The syntax of xyw is deliberately minimalist and simple, and a cross between 6502/6809-style assembly and Forth. Only the following token types are defined: comments numbers instructions directives symbols === Comments Comments are line-oriented only, and prefixed with a semicolon:

; this is a comment (till end of line)

=== Numbers Only one or two byte integers are supported. They can be either unsigned or signed (via two's complement), and they range between: 0 and 65535 (unsigned) -32768 and 32767 (signed) Numbers can be written either in base 16 (hexadecimal) without sign and prefixed with a dollar:

$F2   ; byte   (one byte)
$34BB ; word   (two bytes)

or in base 2 (binary) without sign and prefixed with a percent:

%11011000
%00100101

Note that: bytes and words can be represented in hexadecimal format only single bytes can be represented in binary format === Instructions xyw has 32 instructions that are written as uppercase three-letter mnemonics and may be followed by 1-3 lowercase mode flags (x, y, or w):

ADDxy ; add register x with y
LDBx  ; load one byte from register x
STByw ; store a byte from address in register yw

=== Directives xyw provides some directives that are used to instruct the assembler to do certain operations, like create a label or reserve bytes. They are single words prefixed with a dot, and they may or may not have arguments, but they cannot span more than one line:

.macro square DUP MUL
.label main
.string "Hello!\n"

=== Symbols Symbols are user-created identifiers that are defined via either the .label or .macro directive:

main
fact.loop