7

FORGE

in our examples, but they are one of the features
that make the Z80 so powerful and popular.
In the hardware article, we saw the address and
data buses. Recall that they are the way the CPU
interacts with the rest of the system: the address
bus identifies where in memory (or which I/O port)
the CPU wants to read/write data from/to, and the
data bus moves the data between the system and
the CPU.
The buses show up inside the CPU as well.
Various registers have their contents routed to/from
one or the other buses in order to move data around,
as defined by our code.
The ALU (arithmetic logic unit) is where maths and
logic operations are performed.
The instruction register, decode, and control
blocks are where the instruction being executed is
stored, and processed. Each instruction causes a
specific sequence of events to occur in the CPU.
The exact sequence will depend on the instruction
and will move data around the CPU and between
CPU and memory (or I/O), as required to implement
the instruction.
So, registers hold data being worked on, the ALU
does the work, the buses communicate with the rest
of the system, and the instruction decode/control
module orchestrates everything.

HELLO WORLD
‘Hello World’ is the canonical first program, so why
don’t we start our exploration of Z80 assembly

language with it? We’re going to dive in head first

and then walk through and explain the code.

We want a program to output ‘Hello, World!’,

followed by a new line. We put the code below in a

file called hello.asm. Each line is a single instruction

that is made up of an opcode (what to do) and zero

or more operands (what data to use).

title ‘Hello, world’

ld bc, hello

loop:

ld a, (bc)

or 0

jp z, end

out (1), a

inc bc

jp loop

end:

halt

Right

The internals

of the Z80

CPU AND

SYSTEM

CONTROL

SIGNALS

INSTRUCTION

DECODE

AND

CPU

CONTROL

CPU

CONTROL

INST.

REG

8-BIT

DATA BUS

INTERNAL DATA BUS

16-BIT

ADDRESS BUS

DATA BUS

CONTROL

CPU

REGISTERS

ADDRESS

CONTROL

ALU

+5 V GND CLOCK

The idea behind assembly language is to provide a

one-to-one mapping to machine code while being far

more readable and writable than a series of bytes

”

”