The BBC and Master Computer Public Domain Library

AD36 Evaluate Value/Variable/Open Bracket/BASIC Function

Submitted by Steve Fewell

Routine:evalValue
Name: Evaluate Value/Variable/Open Bracket/BASIC Function
Starting Address: &AD36
Entry criteria: &19, &1A & &1B (Text Pointer B) point to the program text location
at which we should begin our evaluation.
Exit: Any variable/value found will be loaded, or the function will have been executed,
Or the value inside the brackets ['(', ')'] will have been evaluated.

Description:

Load the ASCII value of the next character into the accumulator & increment the
Text pointer B offset (keeping a previous copy of it in Y). Skip any spaces in the text
by incrementing the Text Pointer B offset (past the space character) and loading the next character.

Now, we have our first non-space character in the accumulator.
If the character is '-', then we have a leading [unary] '-', so call &ACD7 to evaluate the
expression after the '-', and complement the result (or Type mismatch error if result is a string).
If the character is '"' [open quote], then call &AD19 to extract string into the SWA & exit.
If the character is '+', leading [unary] '+', then skip any spaces (after the '+') and load the next
non-space character after the '+'.

If the character is a BASIC Statement Keyword (non-function, i.e. >= 198 'AUTO') then
generate a 'No such variable' error as a BASIC Statement keyword can only occur
at the beginning of a line, and not in the middle of an expression!

If the character is >= 142 &8E (OPENIN) [& < 198 &C5], then this represents a BASIC Function Keyword,
so execute the portion of code (in the BASIC ROM) which deals with the appropriate function.
[9019] To do this, the character ASCII Code is multiplied by 2 & added to the base address
&874D to form a pointer to the execution address (LSB first, MSB next) of the required
function. This resulting address is jumped to.
Example 1: Character = &8E [OPENIN token] = 10001110 multiply by 2 = 00011100 (which is &1C in hex).
So, &874D + &1C = &8769 (The LSB of the execution address for the OPENIN function (&876A is
the MSB of the address)).
Example 2: Character = &C5 [EOF token] = 11000101 multiply by 2 = 10001010 (which is &8A in hex).
So, &874D + &8A = &87D7 (The LSB of the execution address for the EOF function (&87D8 is
the MSB of the address)).

BASIC Keywords between 128 and 141 are not considered, as these Keywords are used
in the middle of statements (and are not functions), so the statements/expression handler will
deal with these values. These keywords are as follows: AND, DIV, EOR, MOD, OR, ERROR,
LINE, OFF, STEP, SPC, TAB(, ELSE and THEN.

If the character is >&eq;"." and <"?" then call routine &A2E1 to evaluate the numeric
value at Text Pointer B. This routine will place the result either into the IWA or FWA (depending
on how large the value is, or whether it contains a fractional part).
If the carry flag is clear then the routine failed to convert the ASCII text to a numeric
value, so issue a 'No such variable' error. Otherwise exit.

If the character is "&" then call routine &ADB7 to extract the Hex Number and exit.
If the character is "(" then call routine &ADAC to get the result of the expression (inside
the brackets) & check for a closing bracket ')' at the end (issue a 'Missing )' error if not present) & exit.

If the character is anything else, then it must be a variable.
So, call routine &9909 to evaluate the variable name (or array reference) and
set (&2A, &2B) to the address of the variable's value (or the address of the value
of the specified Array element).
If &9909 returns with the zero flag clear, then the variable was evaluated sucessfully
so exit via routine &B1A0 to Load the variables value into the appropriate location (IWA/FWA/SWA).
Otherwise (zero flag was set), the variable either wasn't found, or a valid variable
name was not found, so check the value of &28, which is the OPT flag.
if Bit 1 of this flag (the second bit from the right) [Relocate is on?] is not set then issue a
'No such variable' error. If the carry flag (on exit from routine &9909) is set then
the text was not a valid variable name, so issue a 'No such variable' error.
Otherwise, (the second bit is set, meaning that OPT = Relocate?) Store back the Text pointer
B offset in &1B and set the IWA to the 2-byte address value from locations &0440-&0441
(&0440 is the LSB), this is the value of the address stored in variable P%, then exit.

Disassembly for the Evaluate Value/Variable/Open Bracket/BASIC Function routine