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Atom Book 1 — Assembler Reference01

Getting Started with Atom ​

An Atom source file contains Z80 instructions, labels and assembler directives. Instructions produce machine code. Labels name addresses. Directives define constants, place code or reserve storage.

For installation and build commands, use the Node guide or CP/M guide.

A first Atom program ​

This program increments a byte eight times and halts. It uses a generic memory layout beginning at 4000H. It can be traced in a Z80 debugger but is not a CP/M transient program, which would start at 100H and return to CP/M.

asm
ORG 4000H

LIMIT EQU 8

START:
    LD B,LIMIT
    LD HL,COUNTER
.LOOP:
    INC (HL)
    DJNZ .LOOP
    HALT

COUNTER:
    DB 0

ORG sets the address of the first instruction. LIMIT EQU 8 declares a constant without producing bytes. START names the first instruction and .LOOP is a private label within that routine. COUNTER names the byte initialised by DB 0.

The first instruction is $06 $08 at $4000. LD HL,COUNTER is $21 $09 $40, encoding $4009 in little-endian order. INC (HL) is $34, DJNZ .LOOP is $10 $FD and HALT is $76. The final DB 0 places one zero byte at $4009.

Although COUNTER is declared after the instruction that uses it, Atom can assemble that instruction. It reserves the address field and fills it when the label is declared. Chapter 3 explains the restrictions on forward references in expressions.

The next chapter defines source lines and symbol scope.