68000 Assembly Programming for the Sega Genesis (Mega Drive)

The Genesis is Sega's 16 bit sucessor to the Master System... With some backwards compatibility at a hardware level, and much more powerful graphics and the CPU... the Genesis was a huge step up from the 8 bit generation While the 68K CPU was massivly superior to that of the Super nintendo, unfortunately the Mode 7 capabilities of the SNES hardware more than made up for it, and the FX chip pretty much doomed the Genesis... but for the programmer, the Genesis is a great system giving the ease and power of the 68000, and a more conventional graphics system than the ROM based NeoGeo!

Specs: Sega Genesis Cpu 8mhz 68000 Ram 64K Vram 64K Resolution 320x224 Bitmap planes 2 x 16 color Max Sprites 80 sprites 16 color  (8x8 px 20 per line) Sound chip Z80 (8K Ram) + YM2612 FM + PSG Power Adaptor Genesis 1 (large version) 9V Center Negative (10V Famicom adaptor works fine) Genesis 2 (smaller model) 10V Center Positive

Tilemaps... Scroll-A,Scroll-B and Window

There are 3 Tilemaps on the Genesis... Scroll-A and Scroll-B are 2 scrolling layers of tilemap...

There is a 3rd known as the 'Window' which cannot scroll - it's intended as a HUD and replaces Scroll-A.  a 'Fixed point' is defined on the screen, and everything above or below, and to the Left or Right of that point is the window (meaning it cannot be in the centre). Depending on the tile priority Window can be in front of, or behind Scroll-B

The VRAM address of these is defined by various registers, here are the suggested defaults.

Scrolling!

Scrolling on the Genesis is odd!

Normally we would scroll the whole tilemap horizontally and vertically by a number of pixels... but we can actually do more!

If we wish,We can scroll individual 'Strips' of the tilemap by different amounts!...

We can scroll the Vertical strips in 'chunks' of 16 pixels (Two tiles)

We can scroll the horizontal strips in 'chunks' of 8 pixels (one tile) OR even one line!

These are defined by reg R11... a setting of $00 scrolls the whole tilemap in one unit. $06 scrolls in blocks, $07 scrolls horizontally in lines (vertically always scrolls in 'Chunks'

*To select a VSRAM address put the desired address in #$4xxx0010 and write to VDP_Ctrl port

*The VRAM address used for Hscroll is defined by R13... if R13=$37 then the base will be $DC00

Vram Addressing

Genesis Joystick Ports
Each joystick has two ports - one for Reading/Writing Data, and one Control port. To set the direction (Read or Write) of each bit of the data port (0=Read, 1=Write)
We will need to set bit 6 to Write to read all the Genesis Buttons

* We should perform a write of 0 to TH afterwards to reset sequence (1,0,1,0,1,0)

Sound

Tilemap A is defined by addresses &C000 onwards - each tile is defined by 2 bytes

The tilemap is 64 tiles wide and 32 tiles tall

Tile pattern data is NOT in bitplane format, it's in the same format as 16 color systems like the MSX2 or SAM coupe. Two pixels color information is held in a single byte, so one 32 bit long defines all 8 pixels of a tile

In H40 mode (the mode used in these tutorials), we can have up to 80 sprites onscreen...  (H32 mode can do only 64)
Each sprite can be 8x8 to 32x32
The position (and other settings) are defined by 8 by (4 Words) per sprite.

The first visible pixel is at sprite (X,Y) pos (128,128)

Multi pattern Sprites
A basic sprite is 8x8, but sprites can be enlarged up to 32x32... however this sprite will still be made up of 8x8 pattern data...

If we want a larger sprite, we need to break up the sprite into 8x8 blocks, and save those in VRAM in the correct order - the tiles go down first, and across second, as shown below:

Mouse Reading

The Genesis mouse can be read from the standard joystick ports.

We need to write #%01100000 to $A10009 for Joy Port 1, or $A1000B for Joy Port 2
We then send and receive bytes from port $A10003 for Joy Port 1, or $A10005 for Joy Port 2

Here is the sequence of bytes we send, and bits we receive back:

Sound on the Genesis via SN76489

We looked at using the SN76489 on the Genesis in the Z80 Tutorials here!

Each channel runs an 'Algorithm made up of multiple 'Operators', these all combine to make a particular type of sound. Algorithms: Alg1: Four serial connection mode  Alg2: Three double modulation serial connection mode Alg3: Double modulation mode Alg4: Two serial connection and two parallel modes Alg5: Common modulation 3 parallel mode Alg6: Two serial connection + two sine mode Alg7: Four parallel sine synthesis mode

The sound changes over time depending on various Operator registers... Sound Terminology: Attack: Increase in volume to max after Key down (Reg $50-$5E) Decay: Short term Sound Fade rate over time while key down (Reg $60-$6E) Sustain: Long term Sound Fade rate (Reg $70-$7E) Release: Sound Fade rate after key released (Reg $80-$8E) Total Level: Effective volume - the loudest the tone ever gets ($40-$4E) Sustain Level: Volume level at which Decay ends, and Sustain starts ($80-$8E)

YM2612 Registers