| Reg. |
Name |
Bits |
Code
|
| 00
|
Mode Register 1
|
|
move.w #$80xx,($c00004).l
|
| 01
|
Mode Register 2
|
- VR: 1 = use 128kB of VRAM. Will not work correctly on standard consoles with 64kB VRAM.
- DE: 1 = enable display; 0 = fill display with background colour.
- IE0: 1 = enable vertical interrupts.
- M1: 1 = enable DMA operations; 0 = ignore DMA operations.
- M2: 1 = 240 pixel (30 cell) PAL mode; 0 = 224 pixel (28 cell) NTSC mode.
- M5: 1 = Mega Drive (mode 5) display; 0 = Master System (mode 4) display.
|
move.w #$81xx,($c00004).l
|
| 02
|
Plane A Name Table Location
|
- PA5-PA3: Bits 15-13 of foreground (plane A) nametable address in VRAM. Effectively the address (which must be a multiple of $2000) divided by $400.
- PA6: Used for 128kB VRAM.
|
move.w #$8200+($xxxx>>10),($c00004).l
|
| 03
|
Window Name Table Location
|
- W5-W1: Bits 15-11 of window nametable address in VRAM. Effectively the address (which must be a multiple of $800) divided by $400.
- W1: Ignored in 320 pixel wide mode, limiting the address to a multiple of $1000.
- W6: Used for 128kB VRAM.
|
move.w #$8300+($xxxx>>10),($c00004).l
|
| 04
|
Plane B Name Table Location
|
- PB2-PB0: Bits 15-13 of foreground (plane B) nametable address in VRAM. Effectively the address (which must be a multiple of $2000) divided by $2000.
- PB3: Used for 128kB VRAM.
|
move.w #$8400+($xxxx>>13),($c00004).l
|
| 05
|
Sprite Table Location
|
- ST6-ST0: Bits 15-9 of sprite table address in VRAM. Effectively the address (which must be a multiple of $200) divided by $200.
- ST0: Ignored in 320 pixel wide mode, limiting the address to a multiple of $400.
- ST7: Used for 128kB VRAM.
|
move.w #$8500+($xxxx>>9),($c00004).l
|
| 06
|
|
|
move.w #$8600+($xxxxxx>>11),($c00004).l
|
| 07
|
Background Colour
|
- PL5-PL4: Palette line.
- C3-C0: Colour.
|
move.w #$8700+(xx<<4)+yy,($c00004).l
|
| 08
|
Unused
|
Master System horizontal scroll register
|
|
| 09
|
Unused
|
Master System vertical scroll register
|
|
| 0A
|
Horizontal Interrupt Counter
|
|
move.w #$8Axx,($c00004).l
|
| 0B
|
Mode Register 3
|
7 6 5 4 3 2 1 0
0 0 0 0 IE2 VS HS1-HS2 - IE2: 1 = enable external interrupts, caused by the TH pin being set to input mode and having the TH interrupt enable bit set. (Both of these are controlled by the Mega Drive' I/O registers).
- VS: Vertical scrolling mode: 1 = 16 pixel columns (1 word per column in VSRAM); 0 = full screen (1 longword only in VSRAM).
- HS1-HS2: Horizontal scrolling mode: 00 = full screen; 01 = invalid; 10 = 8 pixel rows; 11 = single pixel rows.
|
move.w #$8Bxx,($c00004).l
|
| 0C
|
Mode Register 4
|
7 6 5 4 3 2 1 0
RS1 VS HS EP SH LS1-LS0 RS0 - RS1/RS0: 1 = 320 pixel (40 cell) wide mode; 0 = 256 pixel (32 cell) wide mode. Both bits must be the same.
- VS: Appears to do something relating to the horizontal sync, which seems to freeze it. (source md.railgun.works)
- HS: Replaces the VSync signal with a pixel clock signal when set. (source md.railgun.works)
- EP: 1 = enable external pixel bus.
- SH: 1 = enable shadow/highlight mode.
- LS1-LS0: Interlace mode: 00 = no interlace; 01 = interlace normal resolution; 10 = no interlace; 11 = interlace double resolution. Changes do not take effect until the next vertical blank.
|
move.w #$8Cxx,($c00004).l
|
| 0D
|
Horizontal Scroll Data Location
|
- HS5-HS0: Bits 15-10 of horizontal scroll data address in VRAM. Effectively the address (which must be a multiple of $400) divided by $400.
- HS6: Used for 128kB VRAM.
|
move.w #$8D00+($xxxx>>10),($c00004).l
|
| 0E
|
|
- PB4: Bit 16 of background (plane B) nametable address in 128kB VRAM.
- PA0: Bit 16 of foreground (plane A) nametable address in 128kB VRAM.
|
move.w #$8E00+(($xxxxxx&$10000)>>12)+($yyyyyy>>16),($c00004).l
|
| 0F
|
Auto-Increment Value
|
- INC7-INC0: Value to be added to the VDP address register after each read/write to the data port. 2 is most common.
|
move.w #$8Fxx,($c00004).l
|
| 10
|
Plane Size
|
- H1-H0: Height setting for background and foreground (planes A & B). 00 = 256 pixels (32 cells); 01 = 512 pixels (64 cells); 10 = invalid; 11 = 1024 pixels (128 cells).
- W1-W0: Width setting for background and foreground (planes A & B). Same as above.
- Height/width settings of 64x128 or 128x128 cells are invalid, due to a maximum plane size of $2000 bytes.
|
move.w #$90xx,($c00004).l
|
| 11
|
Window Plane Horizontal Position
|
- R: 1 = draw window from HP to right edge of screen; 0 = draw window from HP to left edge of screen.
- HP4-HP0: Horizontal position on screen to start drawing the window plane (in units of 8 pixels).
|
move.w #$91xx,($c00004).l
|
| 12
|
Window Plane Vertical Position
|
- D: 1 = draw window from VP to bottom edge of screen; 0 = draw window from VP to top edge of screen.
- VP4-VP0: Vertical position on screen to start drawing the window plane (in units of 8 pixels).
|
move.w #$92xx,($c00004).l
|
| 13-14
|
DMA Length
|
- L7-L0: Low byte of DMA length in bytes, divided by 2.
- H7-H0: High byte of DMA length in bytes, divided by 2.
|
move.w #$9300+(($xxxx>>1)&$FF),($c00004).l
move.w #$9400+((($xxxx>>1)&$FF00)>>8),($c00004).l
|
| 15-17
|
DMA Source
|
- L7-L0: Low byte of DMA source address, divided by 2.
- M7-M0: Middle byte of DMA source address, divided by 2.
- H5-H0: High byte of DMA source address, divided by 2.
- T1/T0: DMA type. 0x = 68k to VRAM copy (T0 is used as the highest bit in source address); 10 = VRAM fill (source can be left blank); 11 = VRAM to VRAM copy.
|
move.w #$9500+(($xxxxxx>>1)&$FF),($c00004).l
move.w #$9600+((($xxxxxx>>1)&$FF00)>>8),($c00004).l
move.w #$9700+((($xxxxxx>>1)&$7F0000)>>16),($c00004).l
|
|
|
Status Register
|
7 6 5 4 3 2 1 0
VI SO SC OD VB HB DMA PAL - E: 1 = FIFO is empty.
- F: 1 = FIFO is full.
- VI: 1 = vertical interrupt occurred.
- SO: 1 = sprite limit has been hit on current scanline. i.e. 17+ in 256 pixel wide mode or 21+ in 320 pixel wide mode.
- SC: 1 = any two sprites have non-transparent pixels overlapping. Used for pixel-accurate collision detection.
- OD: 1 = odd frame displayed in interlaced mode; 0 = even frame displayed in interlaced mode.
- VB: 1 = vertical blank in progress.
- HB: 1 = horizontal blank in progress.
- DMA: 1 = DMA in progress.
- PAL: 1 = PAL system; 0 = NTSC system.
|
move.w ($c00004).l,d0
|