Sega MegaDrive - Revision history

Leaq: fix typos

2025-12-20T15:32:45Z

fix typos

Leaq: Finish adding introductory info about the hardware and info sizecoders should know about TMSS, interrupts, setting up the console, etc

2025-12-20T15:10:03Z

Finish adding introductory info about the hardware and info sizecoders should know about TMSS, interrupts, setting up the console, etc

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Leaq: Added more correct information and polished introductory explanations a bit. Will add a few chapters and keep refining info later today :)

2025-12-19T14:40:00Z

Added more correct information and polished introductory explanations a bit. Will add a few chapters and keep refining info later today :)

Superogue at 23:28, 18 December 2025

2025-12-18T23:28:36Z

Superogue at 23:25, 18 December 2025

2025-12-18T23:25:28Z

Superogue at 23:15, 18 December 2025

2025-12-18T23:15:48Z

Superogue at 22:51, 18 December 2025

2025-12-18T22:51:08Z

Superogue at 22:46, 18 December 2025

2025-12-18T22:46:19Z

Superogue at 22:44, 18 December 2025

2025-12-18T22:44:46Z

Superogue at 22:42, 18 December 2025

2025-12-18T22:42:25Z

@@ Line 250: / Line 250: @@
 ==== Sprites ====
-Sprites are freely moving objects independent from the 2 background planes. They can be configured to be any size ranging from 1x1 tile to 4x4 tiles (8x8 px tiles, to 8x8 to 32x32 px). Any combination of width and height as long as it's within the 1x1 to 4x4 limit is allowed.
+Sprites are freely moving objects independent from the 2 background planes. They can be configured to be any size ranging from 1x1 tile to 4x4 tiles (8x8 px tiles, so 8x8 to 32x32 px). Any combination of width and height as long as it's within the 1x1 to 4x4 limit is allowed.
 Sprites reside in VRAM as a sort of display list. Every sprite field has x,y position information, tile index for where to find the first tile of the sprite in VRAM, horizontal and vertical flipping, which palette (PAL 0, 1, 2, or 3) to use and finally the "link" value. The link value is used to tell the VDP which sprite in the Sprite Attribute Table (SAT) to process next. The VDP will always start processing the first sprite slot found in the SAT, then it reads the link value for the next sprite and jumps to its slot. Sprite processing ends when the VDP finds a sprite with a link value of 0.
@@ Line 277: / Line 277: @@
 * From the VDP registers
-It's impossible to get an interrupt on startup unless you enable it from both sides, because even if interrupts were to be enabled though VDP registers, the 68k always initializes to IPL level 7 on startup, so all interrupts are ignored.
+It's impossible to get an interrupt on startup unless you enable it from both sides, because even if interrupts were to be enabled through VDP registers, the 68k always initializes to IPL level 7 on startup, so all interrupts are ignored.
 To implement raster effects without interrupts, you can poll the VDP's HVCOUNT port ($C00008). You can get an approximate of what line the VDP is currently rendering there.

@@ Line 1: / Line 1: @@
 == SEGA MegaDrive / Genesis ==
 The SEGA MegaDrive / Genesis is a 16-bit videogame console released by SEGA in 1989.
 It features:
-* 320x240 resolution (PAL)
+* 320x224 screen resolution by default, read "Screen Resolutions" for more information.
-* 4x 16 color palettes
+* 4x 16 color palettes.
-* Tile based display chip with 2 scrolling tilemaps
+* Tile based display chip with 2 scrolling tilemaps.
-* 64 Sprites (up to 4x4tiles in size each)
+* Full plane or 16 pixel column independent vertical scroll.
 * 64K of dedicated Video Memory (VRAM)
-* YM2612 Sound Chip
+* YM2612 FM Sound Chip + SN76489 PSG Sound Chip (Integrated in the VDP)
 === Setting up ===
@@ Line 14: / Line 21: @@
 * Emulator(s): BlastEm ( https://www.retrodev.com/blastem/ )
 * Tool(s): MDTools ( https://github.com/sikthehedgehog/mdtools )
-* Hardware: Sega MegaDrive / Genesis console with a way to access ROM files.
+* Hardware: Sega Mega Drive / Genesis console with an everdrive.
-The real hardware and BlastEm emulator don't require exact ROM headers, but you might want to distribute an emulator friendly version of your production as well for broader emulator compatibility.
+Cartridges have a ROM header that can normally be ditched entirely, since it's only used by emulators to display information or choose correct emulation settings / peripherals. This is not true for all console revisions, though; one part of the header is mandatory for systems equiped with the TradeMark Security System. Read "ROM Header" and "TMSS" for more information.
 === CPU Vector table ===
-This must be the very first thing in the ROM, since the CPU reads it from 0x0000 on bootup.
+This must be the very first thing in the ROM. On power up or reset the CPU initializes the stack pointer held at address $000000 and jumps to the reset vector held at address $000004. The rest of the vectors can be discarded competely, since they are only used for exception handling and video interrupts (more information about interrupts can be found in the "VDP" section).
-The Vector table is a table of addresses that the CPU needs to know about - things like stack address, "main()" function address,
+If a cartridge doesn't want to process a particular interrupt, or wants to ignore it, standard procedure is to set that vector to a dummy handler that simply returns from the exception (RTE). For our purposes, if only the stack pointer and reset vector will be used, we can use the remaining address space used by the vector table as executable code space.
-vertical/horizontal interrupt addresses, etc.
-For any interrupts we don't want to handle, we specify INT_Null (an interrupt at the bottom of the file that doesn't do anything).
+Below are examples that illustrate how the vector table is conventionally set up and how we might want to do it.
 <syntaxhighlight lang="">
-        dc.l   0x00FFE000			; Initial stack pointer value
+	; Example 1: Standard procedure, where all vectors are initialized to something.
 	dc.l   CPU_EntryPoint		; Start of program
-	dc.l   CPU_Exception 		; Bus error
+	dc.l   CPU_Exception		; Bus error
-	dc.l   CPU_Exception 		; Address error
+	dc.l   CPU_Exception		; Address error
-	dc.l   CPU_Exception 		; Illegal instruction
+	dc.l   CPU_Exception		; Illegal instruction
-	dc.l   CPU_Exception 		; Division by zero
+	dc.l   CPU_Exception		; Division by zero
-	dc.l   CPU_Exception 		; CHK CPU_Exception
+	dc.l   CPU_Exception		; CHK CPU_Exception
-	dc.l   CPU_Exception 		; TRAPV CPU_Exception
+	dc.l   CPU_Exception		; TRAPV CPU_Exception
-	dc.l   CPU_Exception 		; Privilege violation
+	dc.l   CPU_Exception		; Privilege violation
 	dc.l   INT_Null				; TRACE exception
 	dc.l   INT_Null				; Line-A emulator
@@ Line 89: / Line 98: @@
 	dc.l   INT_Null				; Unused (reserved)
 	dc.l   INT_Null				; Unused (reserved)
 </syntaxhighlight>
 This vector table is then followed by the ROM header at 0x100

@@ Line 178: / Line 178: @@
 So after the TMSS boot sequence you'll have a black screen but some leftover stuff in VRAM
 You'll have to enable display which you already need to to when setting the VDP registers to your desired configuration And no, the TMSS ROM doesn't use any interrupts
-It doesn't even use DMA, the 68k copies all the graphics and all
+It doesn't even use DMA, the 68k copies all the graphics and all.
 ==== VDP Setup ====

@@ Line 176: / Line 176: @@
 === Video display ===
-To be added.
+So after the TMSS boot sequence you'll have a black screen but some leftover stuff in VRAM
-==== VDP ====
+==== VDP Setup ====
-To be added.
+VDP Access is handled by the following 3 ports and VDP Register constants:
 <syntaxhighlight lang="">
-  to be added
+   lea     (VdpCtrl), a0
 </syntaxhighlight>
 For more information, check out https://plutiedev.com/vdp-setup
 ==== Sprites ====

@@ Line 158: / Line 158: @@
 $FF0000 	$FFFFFF 	68000 RAM
 </syntaxhighlight>
 === Video display ===

← Older revision		Revision as of 22:46, 18 December 2025
Line 100:		Line 100:

	<syntaxhighlight lang="">		<syntaxhighlight lang="">
		+	Start address End address Description 32X SH-2 address
		+	$000000 $3FFFFF Cartridge ROM/RAM $2000000-$23FFFFF
		+	$400000 $7FFFFF Reserved (used by the Mega-CD and 32X)
		+	$800000 $9FFFFF Reserved (used by the 32X)
		+	$840000 $85FFFF 32X frame buffer $4000000-$401FFFF
		+	$860000 $87FFFF 32X frame buffer overwrite mode $4020000-$403FFFF
		+	$880000 $8FFFFF 32X cartridge ROM (first 512kB bank only)
		+	$900000 $9FFFFF 32X cartridge bankswitched ROM (any 512kB bank, controlled by 32X registers)
		+	$A00000 $A0FFFF Z80 memory space
		+	$A10000 $A10001 Version register
		+	$A10002 $A10003 Controller 1 data
		+	$A10004 $A10005 Controller 2 data
		+	$A10006 $A10007 Expansion port data
		+	$A10008 $A10009 Controller 1 control
		+	$A1000A $A1000B Controller 2 control
		+	$A1000C $A1000D Expansion port control
		+	$A1000E $A1000F Controller 1 serial transmit
		+	$A10010 $A10011 Controller 1 serial receive
		+	$A10012 $A10013 Controller 1 serial control
		+	$A10014 $A10015 Controller 2 serial transmit
		+	$A10016 $A10017 Controller 2 serial receive
		+	$A10018 $A10019 Controller 2 serial control
		+	$A1001A $A1001B Expansion port serial transmit
		+	$A1001C $A1001D Expansion port serial receive
		+	$A1001E $A1001F Expansion port serial control
		+	$A10020 $A10FFF Reserved
		+	$A11000 Memory mode register
		+	$A11002 $A110FF Reserved
		+	$A11100 $A11101 Z80 bus request
		+	$A11102 $A111FF Reserved
		+	$A11200 $A11201 Z80 reset
		+	$A11202 $A12FFF Reserved
		+	$A13000 $A130FF TIME registers; used to send signals to the cartridge
		+	$A130EC $A130EF "MARS" when 32X is attached
		+	$A130F1 SRAM access register
		+	$A130F3 Bank register for address $80000-$FFFFF
		+	$A130F5 Bank register for address $100000-$17FFFF
		+	$A130F7 Bank register for address $180000-$1FFFFF
		+	$A130F9 Bank register for address $200000-$27FFFF
		+	$A130FB Bank register for address $280000-$2FFFFF
		+	$A130FD Bank register for address $300000-$37FFFF
		+	$A130FF Bank register for address $380000-$3FFFFF
		+	$A14000 $A14003 TMSS "SEGA"
		+	$A14101 TMSS/cartridge register
		+	$A14102 $BFFFFF Reserved
		+	$C00000 $C00001 VDP data port
		+	$C00002 $C00003 VDP data port (mirror)
		+	$C00004 $C00005 VDP control port
		+	$C00006 $C00007 VDP control port (mirror)
		+	$C00008 $C00009 VDP H/V counter
		+	$C0000A $C0000F VDP H/V counter (mirror)
		+	$C00011 PSG output
		+	$C00013 $C00017 PSG output (mirror)
		+	$C0001C $C0001D Debug register
		+	$C0001E $C0001F Debug register (mirror)
		+	$C00020 $FEFFFF Reserved
		+	$FF0000 $FFFFFF 68000 RAM
	</syntaxhighlight>		</syntaxhighlight>

@@ Line 1: / Line 1: @@
 == SEGA MegaDrive / Genesis ==
-The Sinclair QL (for Quantum Leap) is a personal computer launched by Sinclair Research in 1984, as an upper-end counterpart to the ZX Spectrum.
+The SEGA MegaDrive / Genesis is a 16-bit videogame console released by SEGA in 1989.
-It was aimed at the serious home user and professional and executive users markets from small to medium-sized businesses and higher educational establishments, but failed to achieve commercial success.
-The Sinclair QL uses a Motorola 68008.CPU with 32-bit internal data registers, but an 8-bit external data bus.
+It features:
 === Setting up ===
@@ Line 84: / Line 90: @@
 	dc.l   INT_Null				; Unused (reserved)
 </syntaxhighlight>
 === ROM Header ===
-The ROM Header is a structure that specifies some metadata about the ROM, like its name, author, version number, release date, region,
+The ROM Header is a structure located at 0x100 that specifies some metadata about the ROM, like its name, author, version number, release date, region, and any special peripherals used.
 Note that the Mega Drive console itself doesn't read any of this, it's more a convenience for the programmer, but most emulators will read the title and region. Further header information at: https://plutiedev.com/rom-header