The BBC and Master Computer Public Domain Library

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8FAE Execute next command line / program statement

Submitted by Steve Fewell

Description:

This routine has many entry points (depending on how the last statement finished and what needs to be done to tidy up
before the next statement can be executed. The entry points are listed between square brackets ([]).
Therefore, this routine may not necessarily be executed in the order described.

[&8FAE] Skip the rest of the line. This routine is called when a DATA keyword is found (as DATA statements are skipped
when reached and not executed (as the READ command controls how they are handled). It is also called when the DEF keyword
is found (as function and procedure definitions are not directly executed until they are called by an FN or PROC keyword).
This routine is also called when a REM keyword is found, to skip the rest of the line - as it is a comment line.
Additionally, this routine is also executed when a '*' command is found (the '*' command is first passed to the Operating
System OSCLI routine and then this routine skips the line - so that BASIC doesn't try to execute the '*' command.
The rest of the line is skipped as follows:
* Set A to #&0D (the end of line character - '<cr>').
* Set Y to the BASIC Text Pointer A offset (&0A) - 1.
* Keep incrementing Y and checking the character at BASIC Text pointer A (&0B-&0C) plus Y (offset) with the
character in A (the '<cr>' character) until the characters match.
* Call routine &9BBC to update the BASIC Text Pointer A address (to include the Y offset value) and to reset the offset
(&0A) to 1. (basically, this adds Y to the Text Pointer A address (&0B-&0C)).

[&8FBD] This routine checks whether the last character read (in A) is '<cr>', if it isn't then jump to &8FAE
to skip the rest of the line.
Now, we are at the end of the line (i.e. we have reached the '<cr>' character).

[&8FC1] If the BASIC Text pointer MSB address is #&07 then there are no more program lines to execute (as we were
executing a statement from the command line - and not from within a program), so jump to &8F86 to prompt for the next
command line input.

Set Y to #&01.
Load the next character (offset Y) from the BASIC text pointer A location.
If the character is negative (i.e. #&FF) then we have reached the end of the program, so jump to &8F86 to prompt
for the next command line input.

If the TRACE flag (location &20) is on (i.e. it's value is not zero), then:

* Set IWA to the Line number (MSB byte (&2B) = the first byte at BASIC Text pointer A (offset 1) and the LSB byte (&2A)

    = the second byte at BASIC text pointer A location (offset 2)).

* Call routine &9C4B to display the TRACE line number (in the IWA) on the screen.

* Otherwise, (routine &9909 returned zero) the variable is either invalid or hasn't been created yet, so:

* Check the carry flag status (as returned by routine &9909). If carry is set then jump to &8FEB to

   check whether the BASIC statement begins with a special character ('*', '[', '=' or 'EXT keyword',

   and issue 'Syntax error' [via check end of statement routine &9BA6] if none of these special

   characters match the character at the start of the statement).

* Store the BASIC text pointer B offset (in X) back to location &1B.

* Call routine &9B86 to check whether the next non-space character after the variable name is an '=' character.

   If it isn't then issue the 'Mistake' error, as the variable assignment is not correct and a variable

   cannot appear at the start of a statement unless it is being assigned a value.

* Otherwise, '=' was found sucessfully.

* Call routine &9854 to add the new variable name to the variable pointer table.

* If the variable type is a float (location &2C contains #&05) then set X to #&06; otherwise, set X to

   #&05. This specifies the amount of space to allocate for the variable's value.

* Call routine &9883 to allocate space for the variable (and initialise it's value to zero/null).

* Decrement the BASIC text pointer A offset (so that BASIC Text pointer A points to the first character of the

   variable name) and continue to the LET keyword routine to evaluate the variable name (again!) and

   assign the value specified after the '=' to the variable.

Disassembly for the Execute next command line / program statement routine

Disassembly for 9B86 Check for '=' routine