The BBC and Master Computer Public Domain Library

975F MODE

Submitted by Steve Fewell

Description:

Get the result of the expression at BASIC Text Pointer A, convert the result to Integer (Type mismatch error if String),
and Set BASIC Text Pointer A to BASIC Text Pointer B value.
Call &9BA6 to check that the Statement terminates correctly ('Syntax' error is issued if end of statement marker is
not found).

Use OSBYTE #&82 to obtain the Machine high order address.
If the Machine high order address is not &FFFF then jump to &9797 (skip memory restriction tests).

Compare the Stack Pointer (&04-&05) with HIMEM (&06-&07), if they are not equal (i.e. we have items on
Stack, i.e. as the result of local variables inside a PROC/FN call) then issue 'Bad MODE' error, as BASIC will not allow the
screen mode to be altered if there are items on the Stack (as the stack could then get corrupted).
Note: as the stack contains the return details for a PROC/FN call, MODE cannot be issued from within
a proedure or function, even if no local variables have been declared.

Use OSBYTE #&85 to read the top of User RAM for the specified screen mode (&2A) [X=address LSB & Y=address MSB].
Compare the 'Top of User RAM for the specified Mode' with VARTOP (i.e. the heap, or list of declared variables).
If the 'Top of User RAM for the specified Mode' would fall below the VARTOP value, then the program variables would be
corrupted if the Mode was changed - so we are not allowed to change to that mode, and a 'Bad MODE' error is issued.

Compare the 'Top of User RAM for the specified Mode' with TOP (i.e. the end of the program).
If the 'Top of User RAM for the specified Mode' would fall below the TOP value, then the part of the program would be
corrupted if the Mode was changed - so we are not allowed to change to that mode, and a 'Bad MODE' error is issued.

Set HIMEM (&06-&07) and the Stack Pointer to the 'Top of User RAM for the specified Mode' value.
Zero COUNT (&1E), as after a mode change, the screen is clear, and no characters will have been output on the current
line yet.
Output special character 22 (the operating system's Mode change operator (VDU 22)).
Next, output the contents of location &2A (The IWA LSB, which contains the MODE number), this supplies the mode parameter
to the VDU 22 call, and allows the operating system to execute a MODE change to the MODE specified in &2A.
Lastly, jump to &9005 to start processing the next program statement.

Disassembly for the MODE routine