Source Syntax and Symbols

Atom source is case-insensitive. The spellings START, Start, and start name the same symbol, and the same rule applies to instruction mnemonics, registers, assembler directives, hexadecimal digits, and host-preprocessor names. This book uses uppercase assembly consistently.

Line structure

One source line contains at most one label, equate, directive, or instruction. A label may precede an instruction or data directive on the same line:

START:  LD A,0
BUFFER: DS 32

A label may also occupy a line by itself:

START:
    LD A,0

Spaces and tabs separate tokens. Commas separate instruction operands and lists accepted by DB and DW.

Comments

A semicolon starts a comment extending to the end of the line:

; THIS WHOLE LINE IS A COMMENT.
LD A,0      ; THE COMMENT STARTS AFTER THE OPERAND.

Atom has no block comment or line-continuation syntax. One physical line is one statement boundary.

Global labels

A global label is a name followed by a colon. It receives the current logical output address:

READBYTE:
    LD A,(HL)
    INC HL
    RET

An Atom name begins with an ASCII letter or underscore and continues with ASCII letters, digits, or underscores. A global name contains one through eight characters. Names are exact RADIX-40 values; Atom reports an overlength name instead of truncating or hashing it.

Global labels remain available for the complete build. Two definitions of the same case-insensitive name are an error.

Private labels

A leading period marks a private name. The period is syntax and does not count towards the eight significant characters:

COPYROW:
    LD B,8
.LOOP:
    LD A,(DE)
    LD (HL),A
    INC DE
    INC HL
    DJNZ .LOOP
    RET

CLEARROW:
    LD B,8
.LOOP:
    LD (HL),0
    INC HL
    DJNZ .LOOP
    RET

The two .LOOP declarations have different scopes. A global label begins a new private scope and evicts the private symbols belonging to the previous global. A private reference requires a preceding global label.

Private scope crosses source-part boundaries. An included part may continue the current scope until the next global label appears in compilation order. Atom reports an undefined-private-symbol error if a scope transition would evict an unresolved private reference.

Constants with EQU

EQU binds a name to a resolved assembler-time value:

WIDTH EQU 32
HEIGHT EQU 24
BUFFER_SIZE EQU WIDTH*HEIGHT

Atom accepts a colon before EQU because that punctuation is a common source habit:

LIMIT: EQU 100

Both forms declare a constant. The colon in the second form does not create an address label or change private scope. A global EQU also leaves the current private scope open.

An equate expression must already be resolved. Atom does not implement forward equates:

; ERROR: ENDPOINT IS NOT YET DEFINED.
LIMIT EQU ENDPOINT-START

A private equate uses the ordinary private-name rules and therefore needs a current global scope.

Reserved words

Assembler directives are bare words such as ORG, DB, and ALIGN. A dotted word is always a private symbol, never a directive:

ORG 4000H       ; DIRECTIVE
.ORG:           ; PRIVATE LABEL NAMED ORG

Dotted directive aliases such as .ORG are deliberately absent. Host directives use a separate % prefix and are removed before native assembly; Chapter 5 covers them.