Category Archives: Tutorials

In this post I will describe the process of reading the joystick, changing the sprite position, horizontal flip, Sega Genesis emulator and potentially the console itself.

Reading of presses, processing of “events” of the Sega joystick occurs according to the following scheme:

1. Request for combination of bits of pressed buttons
2. Reading bits of pressed buttons
3. Processing at the game logic level

To move the skeleton sprite we need to store the current position variables.

RAM

Game logic variables are stored in RAM, people haven’t come up with anything better yet. Let’s declare variable addresses, change the rendering code:

skeletonYpos = $FF0002 
frameCounter = $FF0004 
skeletonHorizontalFlip = $FF0006

    move.w #$0100,skeletonXpos 
    move.w #$0100,skeletonYpos 
    move.w #$0001,skeletonHorizontalFlip 

FillSpriteTable: 
    move.l #$70000003,vdp_control_port 
    move.w skeletonYpos,vdp_data_port  
    move.w #$0F00,vdp_data_port 
    move.w skeletonHorizontalFlip,vdp_data_port 
    move.w skeletonXpos,vdp_data_port 

As you can see, the address available for work starts at 0xFF0000 and ends at 0xFFFFFF, so we have 64 KB of memory available. Skeleton positions are declared at skeletonXpos, skeletonYpos, horizontal flip at skeletonHorizontalFlip.

Joypad

Similar to VDP, joypads are handled via two separate ports – the control port and the data port, for the first one it’s 0xA10009 and 0xA10003 respectively. When working with a joypad, there’s one interesting feature – first you need to request a combination of buttons for polling, and then, after waiting for the bus update, read the required presses. For the C/B buttons and the cross, it’s 0x40, an example below:

  move.b #$40,joypad_one_control_port; C/B/Dpad 
  nop ; bus sync 
  nop ; bus sync 
  move.b joypad_one_data_port,d2 
  rts 

The state of the buttons pressed or not pressed will remain in the d2 register, in general, what was requested via the data port will remain. After that, go to the Motorola 68000 register viewer of your favorite emulator, see what the d2 register is equal to depending on the presses. You can find this out in the manual in a smart way, but we don’t take your word for it. Next, processing the pressed buttons in the d2 register

    cmp #$FFFFFF7B,d2; handle left 
    beq MoveLeft  
    cmp #$FFFFFF77,d2; handle right  
    beq MoveRight  
    cmp #$FFFFFF7E,d2; handle up  
    beq MoveUp  
    cmp #$FFFFFF7D,d2; handle down  
    beq MoveDown  
    rts

    move.w skeletonXpos,d0 
    sub.w #1,d0 
    move.w d0,skeletonXpos 
    move.w #$0801,skeletonHorizontalFlip 
    jmp FillSpriteTable

  move.w #512,frameCounter 
WaitFrame: 
  move.w frameCounter,d0 
  sub.w #1,d0 
  move.w d0,frameCounter 
  dbra d0,WaitFrame 
GameLoop: 
  jsr ReadJoypad 
  jsr HandleJoypad 
  jmp GameLoop 

In this note I will describe how to draw sprites using the VDP emulator of the Sega Genesis console.
The process of rendering sprites is very similar to rendering tiles:

1. Loading colors into CRAM
2. Unloading 8×8 sprite parts into VRAM
3. Filling Sprite Table in VRAM

For example, let’s take a sprite of a skeleton with a sword 32×32 pixels

Skeleton Guy [Animated] by Disthorn

CRAM

Using ImaGenesis we will convert it into CRAM colors and VRAM patterns for assembler. After that we will get two files in asm format, then we will rewrite the colors to word size, and the tiles should be put in the correct order for drawing.
Interesting information: you can switch the VDP autoincrement via register 0xF to the word size, this will allow you to remove the address increment from the CRAM color fill code.

VRAM

The Sega manual has the correct tile order for large sprites, but we’re smarter, so we’ll take the indexes from the ChibiAkumas blog, starting the count from index 0:

0 4 8 12

1 5 9 13

2 6 10 14

3 7 11 15

Why is everything upside down? What do you expect, the prefix is ​​Japanese! It could have been from right to left!
Let’s change the order manually in the sprite asm file:

	dc.l	$11111111	; Tile #0 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111	; Tile #4 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111
	dc.l	$11111111	; Tile #8 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111122 
	dc.l	$11111122 
	dc.l	$11111166 
	dc.l	$11111166	; Tile #12 
	dc.l	$11111166 
	dc.l	$11111166 
	и т.д. 

Load the sprite like regular tiles/patterns:

  lea Sprite,a0 
  move.l #$40200000,vdp_control_port; write to VRAM command 
  move.w #128,d0 ; (16*8 rows of sprite) counter 
SpriteVRAMLoop: 
  move.l (a0)+,vdp_data_port; 
  dbra d0,SpriteVRAMLoop 

To draw the sprite, it remains to fill the sprite table (Sprite Table)

Sprite Table

The sprite table is filled in VRAM, its location address is set in VDP register 0x05, the address is again tricky, you can look it up in the manual, an example for address F000:

Ок, теперь запишем наш спрайт в таблицу. Для этого нужно заполнить “структуру” данных состоящую из четырех word. Бинарное описание структуры вы можете найти в мануале. Лично я сделал проще, таблицу спрайтов можно редактировать вручную в эмуляторе Exodus.
The parameters of the structure are obvious from the name, for example XPos, YPos – coordinates, Tiles – the number of the starting tile for drawing, HSize, VSize – the size of the sprite by adding parts 8×8, HFlip, VFlip – hardware rotations of the sprite horizontally and vertically.

It is very important to remember that sprites can be off-screen, this is correct behavior, since unloading off-screen sprites from memory is quite a resource-intensive task.
After filling the data in the emulator, it needs to be copied from VRAM to address 0xF000, Exodus also supports this feature.
By analogy with drawing tiles, first we access the VDP control port to start writing at address 0xF000, then we write the structure to the data port.
Let me remind you that the description of VRAM addressing can be read in the manual or in the blog Nameless Algorithm.

In short, VDP addressing works like this:
[..DC BA98 7654 3210 …. …. …. ..FE]
Where hex is the bit position in the desired address. The first two bits are the type of command requested, for example 01 – write to VRAM. Then for address 0XF000 you get:
0111 0000 0000 0000 0000 0000 0000 0011 (70000003)

As a result we get the code:

  move.l #$70000003,vdp_control_port 
  move.w #$0100,vdp_data_port 
  move.w #$0F00,vdp_data_port 
  move.w #$0001,vdp_data_port 
  move.w #$0100,vdp_data_port 

After this, the skeleton sprite will be displayed at coordinates 256, 256. Cool, huh?

Links

https://gitlab.com/demensdeum /segagenesissamples/-/tree/main/7Sprite/vasm
https://opengameart.org/content/skeleton-guy-animated

Sources

https://namelessalgorithm.com/genesis/blog/vdp/
https://www.chibiakumas.com/68000/platform3.php#LessonP27
https://plutiedev.com/sprites

In this note I will describe how to display an image from tiles on the Sega Genesis emulator using assembler.
The splash image Demens Deum in the Exodus emulator will look like this:

The process of outputting a PNG image using tiles is done step by step:

1. Reduce image to fit Sega screen
2. Convert PNG to assembly data code, with separation into colors and tiles
3. Loading color palette into CRAM
4. Loading tiles/patterns into VRAM
5. Loading tile indices to Plane A/B addresses into VRAM
6. You can reduce the image to the size of the Sega screen using your favorite graphics editor, such as Blender.

PNG conversion

To convert images, you can use the ImaGenesis tool, to work under wine, you need Visual Basic 6 libraries, they can be installed using winetricks (winetricks vb6run), or RICHTX32.OCX can be downloaded from the Internet and placed in the application folder for correct operation.

In ImaGenesis, you need to select 4-bit color, export colors and tiles to two assembler files. Then, in the file with colors, you need to put each color into a word (2 bytes), for this, the opcode dc.w is used.

For example CRAM splash screen:

  dc.w $0000 
  dc.w $0000 
  dc.w $0222 
  dc.w $000A 
  dc.w $0226 
  dc.w $000C 
  dc.w $0220 
  dc.w $08AA 
  dc.w $0446 
  dc.w $0EEE 
  dc.w $0244 
  dc.w $0668 
  dc.w $0688 
  dc.w $08AC 
  dc.w $0200 
  dc.w $0000 

Leave the tile file as is, it already contains the correct format for loading. Example of part of the tile file:

	dc.l	$11111111	; Tile #0 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111	; Tile #1 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 
	dc.l	$11111111 

As you can see from the example above, the tiles are an 8×8 grid of CRAM color palette indices.

Colors in CRAM

Loading into CRAM is done by setting the color load command at a specific CRAM address in the control port (vdp control). The command format is described in the Sega Genesis Software Manual (1989), I will only add that it is enough to add 0x20000 to the address to move to the next color.

Next, you need to load the color into the data port (vdp data); The easiest way to understand the loading is with the example below:

    lea Colors,a0 ; pointer to Colors label 
    move.l #15,d7; colors counter 
VDPCRAMFillLoopStep: 
    move.l  d0,vdp_control_port ;  
    move.w  (a0)+,d1; 
    move.w  d1,(vdp_data_port); 
    add.l #$20000,d0 ; increment CRAM address 
    dbra d7,VDPCRAMFillLoopStep 

Tiles in VRAM

Next comes loading of tiles/patterns into the VRAM video memory. To do this, select an address in VRAM, for example 0x00000000. By analogy with CRAM, we address the VDP control port with a command to write to VRAM and the starting address.

After that, you can upload longwords to VRAM, compared to CRAM, you do not need to specify the address for each longword, since there is a VRAM autoincrement mode. You can enable it using the VDP register flag 0x0F (dc.b $02)

  lea Tiles,a0 
  move.l #$40200000,vdp_control_port; write to VRAM command 
  move.w #6136,d0 ; (767 tiles * 8 rows) counter 
TilesVRAMLoop: 
  move.l (a0)+,vdp_data_port; 
  dbra d0,TilesVRAMLoop 

Tile indexes in Plane A/B

Now we need to fill the screen with tiles by their index. To do this, fill the VRAM at the address Plane A/B, which is set in the VDP registers (0x02, 0x04). More details about the tricky addressing are in the Sega manual, in my example the VRAM address is 0xC000, we will unload the indices there.

Your image will fill the off-screen VRAM space anyway, so after drawing the screen space, your renderer should stop drawing and continue again when the cursor moves to a new line. There are many options for how to implement this, I used the simplest option of counting on two registers of the image width counter, the cursor position counter.

Code example:

  move.w #0,d0     ; column index 
  move.w #1,d1     ; tile index 
  move.l #$40000003,(vdp_control_port) ; initial drawing location 
  move.l #2500,d7     ; how many tiles to draw (entire screen ~2500) 

imageWidth = 31 
screenWidth = 64 

FillBackgroundStep: 
  cmp.w	#imageWidth,d0 
  ble.w	FillBackgroundStepFill 
FillBackgroundStep2: 
  cmp.w	#imageWidth,d0 
  bgt.w	FillBackgroundStepSkip 
FillBackgroundStep3: 
  add #1,d0 
  cmp.w	#screenWidth,d0 
  bge.w	FillBackgroundStepNewRow 
FillBackgroundStep4: 
  dbra d7,FillBackgroundStep    ; loop to next tile 

Stuck: 
  nop 
  jmp Stuck 

FillBackgroundStepNewRow: 
  move.w #0,d0 
  jmp FillBackgroundStep4 
FillBackgroundStepFill: 
  move.w d1,(vdp_data_port)    ; copy the pattern to VPD 
  add #1,d1 
  jmp FillBackgroundStep2 
FillBackgroundStepSkip: 
  move.w #0,(vdp_data_port)    ; copy the pattern to VPD 
  jmp FillBackgroundStep3 

After that, all that remains is to compile the ROM using vasm, run the simulator, and see the picture.

Debugging

Not everything will work out right away, so I want to recommend the following Exodus emulator tools:

1. m68k processor debugger
2. Changing the number of m68k processor cycles (for slow-mo mode in the debugger)
3. Viewers CRAM, VRAM, Plane A/B
4. Carefully read the documentation for m68k, the opcodes used (not everything is as obvious as it seems at first glance)
5. View code/disassembly examples of games on github
6. Implement subroutines of processor exceptions, handle them

Pointers to subroutines of processor exceptions are placed in the ROM header, also on GitHub there is a project with an interactive runtime debugger for Sega, called genesis-debugger.

Use all the tools available, have fun old school coding and may Blast Processing be with you!

Links

https://gitlab.com/demensdeum /segagenesisamples/-/tree/main/6Image/vasm
http://devster.monkeeh.com/sega/imagenesis/
https://github.com/flamewing/genesis-debugger

Sources

https://www.chibiakumas.com/68000/helloworld .php#LessonH5
https://huguesjohnson.com/programming/genesis/tiles-sprites/