| Unlike the
the SMS and
GG have a real Z80 CPU!... Quite surprisingly the Master System and
Game Gear are pretty much the same system!
The Game Gear has a smaller screen than the master system... rather strangely it actually has a better colour palette! but appart from that, they are identical, and can run the same basic games!
Lets take a look at the specs!
The smaller screen is handled by 'skipping the 6 tiles from the left and right side, and 3 from the top and bottom... So we can write a game for our gamegear, and run it on a mastersystem, we just need to design around the screen size difference, and use some different palette code!
Gameboy,Gamegear and Mastersystem screens do not
work like they do on computers like the CPC
Graphics are not just 'bytes' in a memory address... The screen is made up of a 'Tile Layer' and a 'Sprite Layer'
To explain Tiles and sprites, lets look at our imaginary game shown to the right 'The Super Yuusha Siblings', on a theoretical game system the 'GameChibi'... Just to be very clear, we looking at this as a concept, not the actual layout of the gameboy!
Looking at our example, We have a level with some grass, blocks, and some collectable 'stars'... our hero, Yume is controlled by the player...
|The screen is made
up of the Tile layer, and the Sprite
Usually Sprites are drawn above the Tiles... but sometimes they may be drawn below.
It's also possible we could use sprites for the stars.. but sprites are very limited, so the object doesn't move, then tiles will do the job... we can even animate the stars by switching the tile between different patterns
|The tile array on
the systems we'll be looking at is
made up of 8x8 tiles... the array is a 'grid' of these tiles, so the
tiles must line up, a block cannot be at a 'half way boundary'
Tiles are defined by a number (usually 0-255)... we define the bitmap (image) data for that tile (we'll call it a pattern), then tell the hardware what positions in the tile array to use that pattern.
In our example, the black background is pattern 0 ... the blocks are pattern 1... the grass is pattern 2... and the stars are pattern 3
Our 'GameChibi' console has a tile array of 8x8... and 64 bytes is used to define the tile grid...
So to define the stars, we need to set memory locations 10,20 and 15 of the tile array to byte '3'
A real system usually has a tile array bigger than the screen (maybe just by one row and one coumn)... this is to allow smooth scrolling of the screen, where two tiles are 'half shown'
Now in the case of our 'GameChibi' system, with it's 64 tile screen, and 256 pattern definitions, we could just set every visible tile to a different pattern, and treat the screen as a plain bitmap again... We can do that on the MSX1, but unfortunately the Gameboy and Mastersystem have too few tiles for their screen size, so some parts of the screen must contain the same tile!
|Because our system is using hardware sprites, we have
to design our game sprites in a way that can be drawn with the hardware
sprites... for example lets look at our Yume sprite... if our
'GameChibi' used 8x8 hardware sprites, we would have to use 48 of them
to make this image!... we can save 5 (marked green)... these have no
data, so we can just not draw them...
When it comes to moving our character, the software will have to move the hardware sprites all together, so the user does not realise they are made up of many sprites!... on systems with more onscreen colors than sprite colors, two or more sprites may be overlapped to make the sprite appear more colorful
Most systems will have one color (usually 0) which marks the transparent colour.
but there is a problem! with software sprites on a bitmap screen, we can draw as much as we want, it will just get slow.... but with hardware sprites, we have a fixed limit of how many sprites can be shown onscreen at once! sounds bad? well actually it's worse, even though a system like the master system can show 64 sprites onscreen, there can only be 8 on a line... if more than 10 appear on the same line, some will flicker, or not appear... there's nothing we can do about it, we just have to design our game to avoid this problem!
|The Sega Mastersystem and Sega game gear are almost the
system, the major difference is the screen size, The SMS screen is
256x192, but the GG screen is 160x144...
If we draw the same data to the screen, we'll realize that the gamegear is drawing only the center area of the SMS screen area, with 3 lines missing from the top and bottom, and 6 columns missing from the left and right sides.
While both systems are 16 color, The Gamegear has a better color palette!
The MasterSystem uses a single byte palette in the format: --BBGGRR
The GameGear uses 2 bytes, in the format ----BBBB GGGGRRRR
|The Gamegear rom is
pretty simple, when the console starts, execution begins at memory
The 'Header' which defines the cartridge is 16 bytes, and typically appears at &7FF0
some of the attributes are not a full byte, and use half a byte (a nibble)
if you're using an emulator, you do not need need a header! even without the 'System type' the 'Fusion' emulator will use the file extension (SMS or GG) to detect the system to use.
|Note, some of these ports also respond to alternate addresses, so you may see different port numbers in other programs.||
|Vram is separate so must be accessed by OUT commands on the SMS/GG... Note, Palette data is addressed using &C000... however this is actually not a memory address in the VDP, as the VDP only has 16k ram.||
|The GameGear and
Mastersystem use 2x 16 color
Background Tiles can use either of the 2 palettes... sprites always use the second one.
The Mastersystem uses just 2 bits per color channel... meaning 1 byte per definition
The GameGear uses 1 nibble per channel.. meaning 2 bytes per definition... this means the memory locations are different on the SMS and GG
SMS Palette Definition:
GG Palette Definition:
|The SMS/GG memory
supports up to 64 sprites,
There are 3 memory bytes for each hardware sprite, but they are not consecutive!
Look at sprite 0 - shown in black... it's X co-ordinate is at &3F00 ... it's Y co-ordinate is at &3F80... it's Sprite Number is at &3F81
Sprite Numbers can come from memory address &2000, or &0000 (The Tile Patterns) depending on the setting of register &06
|Register Number||Purpose||Bits||Bit meaning|
|&00||Mode Control No. 1|
|&01||Mode Control No. 2|
|&02||Name Table Base Address|
|&03||Color Table Base Address|
|&04||Pattern Generator Table Base Address|
|&05||Sprite Attribute Table Base Address|
|&06||Sprite Pattern Generator Table Base Address||- - - - - S - -||S=Sprite Table Address (0= use patterns for sprites, 1=sprites at &2000)|
|&08||Background X Scroll||T T T T T P P P||T = Tile offset (0-31), P=Pixel offset|
|&09||Background Y Scroll||T T T T T P P P||T = Tile offset (0-27), P=Pixel offset|
|The GameGear and
Master System use 3 ports in total, but there are differences!
The Gamegear has an extra button! 'Start' accessible from bit 7 of Port &00... The Gamegear does not have any player 2 controls
The Mastersystem has no start button, but it does have a second player paddle!
note:Button TH has no use on the normal SMS gamepads, so is not used in these rexamples.