables area (or in the program area at DEF FN statements for a user-defined function FN). If an appropriate numeric value is found then it is copied to the calculator stack using STACK_NUM. However a string or string array entry has to have the appropriate parameters passed to the calculator stack by STK_VAR (or in the case of a user-defined function, by STK_F_ARG as called from LOOK_VARS).
S_LETTER	26C9	CALL LOOK_VARS	Look in the existing variables for the matching entry.
	26CC	JP C,REPORT_2	An error is reported if there is no existing entry.
	26CF	CALL Z,STK_VAR	Stack the parameters of the string entry/return numeric element base address.
	26D2	LD A,($5C3B)	Fetch FLAGS.
	26D5	CP $C0	Test bits 6 and 7 together.
	26D7	JR C,S_CONT_1	Jump if one or both bits are reset.
	26D9	INC HL	A numeric value is to be stacked.
	26DA	CALL STACK_NUM	Move the number.
This entry point is used by the routine at S_DECIMAL.
S_CONT_1	26DD	JR S_CONT_2	Jump forward.
This entry point is used by the routine at S_ALPHNUM. The character is tested against the code for '-', thus identifying the 'unary minus' operation. Before the actual test the B register is set to hold the priority +09 and the C register the operation code +DB that are required for this operation.
S_NEGATE	26DF	LD BC,$09DB	Priority +09, operation code +DB.
	26E2	CP "-"	Is it a '-'?
	26E4	JR Z,S_PUSH_PO	Jump forward if it is 'unary minus'.
Next the character is tested against the code for 'VAL$', with priority +10 and operation code +18.
	26E6	LD BC,$1018	Priority +10, operation code +18.
	26E9	CP $AE	Is it 'VAL$'?
	26EB	JR Z,S_PUSH_PO	Jump forward if it is 'VAL$'.
The present character must now represent one of the functions CODE to NOT, with codes +AF to +C3.
	26ED	SUB $AF	The range of the functions is changed from +AF to +C3 to range +00 to +14.
	26EF	JP C,REPORT_C	Report an error if out of range.
The function 'NOT' is identified and dealt with separately from the others.
	26F2	LD BC,$04F0	Priority +04, operation code +F0.
	26F5	CP $14	Is it the function 'NOT'?
	26F7	JR Z,S_PUSH_PO	Jump if it is so.
	26F9	JP NC,REPORT_C	Check the range again.
The remaining functions have priority +10. The operation codes for these functions are now calculated. Functions that operate on strings need bit 6 reset and functions that give string results need bit 7 reset in their operation codes.
	26FC	LD B,$10	Priority +10.
	26FE	ADD A,$DC	The function range is now +DC to +EF.
	2700	LD C,A	Transfer the operation code.
	2701	CP $DF	Separate CODE, VAL and LEN which operate on strings to give numerical results.
	2703	JR NC,S_NO_TO_S
	2705	RES 6,C
S_NO_TO_S	2707	CP $EE	Separate STR$ and CHR$ which operate on numbers to give string results.
	2709	JR C,S_PUSH_PO
	270B	RES 7,C	Mark the operation codes. The other operation codes have bits 6 and 7 both set.
This entry point is used by the routine at S_INKEY. The priority code and the operation code for the function being considered are now pushed on to the machine stack. A hierarchy of operations is thereby built up.
S_PUSH_PO	270D	PUSH BC	Stack the priority and operation codes before moving on to consider the next part of the expression.
	270E	RST $20
	270F	JP S_LOOP_1
This entry point is used by the routines at S_QUOTE, S_BRACKET, S_INKEY and S_FN_SBRN. The scanning of the line now continues. The present argument may be followed by a '(', a binary operator or, if the end of the expression has been reached, then e.g. a carriage return character or a colon, a separator or a 'THEN'.
S_CONT_2	2712	RST $18	Fetch the present character.
S_CONT_3	2713	CP "("	Jump forward if it is not a '(', which indicates a parenthesised expression.
	2715	JR NZ,S_OPERTR
If the 'last value' is numeric then the parenthesised expression is a true sub-expression and must be evaluated by itself. However if the 'last value' is a string then the parenthesised expression represents an element of an array or a slice of a string. A call to SLICING modifies the parameters of the string as required.
	2717	BIT 6,(IY+$01)	Jump forward if dealing with a numeric parenthesised expression (bit 6 of FLAGS set).
	271B	JR NZ,S_LOOP
	271D	CALL SLICING	Modify the parameters of the 'last value'.
	2720	RST $20	Move on to consider the next character.
	2721	JR S_CONT_3	Move on to consider the next character.
If the present character is indeed a binary operator it will be given an operation code in the range +C3 to +CF, and the appropriate priority code.
S_OPERTR	2723	LD B,$00	Original code to BC to index into the table of operators.
	2725	LD C,A	Original code to BC to index into the table of operators.
	2726	LD HL,$2795	The pointer to the table.
	2729	CALL INDEXER	Index into the table.
	272C	JR NC,S_LOOP	Jump forward if no operation found.
	272E	LD C,(HL)	Get required code from the table.
	272F	LD HL,$26ED	The pointer to the priority table (26ED+C3 gives PRIORITIES as the first address).
	2732	ADD HL,BC	Index into the table.
	2733	LD B,(HL)	Fetch the appropriate priority.
The main loop of this subroutine is now entered. At this stage there are: i. A 'last value' on the calculator stack. ii. The starting priority marker on the machine stack below a hierarchy, of unknown size, of function and binary operation codes. This hierarchy may be null. iii. The BC register pair holding the 'present' operation and priority, which if the end of an expression has been reached will be priority zero. Initially the 'last' operation and priority are taken off the machine stack and compared against the 'present' operation and priority.
S_LOOP	2734	POP DE	Get the 'last' operation and priority.
	2735	LD A,D	The priority goes to the A register.
	2736	CP B	Compare 'last' against 'present'.
	2737	JR C,S_TIGHTER	Exit to wait for the argument.
	2739	AND A	Are both priorities zero?
	273A	JP Z,GET_CHAR	Exit via GET_CHAR thereby making 'last value' the required result.
Before the 'last' operation is performed, the 'USR' function is separated into 'USR number' and 'USR string' according as bit 6 of FLAGS was set or reset when the argument of the function was stacked as the 'last value'.
	273D	PUSH BC	Stack the 'present' values.
	273E	LD HL,$5C3B	This is FLAGS.
	2741	LD A,E	The 'last' operation is compared with the code for USR, which will give 'USR number' unless modified; jump if not 'USR'.
	2742	CP $ED
	2744	JR NZ,S_STK_LST
	2746	BIT 6,(HL)	Test bit 6 of FLAGS.
	2748	JR NZ,S_STK_LST	Jump if it is set ('USR number').
	274A	LD E,$99	Modify the 'last' operation code: 'offset' +19, plus +80 for string input and numerical result ('USR string').
S_STK_LST	274C	PUSH DE	Stack the 'last' values briefly.
	274D	CALL SYNTAX_Z	Do not perform the actual operation if syntax is being checked.
	2750	JR Z,S_SYNTEST
	2752	LD A,E	The 'last' operation code.
	2753	AND $3F	Strip off bits 6 and 7 to convert the operation code to a calculator offset.
	2755	LD B,A
	2756	RST $28	Now use the calculator.
	2757	DEFB $3B	fp_calc_2: (perform the actual operation)
	2758	DEFB $38	end_calc
	2759	JR S_RUNTEST	Jump forward.
An important part of syntax checking involves the testing of the operation to ensure that the nature of the 'last value' is of the correct type for the operation under consideration.
S_SYNTEST	275B	LD A,E	Get the 'last' operation code.
	275C	XOR (IY+$01)	This tests the nature of the 'last value' (bit 6 of FLAGS) against the requirement of the operation. They are to be the same for correct syntax.
	275F	AND $40
S_RPORT_C_2	2761	JP NZ,REPORT_C	Jump if syntax fails.
Before jumping back to go round the loop again the nature of the 'last value' must be recorded in FLAGS.
S_RUNTEST	2764	POP DE	Get the 'last' operation code.
	2765	LD HL,$5C3B	This is FLAGS.
	2768	SET 6,(HL)	Assume result to be numeric.
	276A	BIT 7,E	Jump forward if the nature of 'last value' is numeric.
	276C	JR NZ,S_LOOPEND	Jump forward if the nature of 'last value' is numeric.
	276E	RES 6,(HL)	It is a string.
S_LOOPEND	2770	POP BC	Get the 'present' values into BC.
	2771	JR S_LOOP	Jump back.
Whenever the 'present' operation binds tighter, the 'last' and the 'present' values go back on the machine stack. However if the 'present' operation requires a string as its operand then the operation code is modified to indicate this requirement.
S_TIGHTER	2773	PUSH DE	The 'last' values go on the stack.
	2774	LD A,C	Get the 'present' operation code.
	2775	BIT 6,(IY+$01)	Do not modify the operation code if dealing with a numeric operand (bit 6 of FLAGS set).
	2779	JR NZ,S_NEXT
	277B	AND $3F	Clear bits 6 and 7.
	277D	ADD A,$08	Increase the code by +08.
	277F	LD C,A	Return the code to the C register.
	2780	CP $10	Is the operation 'AND'?
	2782	JR NZ,S_NOT_AND	Jump if it is not so.
	2784	SET 6,C	'AND' requires a numeric operand.
	2786	JR S_NEXT	Jump forward.
S_NOT_AND	2788	JR C,S_RPORT_C_2	The operations -, *, /, ↑ and OR are not possible between strings.
	278A	CP $17	Is the operation a '+'?
	278C	JR Z,S_NEXT	Jump if it is so.
	278E	SET 7,C	The other operations yield a numeric result.
S_NEXT	2790	PUSH BC	The 'present' values go on the machine stack.
	2791	RST $20	Consider the next character.
	2792	JP S_LOOP_1	Go around the loop again.