Spectrum ROM: Routine at 09929

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	09929	CALL LOOK_VARS	Look in the existing variables for the matching entry.
	09932	JP C,REPORT_2	An error is reported if there is no existing entry.
	09935	CALL Z,STK_VAR	Stack the parameters of the string entry/return numeric element base address.
	09938	LD A,(23611)	Fetch FLAGS.
	09941	CP 192	Test bits 6 and 7 together.
	09943	JR C,S_CONT_1	Jump if one or both bits are reset.
	09945	INC HL	A numeric value is to be stacked.
	09946	CALL STACK_NUM	Move the number.
This entry point is used by the routine at S_DECIMAL.
S_CONT_1	09949	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 9 and the C register the operation code 219 that are required for this operation.
S_NEGATE	09951	LD BC,2523	Priority 9, operation code 219.
	09954	CP "-"	Is it a '-'?
	09956	JR Z,S_PUSH_PO	Jump forward if it is 'unary minus'.
Next the character is tested against the code for 'VAL$', with priority 16 and operation code 24.
	09958	LD BC,4120	Priority 16, operation code 24.
	09961	CP 174	Is it 'VAL$'?
	09963	JR Z,S_PUSH_PO	Jump forward if it is 'VAL$'.
The present character must now represent one of the functions CODE to NOT, with codes 175 to 195.
	09965	SUB 175	The range of the functions is changed from 175 to 195 to range 0 to 20.
	09967	JP C,REPORT_C	Report an error if out of range.
The function 'NOT' is identified and dealt with separately from the others.
	09970	LD BC,1264	Priority 4, operation code 240.
	09973	CP 20	Is it the function 'NOT'?
	09975	JR Z,S_PUSH_PO	Jump if it is so.
	09977	JP NC,REPORT_C	Check the range again.
The remaining functions have priority 16. The operation codes for these functions are now calculated. Functions that operate on strings need bit 6 reset and functions that give string results need bit 7 reset in their operation codes.
	09980	LD B,16	Priority 16.
	09982	ADD A,220	The function range is now 220 to 239.
	09984	LD C,A	Transfer the operation code.
	09985	CP 223	Separate CODE, VAL and LEN which operate on strings to give numerical results.
	09987	JR NC,S_NO_TO_S
	09989	RES 6,C
S_NO_TO_S	09991	CP 238	Separate STR$ and CHR$ which operate on numbers to give string results.
	09993	JR C,S_PUSH_PO
	09995	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	09997	PUSH BC	Stack the priority and operation codes before moving on to consider the next part of the expression.
	09998	RST 32
	09999	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	10002	RST 24	Fetch the present character.
S_CONT_3	10003	CP "("	Jump forward if it is not a '(', which indicates a parenthesised expression.
	10005	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.
	10007	BIT 6,(IY+1)	Jump forward if dealing with a numeric parenthesised expression (bit 6 of FLAGS set).
	10011	JR NZ,S_LOOP
	10013	CALL SLICING	Modify the parameters of the 'last value'.
	10016	RST 32	Move on to consider the next character.
	10017	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 195 to 207, and the appropriate priority code.
S_OPERTR	10019	LD B,0	Original code to BC to index into the table of operators.
	10021	LD C,A	Original code to BC to index into the table of operators.
	10022	LD HL,10133	The pointer to the table.
	10025	CALL INDEXER	Index into the table.
	10028	JR NC,S_LOOP	Jump forward if no operation found.
	10030	LD C,(HL)	Get required code from the table.
	10031	LD HL,9965	The pointer to the priority table (9965+195 gives PRIORITIES as the first address).
	10034	ADD HL,BC	Index into the table.
	10035	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	10036	POP DE	Get the 'last' operation and priority.
	10037	LD A,D	The priority goes to the A register.
	10038	CP B	Compare 'last' against 'present'.
	10039	JR C,S_TIGHTER	Exit to wait for the argument.
	10041	AND A	Are both priorities zero?
	10042	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'.
	10045	PUSH BC	Stack the 'present' values.
	10046	LD HL,23611	This is FLAGS.
	10049	LD A,E	The 'last' operation is compared with the code for USR, which will give 'USR number' unless modified; jump if not 'USR'.
	10050	CP 237
	10052	JR NZ,S_STK_LST
	10054	BIT 6,(HL)	Test bit 6 of FLAGS.
	10056	JR NZ,S_STK_LST	Jump if it is set ('USR number').
	10058	LD E,153	Modify the 'last' operation code: 'offset' 25, plus 128 for string input and numerical result ('USR string').
S_STK_LST	10060	PUSH DE	Stack the 'last' values briefly.
	10061	CALL SYNTAX_Z	Do not perform the actual operation if syntax is being checked.
	10064	JR Z,S_SYNTEST
	10066	LD A,E	The 'last' operation code.
	10067	AND 63	Strip off bits 6 and 7 to convert the operation code to a calculator offset.
	10069	LD B,A
	10070	RST 40	Now use the calculator.
	10071	DEFB 59	fp_calc_2: (perform the actual operation)
	10072	DEFB 56	end_calc
	10073	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	10075	LD A,E	Get the 'last' operation code.
	10076	XOR (IY+1)	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.
	10079	AND 64
S_RPORT_C_2	10081	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	10084	POP DE	Get the 'last' operation code.
	10085	LD HL,23611	This is FLAGS.
	10088	SET 6,(HL)	Assume result to be numeric.
	10090	BIT 7,E	Jump forward if the nature of 'last value' is numeric.
	10092	JR NZ,S_LOOPEND	Jump forward if the nature of 'last value' is numeric.
	10094	RES 6,(HL)	It is a string.
S_LOOPEND	10096	POP BC	Get the 'present' values into BC.
	10097	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	10099	PUSH DE	The 'last' values go on the stack.
	10100	LD A,C	Get the 'present' operation code.
	10101	BIT 6,(IY+1)	Do not modify the operation code if dealing with a numeric operand (bit 6 of FLAGS set).
	10105	JR NZ,S_NEXT
	10107	AND 63	Clear bits 6 and 7.
	10109	ADD A,8	Increase the code by 8.
	10111	LD C,A	Return the code to the C register.
	10112	CP 16	Is the operation 'AND'?
	10114	JR NZ,S_NOT_AND	Jump if it is not so.
	10116	SET 6,C	'AND' requires a numeric operand.
	10118	JR S_NEXT	Jump forward.
S_NOT_AND	10120	JR C,S_RPORT_C_2	The operations -, *, /, ↑ and OR are not possible between strings.
	10122	CP 23	Is the operation a '+'?
	10124	JR Z,S_NEXT	Jump if it is so.
	10126	SET 7,C	The other operations yield a numeric result.
S_NEXT	10128	PUSH BC	The 'present' values go on the machine stack.
	10129	RST 32	Consider the next character.
	10130	JP S_LOOP_1	Go around the loop again.