E00c070.PDF

MICRO PROCESSOR

GameBoy Development Tools

By Luc Lemmens

The articles on the GameBoy Digital sampling oscilloscope (GBDSO) did

not fail to convince many of you that the Nintendo game console is a

veritable gateway to more serious applications. This month we’ll look at

the tricks of the trade, or what goes into creating your GB application.

Since its introduction on the toy market, the

immensely successful Gameboy computer

game console has been used applications

‘totally different’ from than those envisaged

by its manufacturer, Nintendo. The original

game cartridges containing no more than a

ROM (sometimes accompanied by a RAM),

illegal copies soon appeared on the black

market. The next step was to develop Flash-

RAM cartridges which could be loaded over

and over again (from a PC) with different

games. The Internet abounds with websites

where this (illegal) activity is practised.

On a more serious (and certainly less illegal)

front, people are creating their own games for

the GameBoy. This deserves more respect

than the copying activity because one needs

to know the internal workings of the Game-

boy, and be able to program in the processor

language. The GBDSO takes this one step fur-

ther by adding not only software but also new

hardware to the GameBoy, enabling the ‘toy’ to

perform a task which is completely different

from what it was originally designed for! Not

surprising, really, because the GameBoy is a

compact system with its own control soft-

ware, a graphics display, processor, pushbut-

tons and an attractively styled plastic case, all

a price which defeats home construction.

Like us, you’ll soon get lost if you try to fol-

low all GameBoy links, threads and other ref-

erences on the Internet. In this article we will

endeavour to inform you about ‘places to

visit’ if you are interested in development

tools for the GB. Alas, in view if the huge vol-

ume of GameBoy-related information on the

web, it is impossible to cover it all.

Inside the GameBoy

The heart of the GameBoy is a Z80-

ish processor. The Z80 has been fea-

tured in many Elektor Electronics

projects, and is an offshoot of the

good old Intel 8080. The instruction

set, however, misses out on some

instructions, while others have been

added. Furthermore, the flag regis-

ter looks slightly different because

the sign and overflow bits have

been omitted.

Developing your own software

requires a system memory map like

the one shown in Figure 1 . The

address space of the Gameboy com-

prises several RAM banks (regular

internal memory, video memory,

sprite memory), ROM banks, mem-

ory-mapped I/O and three registers

that allow other memory banks to be

selected. In this way, the physical

memory can be much larger than

64 kBytes as originally specified for

the Z80 processor.

At the end of this article you’ll

find a reference to the document

called ‘GMB-SPEC.SC’ in which all

relevant hardware aspects are dis-

cussed at length. A must for anyone

Elektor Electronics

12/2000

GBDT

MICRO PROCESSOR

General Memory Map

board to be made to enable it to be connected.

Whatever method you use, the ﬁnal test will

have to be on real hardware, and that is espe-

cially true for project designed on emulators, as

well as for designs involving new hardware

added to a cartridge.

In most cases, you’ll also need a helper

program to add the special Nintendo header

to the ROM cartridge and in addition com-

putes the right checksum. The job is best

handled by a program like RGBFIX. At

power-on, the GameBoy ﬁrst runs a couple of

routines from internal ROM to check if the

cartridge contains the correct checksum, and

the Nintendo copyright message is stored in

the right location in the cartridge memory. If

these two conditions are not satisfied the

GameBoy will not get past its own start

screen. If everything is okay, it is up to the

ROM in the cartridge to make the little com-

puter do the necessary.

Interrupt Enable Register

FFFF

Internal RAM

FF80

Empty but unusable for I / O

FF4C

I /O ports

FF00

Empty but unusable for I / O

FEA0

Sprite Attrib Memory (OAM)

FE00

Echo of 8 kB Internal RAM

E000

8 kB Internal RAM

C000

8 kB switchable RAM bank

A000

Shopping list

8 kB Video RAM

8000

First of all you need a GameBoy, at least

that’s what you would think. True and not

true. On the Internet you may find several

emulators that allow GameBoy software to be

tested on many different computer types,

before you even get your hands on the real

thing. However, especially when designing

new hardware, you really have to get your

hands on a GameBoy to make sure your appli-

cation actually works.

Besides the hardware development tools,

you’ll also find an integrated development

system useful, like ‘GameBoy Development

Studio’. This wonderful product bundles all

components of the development software in

one graphics-based package. Finally, the

GameBoy Development Kit (GBDK) is highly

recommended, if only for the extensive col-

lection of examples included.

MBC1 ROM/RAM Select

16 kB switchable ROM bank

6000

RAM Bank Select

4000

ROM Bank Select

RAM Bank Enable

16 kB ROM bank #0

2000

0000

000152 - 11

Figure 1. GameBoy memory map.

serious about developing GameBoy

cartridge hardware.

The graphics

The Gameboy works with tiles,

which in graphics terms translates

into rectangles of 8?6 pixels. The

next large unit, a ‘map’ consists of

32?32 tiles. Graphics screens may be

designed on the PC using tile editors

or map editors like Tile Buddy, GB

Tile Designer (GBTD) and GB Map

Builder (GBMB), all suitable for run-

ning under Windows 95. DOS users

may employ TILE256. Besides these

editors, there are also programs that

enable graphic ﬁles to the BMP, PCX,

GIF or TIF format to be converted to

GameBoy pictures.

Development Software

While there are various superb

assemblers for the Z80, you’ll like

the news that there also is a real C

Compiler for the GameBoy: the

GameBoy Development Kit (GBDK).

None the less, many developers will

stick to their assemblers, mainly

because the C Compiler fails to gen-

erate code which is optimised for

compactness. A number of assem-

blers float around on the Internet,

including RGBDS, TASM (Table

Assembler — not Turbo ASsembler),

ADVancedGBIDE and the assembler

included with the GBDK. Profession-

als typically use ISAS, an assembler

recommended by Nintendo. Unfor-

tunately, it is only made available to

licensed designers.

Each assembler has its own pros

and cons, and your choice will ulti-

mately be governed by personal

taste and requirements.

(000152-1)

Internet addresses:

Jeff Frohwein’s Technical Page:

http://www.devrs.com/gb/

Pascal Felber and Michael Hope’s GBDK:

http://gbdk.sourceforge.net/

Ian James GBDS:

http://www.geocities.com/Eureka/9827/

Dr Pan’s Gameboy technical documentation:

http://www.gbdev.org/news/dl.html

Paul Robson’s GB97 emulator:

http://users.aol.com/autismuk/gameboy.htm

Hardware or software?

Several options are available when it

comes to testing applications. Of

course, you could use a cartridge con-

taining an EPROM or a Flash memory,

but for your ﬁrst (faltering) attempts a

GameBoy emulator is highly recom-

mended. Alternatively you could

resort to an EPROM emulator,

although that requires an adapter

12/2000

Elektor Electronics

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