Sega Game 1000 (SG-1000) Information (Draft)
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This is a copy of an "unofficial" document containing original research, for use as a source on Sega Retro. This page likely exists for historical purposes - the contents should ideally be copy-edited and wikified to make better use of Sega Retro's software. Original source: http://www.smspower.org/uploads/Development/sg1000.txt |
draft
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Sega Game 1000 Specifications by Omar Cornut / Zoop
Project started on the 04/27/99 Last updated on the 04/27/99
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CPU: Zilog Z-80 at 3,579545 mhz
Video: TMS9918. Resolution is 256x192.
Sound: SN76489 Programmable Sound Generator, by Texas Instruments
RAM: 8 kilobytes.
VRAM: 16 kilobytes.
Memory Map
----------
0x0000 ------------------------------------------------
ROM (read-only)
0x8000 ------------------------------------------------
Unused on SG-1000
0xA000 ------------------------------------------------
RAM (read/write)
0xC000 ------------------------------------------------
Mirror of RAM (read/write)
0xFFFF ------------------------------------------------
IO Ports
--------
0xDC / 0xC0 : Joypad Port 1 (read-only)
bit 0 : Joypad 1 Up
bit 1 : Joypad 1 Down
bit 2 : Joypad 1 Left
bit 3 : Joypad 1 Right
bit 4 : Joypad 1 Button 1
bit 5 : Joypad 1 Button 2
bit 6 : Joypad 2 Up
bit 7 : Joypad 2 Down
Low logic port. 0 = pressed, 1 = released
0xDD / 0xC1 : Joypad Port 2 (read-only)
bit 0 : Joypad 2 Left
bit 1 : Joypad 2 Right
bit 2 : Joypad 2 Button 1
bit 3 : Joypad 2 Button 2
bit 4 : Reset Button
Low logic port. 0 = pressed, 1 = released
0x7E : 0x7F : Programmable Sound Generator Output (write)
0xBE : VDP Data (read/write)
Use to read and write to VRAM (not VDP Registers).
0xBF : VDP Address (write)
To set an address, write two bytes:
bbbbbbbb then 01aaaaaa
Where bbbbbbbbaaaaaa is a 14-bit VRAM address.
To write to a VDP register, write two bytes:
bbbbbbbb then 1000aaaa
Where aaaa is the VDP register number and bbbbbbbb the value
to write to it.
0xBF : VDP Status Register (read)
bit 5 : Sprite Collision Flag
Set when two sprites overlap.
bit 6 : Line Interrupt Flag (maybe unused in SG-1000 ?)
Set when a line interrupt occurs.
bit 7 : VSync flag
Set when any interrupt occurs.
Bit 6 and 7 are reset when the port is read.
Interrupts
----------
An interrupts occur every 60th/second = CPU_CLOCK * 1000000 / 60 cycles.
The SG-1000 only supports Z-80 interrupt in mode 1:
- push PC register to stack
- jump to 0x38
When returning from the interrupt with IRET, the PC register is restored.
It's up to the interrupt routine to save other registers and restore them.
Non Maskable Interrupts
-----------------------
A NMI occurs when the Pause button is pressed.
It's behaviour is the same as standard interrupt, except that the jumping
location is 0x66.
VDP Registers
-------------
Register 0
----------
bit 1 : video mode bit 0
bit 2 : video mode bit 1
bit 3 : video mode bit 2
Register 1
----------
bit 0 : zoomed sprites (show 8x8 as 16x16 and 16x16 as 32x32)
bit 1 : double sprite size (8x8 -> 16x16)
bit 3 : video mode bit 3
bit 4 : video mode bit 4
bit 6 : display bit. set to 0 and the screen will stay black.
Register 2
----------
bit 0 -> Tile Map Address bit 10
bit 1 -> Tile Map Address bit 11
bit 2 -> Tile Map Address bit 12
bit 3 -> Tile Map Address bit 13
Register 3
----------
Color Map Address in VRAM.
The bitmask depend on the video mode:
mode 0: 0x00 (text)
mode 1: 0xFF (graphic 1)
mode 2: 0x80 (graphic 2, the most common)
Multiply the ANDed value by 0x40 and you have the address.
Register 4
----------
Tiles Starting Address in VRAM.
The bitmask depend on the video mode:
mode 0: 0x3F (text)
mode 1: 0x3F (graphic 1)
mode 2: 0x3C (graphic 2, the most common)
Multiply the ANDed value by 0x8000 and you have the address.
Register 5
----------
Sprite Table Address
Bits 0 to 6 -> multiply by 128 -> Start address of the table in VRAM
Register 6
----------
Sprite Starting Tile Address
Multiplay value by 0x800 and you've got the address.
Register 7
----------
Bits 0 to 3 code for the transparent color. Setting them to 0 mean a black
color, else the color is taken in the palette.
Palette
-------
The palette is constitued of 16 fixed colors,
which 8-bit RGB values are:
0: transparent
1: black - 0x00, 0x00, 0x00
2: green - 0x20, 0xC0, 0x20
3: bright green - 0x60, 0xE0, 0x60
4: blue - 0x20, 0x20, 0xE0
5: bright blue - 0x40, 0x60, 0xE0
6: dark red - 0xA0, 0x20, 0x20
7: cyan (?) - 0x40, 0xC0, 0xE0
8: red - 0xE0, 0x20, 0x20
9: bright red - 0xE0, 0x60, 0x60
10: yellow - 0xC0, 0xC0, 0x20
11: bright yellow - 0xC0, 0xC0, 0x80
12: dark green - 0x20, 0x80, 0x20
13: pink - 0xC0, 0x40, 0xA0
14: gray - 0xA0, 0xA0, 0xA0
15: white - 0xE0, 0xE0, 0xE0
Graphics Modes
--------------
All graphics modes display sprites after background.
Resolution is 256x192 pixels = 32x24 tiles.
Tile encoding
-------------
A tile is 8x8 pixels large.
Each lines of the tile is stored in one byte, therefore a tile is 8 bytes.
As tiles can use two colors only, each bit set a pixel.
Mode 0 Background
-----------------
...
Mode 1 Background
-----------------
...
Mode 2 Background
-----------------
The screen is splitted in three 32x8 tiles blocks.
Each use a different area to store the tile graphics:
Tiles_Start_Address -------------------------------------
Tiles graphics for the top part
Tiles_Start_Address + 0x0800 ----------------------------
Tiles graphics for the middle part
Tiles_Start_Address + 0x1000 ----------------------------
Tiles graphics for the bottom part
Tiles_Start_Address + 0x1800 ----------------------------
Usually used for sprites tiles graphics
---------------------------------------------------------
Tiles number are raw stored, starting at the address specified by VDP
Register 2.
Tiles colors are raw stored, starting at the address specified by VDP
Register 3. Each byte contains the background color in the first four
bits and the foreground color in the last four bits.
.....to complete
Sprites
-------
Sprite are shown in modes 1 and 2 only. The sprite table is 128 bytes:
0 + (n * 4) : Y coordinate
1 + (n * 4) : X coordinate
2 + (n * 4) : Tile Number. Clear bits 0 and 1 is sprites are doubled.
3 + (n * 4) : Bits 0 to 3 = Color. If color is 0, don't display the sprite.
When bit 7 is set, shift sprite by 8 pixels.
When Y = 208, stop displaying sprites.