7

FORGE

title ‘Hello, world 2’

ld bc, hello
call outstr
ld bc, world
call outstr
halt

hello:
defm ‘Hello, ’
defb 0
world:
defm ‘World!’
defb 13,10,0

outstr:
ld a, (bc)
or 0

ret z

out (1), a

inc bc

jr outstr

Now we can load the address of a string into BC

and call the outstr subroutine. As shown in the code

above, having a subroutine means that we can use it

for different strings without having to repeat the code

each time.

The subroutine is just the code we had before to

output the string. The big difference is that, instead of

jumping past the end of the loop when the zero byte

is found, it returns.

GOING FURTHER

The Z80 has 158 different instructions and ten

addressing modes available; there’s a lot more than

we’ve seen here.

Remember, every CPU has an assembly language,

including the CPU cores in modern MCUs. It’s very

possible to write MCU code in assembly. With

modern C compilers, it’s not necessary to work in

assembly language to get good performance, but it

does give a more detailed understanding of how a

particular MCU or CPU works.

If you want to explore the Z80 and its assembly

language with some real hardware and don’t want to

build your own Z80 computer, you can get RC2014 on

Tindie. The project has a GitHub repo with plenty of

information and tools: github.com/RC2014Z80.

There are a variety of sites dedicated to information

on the Z80 and its usage. The z80pack site has plenty,

and z80.info is a great collection of online material on

the Z80 to boot.

JUMP AROUND

The code uses jp instructions. These jump to a

literal address in memory. There’s another way to

do this: the jr (jump-relative) instructions. jr takes

a relative value, i.e. how far to jump backwards

or forwards. This is done with a single byte, so

it’s limited in how far it can jump to between 126

backward, to 129 forward (relative to the address

of the jr opcode). That’s a small limitation, and

the advantages are that it takes one less byte of

machine code, and it doesn’t use a literal address,

so the code can be loaded into different places in

memory. That can be quite handy at times.

Below

Despite being over 40

years old, Z80 chips

are still made today

The CPU has a

program counter

(PC) register

that contains the

address of the

next instruction to

be executed.

QUICK TIP