The BBC and Master Computer Public Domain Library

B97D READ

Submitted by Steve Fewell

Description:

[&B975] Check for a comma, if found - jump to READ (&B97D) otherwise end of READ statement (jump to &9000)
Call routine &8CE5 to check that the next non-space character is a comma ',', if it is then jump to &B97D
to READ the new data value into the variable specified after the comma.
If the next non-space character was not a comma, then we have reached the end of the READ statement so jump to &9000 to
execute the next program statement.

[&B97D] READ statement

Call &98AE to evaluate the variable name after the READ keyword [or after the comma, if we have already
processed the first variable of the READ statement].
If the zero flag is set on return from &98AE then a valid variable name was not found; so, jump to &B975 to check
whether the next non-space character at PTR A is a comma, if it is then try reading a variable name after the comma;
otherwise, jump to &9000, as the end of the READ statement has been found. Note: &9000 will handle any Syntax
Error messages if the command at PTR A is not reconised.
If the carry flag is 1 then the variable is a String variable; so, jump to &B98F to process the String variable
Otherwise, the variable is numeric, so continue with &B984, to READ the numeric value.

[&B984] READ a value into a Numeric variable
Call &B9AC to set the BASIC Text Pointer B to point to the location before the start of the next DATA element.
Call &BC43 to push the variable address & type information (&2A, &2B and &2C) to the 6502 Stack.
Call &B328 to evaluate the numeric expression at BASIC Text Pointer B, retrieve the variable information from
the 6502 stack and set the variable's value to the result of the numeric expression, or issue 'Type mismatch' error
if a non-numeric (i.e. String) value was found.
Jump to &B99D to update the (&1C, &1D) DATA pointer and to continue processing the READ statement.

[&B98F] READ a value into a String variable
Call &B9AC to set the BASIC Text Pointer B to point to the location before the start of the next DATA element.
Call &BC26 to push the IWA value (which contains the Variable address and type information) to the BASIC stack.
Call &ACF8 to Extract the Next Field starting from the location pointed to by BASIC text pointer B, storing the
result in the SWA.
Store A (the result type) in location &27.
Call &90AB to retrieve the String variable details from the BASIC stack and set the String variable to the SWA value.
Continue to &B99D to update the (&1C, &1D) DATA pointer and to continue processing the READ statement.
Note: Because the Extract Next field routine (&ACF8) is used, a value of any type (Integer, Float or String) can
be READ into a String variable (i.e. This is fine: 10 READ A$    20 DATA 2.34).

[&B99D] Update the DATA Pointer and READ the next value (if any more are specified)
Add the BASIC Text Pointer B value (&19, &1A) to the DATA pointer (&1C, &1D).
Jump back to &B975 to check for a comma after the variable name in the READ Statement (BASIC text pointer A), if a
comma is found then process the READ statement for the next variable, otherwise end of READ statement found, so jump
to &9000 to start executing the next Statement in the Program.

[&B9AC] Move BASIC Text Pointer B to next DATA element location
Call &9275 to update the BASIC Text Pointer A offset to equal the BASIC Text Pointer B offset.
Set BASIC text pointer B to point to the value at locations &1C-&1D (this is either the last DATA element READ
or, if nothing has been ready yet, it will be the start of the BASIC program. Zero the BASIC text pointer B offset
Note: If (&1C-&1D) pointed to the start of the program, then the character pointed to will be the carriage return
('<cr>') character which indicates the start of the program.

Get the next non-space character at the location pointed to by BASIC Text Pointer B.
If the character is a comma then exit (RTS).
If the character is the 'DATA-token' (&DC) then exit (RTS).
If the character is a carriage return ('<cr>') then jump to &B9CF to move to the next line.

[&B9C6] Get the next non-space character at the location pointed to by BASIC Text Pointer B.
If the character is a comma then exit (RTS).
If the character is not '<cr>' then jump back to &B9C6 to check the next character.

[&B9CF] Check the next character pointed to by BASIC Text pointer B (plus offset - &1B), if it is negative
(i.e. #&FF) then the end of the program has been reached without finding the required data, so
issue 'Out of DATA' error; Otherwise (not negative), start processing the next program line by:
      * Incrementing Y (offset) by 2 (to skip the Line Number)
      * Setting X to the character at Text pointer B (plus offset) - this is the pointer to the next program line.
      * Incrementing Y (offset) by 1 (to skip the pointer to the next Program Line)
      * Skipping any spaces at the beginning of the Program Line
      * If the first non-space character on the program line is 'DATA-token' then jump to
         &BA13 to increment Y (offset), store Y back in &1B (PTR B
         offset) and exit.
      * Otherwise, Add X to BASIC Text Pointer B (&19, &1A) and junmp back to &B9CF to move to the next line

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Thus, in other words, &B9AC simply looks for a Comma or DATA-Token at the current DATA pointer location
(&1C, &1D) and, if neither of these are found, then BASIC will skip the rest of the line until a comma is found,
or if a carriage return character is found (or a comma was not found on the current program line) then this routine will
look for a Program Line that begins with a DATA-token (as the first non-space character). 'Out of DATA' error is issued
if no DATA is found.

This format allows comments to be included inside the DATA statement! This is illustrated by the following program:
        10 READ A, B, C
        20 PRINT A, B, C
        30 DATA 2 (This is A) , 3 'This is B, 4 : This is C
This program correctly outputs 2         3          4
The best characters to use to separate the DATA element from the remark is '(' (open bracket), "'" (quote) or ':' (colon).
However, DATA statements will not be reconised if they do not begin at the start of the line (i.e. 30 : DATA 2, 3, 4)

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Disassembly for the READ routine

B975		032 229 140	20 E5 8C	JSR &8CE5 Compare next non-space [PTR A] character with ','
B978		240 003	F0 03	BEQ 3 --> &B97D READ
B97A	L	076 000 144	4C 00 90	JMP &9000   Check end of statement & execute the next statement on the Command/Program line
B97D		032 174 152	20 AE 98	JSR &98AE Evaluate variable name & create it if it's a new variable
B980		240 243	F0 F3	BEQ -13 --> &B975 Check for a comma (if not found then end of READ statement)
B982		176 011	B0 0B	BCS 11 --> &B98F Process String Variable
B984		032 172 185	20 AC B9	JSR &B9AC Set the BASIC Text Pointer B to point to the location before the start of the next DATA element
B987	C	032 067 188	20 43 BC	JSR &BC43 Push &2A, &2B & &2C (the variable info) to the 6502 Stack
B98A	(	032 040 179	20 28 B3	JSR &B328 Evaluate expression and set Numeric variable
B98D		128 014	80 0E	BRA 14 --> &B99D
B98F		032 172 185	20 AC B9	JSR &B9AC Set the BASIC Text Pointer B to point to the location before the start of the next DATA element
B992	&	032 038 188	20 26 BC	JSR &BC26 Push IWA value (variable info) to the BASIC Stack [pushi]
B995		032 248 172	20 F8 AC	JSR &ACF8 Extract next field (PTR B)
B998	'	133 039	85 27	STA &27
B99A		032 171 144	20 AB 90	JSR &90AB Set String variable
B99D		024	18	CLC
B99E		165 027	A5 1B	LDA &1B
B9A0	e	101 025	65 19	ADC &19
B9A2		133 028	85 1C	STA &1C
B9A4		165 026	A5 1A	LDA &1A
B9A6	i	105 000	69 00	ADC#&00
B9A8		133 029	85 1D	STA &1D
B9AA		128 201	80 C9	BRA -55 --> &B975 Check for a comma (if not found then end of READ statement)
B9AC	u	032 117 146	20 75 92	JSR &9275 Set PTR A Offset to PTR B Offset
B9AF		165 028	A5 1C	LDA &1C
B9B1		133 025	85 19	STA &19
B9B3		165 029	A5 1D	LDA &1D
B9B5		133 026	85 1A	STA &1A
B9B7	d	100 027	64 1B	STZ &1B
B9B9		032 235 142	20 EB 8E	JSR &8EEB Get next non-space char (PTR B) & compare with ','
B9BC	X	240 088	F0 58	BEQ 88 --> &BA16 RTS
B9BE		201 220	C9 DC	CMP#&DC
B9C0	T	240 084	F0 54	BEQ 84 --> &BA16 RTS
B9C2		201 013	C9 0D	CMP#&0D
B9C4		240 009	F0 09	BEQ 9 --> &B9CF
B9C6		032 235 142	20 EB 8E	JSR &8EEB Get next non-space char (PTR B) & compare with ','
B9C9	K	240 075	F0 4B	BEQ 75 --> &BA16 RTS
B9CB		201 013	C9 0D	CMP#&0D
B9CD		208 247	D0 F7	BNE -9 --> &B9C6
B9CF		164 027	A4 1B	LDY &1B
B9D1		177 025	B1 19	LDA (&19),Y
B9D3	0	048 028	30 1C	BMI 28 --> &B9F1 Out of DATA error
B9D5		200	C8	INY
B9D6		200	C8	INY
B9D7		177 025	B1 19	LDA (&19),Y
B9D9		170	AA	TAX
B9DA		200	C8	INY
B9DB		177 025	B1 19	LDA (&19),Y
B9DD		201 032	C9 20	CMP#&20
B9DF		240 249	F0 F9	BEQ -7 --> &B9DA
B9E1		201 220	C9 DC	CMP#&DC
B9E3	.	240 046	F0 2E	BEQ 46 --> &BA13
B9E5		138	8A	TXA
B9E6		024	18	CLC
B9E7	e	101 025	65 19	ADC &19
B9E9		133 025	85 19	STA &19
B9EB		144 226	90 E2	BCC -30 --> &B9CF
B9ED		230 026	E6 1A	INC &1A
B9EF		128 222	80 DE	BRA -34 --> &B9CF

Disassembly for BA13 - Increment Y & Update Text Pointer Offset (&1B=Y)

BA13		200	C8	INY
BA14		132 027	84 1B	STY &1B
BA16	`	096	60	RTS

 

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