The BBC and Master Computer Public Domain Library

B618 FOR

Submitted by Steve Fewell

Description:

Call routine &98AE to evaluate the variable name at the BASIC Text Pointer A location, and to create it if it doesn't
exist, and return the variable value's memory address in locations &2A-&2B and its type in &2C.
If the zero flag is set on return from &98AE, then the variable name is not a valid variable name, so issue
'FOR variable' error.
If the carry flag is set on return from &98AE, then the variable is a String variable, so issue 'FOR variable'
error, as a FOR variable must be numeric.

Call routine &BC43 to store the variable information data (&2A-&2C) to the 6502 stack.
Call routine &9B86 to check whether the next non-space character is '=' (equals), if not then issue 'Mistake' error;
otherwise, skip over the '=' character.

Call routine &B328 to evaluate the expression (after the '=' character), retrieve the variable information from the
6502 stack and set the Numeric FOR variable to the result of the expression.
If the next non-space character at BASIC Text Pointer B is not the 'TO-token', then issue 'No TO' error, as the TO-Keyword
must follow the expression giving the variable's start value.

Load Y with the current FOR level (location &26).
If the current FOR level is greater than or equal to #&96 (150) then issue 'Too many FORs' error, as only a maximum
of 10 FOR loops can be nested (each FOR loop uses 15 bytes on the FOR stack - and increases the FOR level by 15).

Add #&0F (15) to the current FOR level, and update location &26 with the new value.
Now, the &26 location has been updated, but Y still contains the original FOR level (before the addition).
Store the variable value address (&37-&38) in locations &0528 + Y and &0529 + Y; and store the
variable type (&39) in location &052A + Y.

If the variable type (&39) is Floating-Point (i.e. it is #&05) then:

* [&B6A1] Call routine &9D3B to evaluate the expression after the TO-Keyword

* Call routine &96DD to convert the result to a Floating-point value if it is Integer, or issue 'Type

    mismatch' error if it is a String value.

* Add #&21 (33) to the current FOR level (location &26) and store the result in location &4A.

* Store #&05 in location &4B

* Store the TO-value (the value in the FWA) in the 5-byte location pointed to by argp (&4A-&4B).

* Call routine &A5D8 to set the FWA to 1.0 (the default FOR STEP value)

* Get the next non-space character after the TO-expression (at the location pointed to by BASIC text

    pointer B)

* If the character is the 'STEP-token' then: Call routine &9D3B to evaluate the expression after the

    STEP-Keyword, Call &96DD to convert the result to Floating-Point (if it was an

    integer, or issue 'Type mismatch' error if it was a String) and Set Y to the BASIC

    text pointer B offset.

* [&B6C7] Update BASIC Text Pointer A to point to the end of the FOR-statement (by setting it to the

    value of Y (which is the current position on the Program Line)).

* Add #&1C to the current FOR level (location &26) and store the result in location &4A

* Set location &4B to #&05

* Call &A519 to store the STEP value (the value in the FWA) to the location pointed to by argp

    (&4A-&4B)

* Jump to &B68F to Check and skip the end of the program statement (issuing 'Syntax error' if the end of

    statement character (':','<cr>','ELSE') wasn't found), Store the current BASIC Text pointer A value

    to &0526 + Y - &0527 + Y (where Y is the FOR level - from location &26), Jump to &900B to

    continue executing the program - starting at the next program statement after the FOR-Statement.

Otherwise, the variable type (&39) must be Integer (i.e. it is #&04), so:

* [&B64D] Call routine &96AF to evaluate the expression after the TO-Keyword and convert the

    expression result to an Integer (if it was a Floating-Point value), or issue 'Type

    mismatch' error if the result was a String value.

* Set Y to the current FOR level (from location &26)

* Store the TO-value (in the IWA) in locations &0521 + Y to &0524 + Y (LSB first)

* Set the IWA to 1 (the default FOR STEP value)

* Get the next non-space character after the TO-expression (at the location pointed to by BASIC text

    pointer B)

* If the character is the 'STEP-token' then: Call routine &96AF to evaluate the expression after the

    TO value, and convert the result to Integer (if it was Floating-Point, or issue 'Type mismatch' error if

    the result was a String value), Set Y to the BASIC text pointer B offset value (&1B).

* [&B677] Update BASIC Text Pointer A to point to the end of the FOR-statement (by setting it to the

    value of Y (which is the current position on the Program Line)).

* Set Y to the current FOR level (from location &26).

* Store the STEP value (the current IWA value) to the FOR stack at starting at location &051C + Y (for the

    LSB byte - &2A) and ending at location &051F + Y (for the MSB byte - &2D).

* [&B68F] Check and skip the end of the program statement (issuing 'Syntax error' if the end of

    statement character (':','<cr>','ELSE') wasn't found) - also reset the BASIC text pointer A base

    value (&0B-&0C) to take into account the offset value (&0A).

* Store the current BASIC Text pointer A value to &0526 + Y and &0527 + Y (where Y is the FOR

    level - from location &26)

* Jump to &900B to continue executing the program - starting at the next program statement after the FOR-

    statement.

The memory layout of the FOR Stack (&0528-&05CB) is as follows:
The stack contains details for a maximum of 10 FOR loops. Each FOR block is 15 bytes long.
In each FOR block, the bytes have the following meaning:
Byte 1 (&0528) is the variable location - LSB.
Byte 2 (&0529) is the variable location - MSB
Byte 3 (&052A) is the variable type
Byte 4 (&052B) - Byte 8 (&052F) is the STEP value (Float (5-byte) / Integer (4-byte))
Byte 9 (&0530) - Byte 13 (&0534) is the TO value (Float (5-byte) / Integer (4-byte))
Byte 14 (&0535) is the program location of the start of the first statement within the FOR loop - LSB
Byte 15 (&0536) is the program location of the start of the first statement within the FOR loop - MSB

Disassembly for the FOR routine

B618		032 174 152	20 AE 98	JSR &98AE Evaluate variable name & create it if it's a new variable
B61B		240 219	F0 DB	BEQ -37 --> &B5F8 'FOR variable' error
B61D		176 217	B0 D9	BCS -39 --> &B5F8 'FOR variable' error
B61F	C	032 067 188	20 43 BC	JSR &BC43 Push &2A, &2B & &2C (the variable info) to the 6502 Stack
B622		032 134 155	20 86 9B	JSR &9B86 Check whether the next non-space character is '=' ('Mistake' error if not)
B625	(	032 040 179	20 28 B3	JSR &B328 Evaluate expression and set Numeric variable
B628		032 213 142	20 D5 8E	JSR &8ED5 Get next non-space character (PTR B)
B62B		201 184	C9 B8	CMP#&B8
B62D		208 226	D0 E2	BNE -30 --> &B611 'No TO' error
B62F	&	164 038	A4 26	LDY &26
B631		192 150	C0 96	CPY#&96
B633		176 207	B0 CF	BCS -49 --> &B604 'Too many FORs' error
B635		152	98	TYA
B636	i	105 015	69 0F	ADC#&0F
B638	&	133 038	85 26	STA &26
B63A	7	165 055	A5 37	LDA &37
B63C	(	153 040 005	99 28 05	STA &0528,Y
B63F	8	165 056	A5 38	LDA &38
B641	)	153 041 005	99 29 05	STA &0529,Y
B644	9	165 057	A5 39	LDA &39
B646	*	153 042 005	99 2A 05	STA &052A,Y
B649		201 005	C9 05	CMP#&05
B64B	T	240 084	F0 54	BEQ 84 --> &B6A1
B64D		032 175 150	20 AF 96	JSR &96AF Get expression result & convert it to Integer
B650	&	164 038	A4 26	LDY &26
B652	*	165 042	A5 2A	LDA &2A
B654	!	153 033 005	99 21 05	STA &0521,Y
B657	+	165 043	A5 2B	LDA &2B
B659	"	153 034 005	99 22 05	STA &0522,Y
B65C	,	165 044	A5 2C	LDA &2C
B65E	#	153 035 005	99 23 05	STA &0523,Y
B661	-	165 045	A5 2D	LDA &2D
B663	$	153 036 005	99 24 05	STA &0524,Y
B666		169 001	A9 01	LDA#&01
B668		032 024 174	20 18 AE	JSR &AE18 Set IWA to the 8-bit value in A
B66B		032 213 142	20 D5 8E	JSR &8ED5 Get next non-space character (PTR B)
B66E		201 136	C9 88	CMP#&88
B670		208 005	D0 05	BNE 5 --> &B677
B672		032 175 150	20 AF 96	JSR &96AF Get expression result & convert it to Integer
B675		164 027	A4 1B	LDY &1B
B677		132 010	84 0A	STY &0A
B679	&	164 038	A4 26	LDY &26
B67B	*	165 042	A5 2A	LDA &2A
B67D		153 028 005	99 1C 05	STA &051C,Y
B680	+	165 043	A5 2B	LDA &2B
B682		153 029 005	99 1D 05	STA &051D,Y
B685	,	165 044	A5 2C	LDA &2C
B687		153 030 005	99 1E 05	STA &051E,Y
B68A	-	165 045	A5 2D	LDA &2D
B68C		153 031 005	99 1F 05	STA &051F,Y
B68F		032 207 155	20 CF 9B	JSR &9BCF Check & skip end of program line
B692	&	164 038	A4 26	LDY &26
B694		165 011	A5 0B	LDA &0B
B696	&	153 038 005	99 26 05	STA &0526,Y
B699		165 012	A5 0C	LDA &0C
B69B	'	153 039 005	99 27 05	STA &0527,Y
B69E	L	076 011 144	4C 0B 90	JMP &900B Process next BASIC program statement
B6A1	;	032 059 157	20 3B 9D	JSR &9D3B Evaluate expression at BASIC Text pointer B
B6A4		032 221 150	20 DD 96	JSR &96DD Check Float value (Convert if Integer, or 'Type mismatch' error if String)
B6A7	&	165 038	A5 26	LDA &26
B6A9		024	18	CLC
B6AA	i!	105 033	69 21	ADC#&21
B6AC	J	133 074	85 4A	STA &4A
B6AE		169 005	A9 05	LDA#&05
B6B0	K	133 075	85 4B	STA &4B
B6B2		032 025 165	20 19 A5	JSR &A519 Store FWA's value to argp address (&4A-&4B)
B6B5		032 216 165	20 D8 A5	JSR &A5D8 Set FWA to 1.0
B6B8		032 213 142	20 D5 8E	JSR &8ED5 Get next non-space character (PTR B)
B6BB		201 136	C9 88	CMP#&88
B6BD		208 008	D0 08	BNE 8 --> &B6C7
B6BF	;	032 059 157	20 3B 9D	JSR &9D3B Evaluate expression at BASIC Text pointer B
B6C2		032 221 150	20 DD 96	JSR &96DD Check Float value (Convert if Integer, or 'Type mismatch' error if String)
B6C5		164 027	A4 1B	LDY &1B
B6C7		132 010	84 0A	STY &0A
B6C9	&	165 038	A5 26	LDA &26
B6CB		024	18	CLC
B6CC	i	105 028	69 1C	ADC#&1C
B6CE	J	133 074	85 4A	STA &4A
B6D0		169 005	A9 05	LDA#&05
B6D2	K	133 075	85 4B	STA &4B
B6D4		032 025 165	20 19 A5	JSR &A519 Store FWA's value to argp address (&4A-&4B)
B6D7		128 182	80 B6	BRA -74 --> &B68F

 

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