The BBC and Master Computer Public Domain Library

9489 AUTO

Submitted by Steve Fewell

Description:

Call routine &934D to read the Starting Line Number and Line increment parameters (if any were specified) as follows:

* Set the IWA to 10 (the default program starting Line Number)

* Call routine &9B1E to detokenise and extract the line number at BASIC text pointer A location and store it

    in the IWA. If no line number was found at BASIC text pointer A then the IWA will retain

    the default value of 10.

* Push the IWA value (the starting line number) to the BASIC stack

* Set the IWA to 10 (the default line number increment)

* If the next non-space character is not comma ',', then no line increment parameter has been specified, so call &9BB0

    to check that the statement is terminated correctly (with a ':', '<cr>' or 'ELSE-token'

    terminator character), [if not then a 'Syntax error' will be issued] and exit routine &934D.

* Otherwise, call routine &9B1E to detokenise and extract the Line number value from the BASIC Text pointer A

    location and store it in the IWA. The IWA now contains the specified Line increment value.

* If byte &2B of the IWA contains a non-zero value, then the increment value is more than 255, so issue error

    'Silly', as the line increment cannot be more than 255.

* If byte &2A (the LSB) of the IWA contains a zero value, then the increment value is zero, so issue error

    'Silly', as the line increment cannot be zero.

* Call &9BA6 to skip any space characters and check that the statement is terminated correctly (with a ':',

    '<cr>' or 'ELSE-token' terminator character), [if not then a 'Syntax error' will be

    issued] and exit routine &934D.

Store the step size (the line increment value) to the 6502 stack.

Retrieve the Starting Line number from the BASIC Stack (and store in the IWA).

[&BC26] Push the IWA Line Number value to the BASIC stack.
Call routine &A085 to display the Line Number (in the IWA) on the screen.

Call routine &BA74 to use OSWORD 0 to read the input line entered by the user.
If Escape is pressed during routine &BA74 then it will be detected and the 'Escape' error will be issued.
Retrieve the Line Number from the BASIC Stack (and store it in the IWA).

Call routine &8DE7 to tokenise the input line.
Set Y (pointer offset) to 0.
Call routine &BB08 to Add the tokenised input line (at the line number position denoted by the IWA value) in the
BASIC program (replacing any previous line of the same number).

Call routine &BBAC to initialise page &7 variables and the program non-resident variable pointer tables
(as a program line change means that all current variables should be lost - due to the Stack location changing).

Retrieve the Step size from the 6502 stack and push the value back to the 6502 stack.
Add the step size (line increment value) to the IWA value.
If the IWA &2B value is positive (i.e. less than &8000) then jump back to &9492 for entry of the next program
line; otherwise, the maximum line number (32767) has been exceeded, so jump to &8F83 to initialise variable space and
prompt for the user to enter another command line input.

Disassembly for the AUTO routine

9489	M	032 077 147	20 4D 93	JSR &934D Read Start line and line increment parameters
948C	*	165 042	A5 2A	LDA &2A
948E	H	072	48	PHA
948F		032 230 188	20 E6 BC	JSR &BCE6 Retrieve IWA value from the BASIC Stack [popi]
9492	&	032 038 188	20 26 BC	JSR &BC26 Push IWA value to the BASIC Stack [pushi]
9495		032 133 160	20 85 A0	JSR &A085 Print Line Number on screen (width 5)
9498	t	032 116 186	20 74 BA	JSR &BA74 Prompt for & get the User's input line (storing it in buffer &0700-&07FF)
949B		032 230 188	20 E6 BC	JSR &BCE6 Retrieve IWA value from the BASIC Stack [popi]
949E		032 231 141	20 E7 8D	JSR &8DE7 Tokenise the input line
94A1		160 000	A0 00	LDY#&00
94A3		032 008 187	20 08 BB	JSR &BB08 Insert tokenised input line & insert it into the program
94A6		032 172 187	20 AC BB	JSR &BBAC Initialise Page 7 & reset Variable pointers, etc...
94A9	h	104	68	PLA
94AA	H	072	48	PHA
94AB		024	18	CLC
94AC	e*	101 042	65 2A	ADC &2A
94AE	*	133 042	85 2A	STA &2A
94B0		144 224	90 E0	BCC -32 --> &9492
94B2	+	230 043	E6 2B	INC &2B
94B4		016 220	10 DC	BPL -36 --> &9492
94B6	L	076 131 143	4C 83 8F	JMP &8F83 Initialise & prompt for next command line

 

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