The BBC and Master Computer Public Domain Library

AB05 EVAL

Submitted by Steve Fewell

Description:

Call routine &AD36 to obtain the value of the expression after the EVAL keyword.
If the expression result is not 0 (i.e. the result is Numeric & not a String value) then issue 'Type mismatch' error.

Increment the SWA length and store a '<cr>' (carriage return) character at the end of the existing SWA value.
Push the SWA value to the BASIC stack.
Store the current BASIC Text pointer B values (&19, &1A & &1B) to the 6502 stack for temporary storage.

Set BASIC Text Pointer B (&19, &1A) and pointer (&37, &38) to the current address of the BASIC Stack pointer + 1,
the BASIC Stack pointer points to the next free location on the stack, so adding the 1 will point to the first character (length)
of the SWA value on the stack.

Set location &3B to #&FF (to set the middle of a statement mode).
Zero byte &3C (as a Line number is not expected).
Call the &8DB2 routine to tokenise the String value.

Zero byte &1B (BASIC Text pointer B offset), to reset BASIC Text Pointer B to the start of the tokenised String value.
Call routine &9D3B to evalualate the tokenised string and obtain the result of the String Expression to the SWA.
Call routine &BCE1 to restore the Stack space used by the String value.
Continue to the ASCII-Numeric routine (&AB3A) to retrieve the BASIC Text pointer B values from the 6502 stack and
convert the ASCII String result (SWA) to a numeric value (either a Floating-Point value or an Integer value) and exit.

Disassembly for the EVAL routine