The BBC and Master Computer Public Domain Library

9D3B Expression Handler

Submitted by Steve Fewell

Description:

If called from &9D2F then set BASIC Text Pointer B = BASIC Text Pointer A.

The lowest level of the expression handler starts at address &9D3B.
There are many levels to the expression handler, each level calls the level above
(to evaluate any higher-level operators) before checking whether any of its operators
need to be evaluated.

The Expression handler is structured as follows:
JSR &9D3B [Entry point (lowest level)] Handle (OR, EOR)
     JSR &9D81 handle (AND)
         JSR &9DA9 handle (<, =, >)
             JSR &9E4C handle (+, -)
                   JSR &9FC7 handle (*, /, MOD, DIV)
                         JSR &A012 handle (Skip spaces and ^)
                                 JSR &AD36 handle (leading -, Variable name/value, Open Bracket, BASIC Keyword (functions))

Some routines have additional entry points which enable a previous value to be stored
on the stack and/or a new value obtained before evaluating the expression.
This is so that any previous value is not lost during the expression handling,
in situations where more than 1 value may be required to be worked on at the same time.

After evaluating the operator, the routine usually jumps back into the expression handler
at the same level as the previous operator - this checks for further operators with the same
priority (precedence) before carrying on with the lower levels. I.e. an expression can contain
many multiplications, one after the other, all which much be evaluated before the lower level is
returned to (i.e. 2 + X * Y * Z). The exception for this, is the relational operators (<, =, >);
these cannot be located one after the other, as this is not allowed in BASIC.

If there are no more operators/values to evaluate (&9D46), then the PTR B offset is decremented,
Y = Value type, and ?&27 is also set to the value type, and then exit.

Note: On exit X contains the next non-space character found after the expression (pointed to
by &1B). Many of the routines that call this expression handler use this as a quick way of
knowing what the next character is.

Disassembly for the Expression Handler routine

9D81 Expression Handler Level 2 [AND]

9DA9 Expression Handler Level 3 [<, =, >]

9E4C Expression Handler Level 4 [+, -]

9FC7 Expression Handler Level 5 [*, /, MOD, DIV]

A012 Expression Handler Level 6 [Skip spaces, ^]

The disassembly for the "AD36 Expression Handler Level 7 [Unary -, Variable/Value, Open bracket, BASIC Keyword (function)]" routine is in the separate description for "AD36 Evaluate Variable/Value"