Expressions

Anywhere a directive or an opcode takes a value, you can give it an expression instead of a literal.

Expressions are parsed by CodingSeb.ExpressionEvaluator with a 65c02 extension. The result has to be a whole number, because the compiler needs an integer. A char becomes its byte value and a bool becomes 1 or 0. Anything that evaluates to a fraction, so most Math.* functions and / where it would not divide evenly, is an error. For real logic and floating-point work, use the Template Engine.

What works:

• integer arithmetic: + - * / % (integer division, so 7 / 2 is 3)
• bitwise: & | ~ << >>
• comparison and logic: == != <= >= && || ! (each gives 1 or 0)
• the ternary ?:
• character literals, so 'A' is 65
• labels and constants, anywhere a number can go

Number formats

Form	Base	Example
$ prefix	hex	$9f20
0x prefix	hex	0x9f20
% prefix	binary	%1010_0000
0b prefix	binary	0b10100000
plain	decimal	40736

_ may be used between digits as a separator. % before binary digits is a literal; between two values it is modulo.

Byte operators

<, > and ^ are redefined. As a prefix, with nothing to their left, they pull one byte out of a value:

Prefix	Returns	Example
<	Low byte, value & $ff	<$123456 is $56
>	High byte, (value & $ff00) >> 8	>$123456 is $34
^	Top byte, (value & $ff0000) >> 16	^$123456 is $12

This is the usual way to split an address:

    lda #<target        ; low byte of target
    ldx #>target        ; high byte of target

A byte operator binds tightly, so <target + 1 is (<target) + 1. Parenthesise for the other grouping: <(target + 1).

Because these three symbols are taken as prefixes, use <= >= == != for comparisons.

Referencing constants

Labels and constants can be used in an expression:

    lda data + 3    ; loads $13

.data:
    .byte $00, $11, $12, $13, $14

Names are resolved through the current scope. A label name that is defined more than once is ambiguous and cannot be used here. See Labels.