7

FORGE

001c 21 15 18

001d 0d 0a 00 16 19 defb

13,10,0

17 20

Look at line 4:

0000 01 0d 00 4 4 ld bc, hello

We can see that the assembly language instruction
ld bc, hello translates directly to the bytes 0x01,
0x10, and 0x00. 0x01 is the opcode for loading the BC
register pair with the two byes immediately following
the opcode. B is loaded with 0x00, and C is loaded
with 0x10. If we look later in the listing to line 13 and
14, we can see that the label hello corresponds to
address 0x0010.

So, let’s have a closer look at this listing file. There
are five columns, four of which are interesting:

LOC: the memory address of the start of the
line/instruction

OBJECT CODE: the bytes that make up the
assembled instruction, the machine code

LINE: the line in the source file

SOURCE CODE: the code that was in the .asm
source file

Each source instruction is converted into one or
more bytes of machine code. The assembler also
provides some pseudo-instructions such as title,
defm, and defb that are part of the assembler, not the
CPU instruction set.

SIMULATION
Before we go any further, let’s execute the assembled
program, using z80sim as shown in the ‘Running the
code’ box’. We load the bin file with the r command,
giving it the name of the file. We can also provide
the memory address at which to load the machine
code. As we said in the hardware article, the Z80

starts executing at address 0x0000

and z80asm/z80sim assumes that’s

where you want it to start

putting the machine code

it generates/loads.

Once the code

is loaded, we can

execute it with the g

command (short for

go). We can provide the

address to start executing

at, in this case 0000.

We can then see it print

‘Hello, World!’ and stop due to the

halt instruction.

In the simulator we can examine the

contents of memory. We can look at it as

code using the l(ist) command.

There are many more commands available in the

simulator, including debugger features like single

stepping, breakpoints, etc. You can see the list using

the? command.

RUNNING THE CODE

>:z80sim

####### ##### ### ##### ### # #

# # # # # # # # ## ##

# # # # # # # # # # #

# ##### # # ##### ##### # # # #

# # # # # # # # #

# # # # # # # # # #

####### ##### ### ##### ### # #

Release 1.36, Copyright (C) 1987-2017 by Udo Munk

CPU speed is unlimited

>>> r hello.bin,0000

Loader statistics for file hello.bin:

START : 0000

END : 001f

LOADED: 0020

>>> g 0000

Hello, World!

HALT Op-Code reached at 000f

PC A SZHPNC I IFF BC DE HL A’F’ B’C’ D’E’ H’L’ IX IY SP

0010 00 010100 00 00 001f 1f94 38a3 d20b f21c c6c8 adb7 ae55 2b53 1261

>>>

The assembler also provides

some pseudo-instructions

such as title, defm, and defb

that are part of the assembler,

not the CPU instruction set

”

”

Below

Originally, code was

loaded into EPROMs

such as this