Before we start, it is very recommended that you have a good knowledge of how the MegaDrive works.

MegaCD Terms

If you're not sure on anything from this thread, you can check out the MegaCD manual here, or ask questions in this thread.

We are going to begin with defining some terms commonly used when programming for the MegaCD.

Main CPU - This refers to the CPU from the MegaDrive side. It's mainly used to handle the game's code. Runs at 7.67 MHz.

Sub CPU - This refers to the CPU from the MegaCD side. It's mainly used to handle the CD functions and can communicate with the BIOS. Runs as 12.5 MHz.

ASIC - Also referred to as the graphics numeric processing chip, this chip can be used to rotate and scale art data, or to handle art data as pixels, rather than tiles. Can only be used by the Sub CPU.

Backup RAM - Commonly referred to as BRAM, this area of 64 kilobits ($1F40 bytes) is backed up by a battery built into the MegaCD to be used to store data. This can be used to save games and can be used by multiple games. Can only be used by the Sub CPU.

Word RAM - This is an area of RAM that is both shared by the Main and Sub CPUs. This area has 2 modes, 1M/1M and 2M mode. The M stands for Megabit, which is $20000 bytes in size. In 2M mode, all 2 megabits are used at once, and can only be accessed by one CPU at a time. In 1M/1M mode, $20000 bytes of Word RAM is split between the CPUs, but only one CPU can access a section at a time. Word RAM can be used for storing file data, or for just acting as a buffer for the Main CPU and Sub CPU to communicate each other with. The ASIC uses Word RAM as a buffer for its image functions.

Work RAM - This is the regular RAM from the MegaDrive. Can only be used by the Main CPU.

Program RAM - Commonly referred to as PRG-RAM or PRAM, this refers to $80000 bytes of RAM that is mainly used by the Sub CPU (can be accessed by the MainCPU when the Sub CPU is halted), that is mainly used for storing larger functions that can't fit into the little space in the Sub CPU program area (found at $1000-$7FFF). This can also be used as a buffer, like in Work RAM.

Boot ROM - This holds the basic operating program for the MegaCD (A.K.A. the BIOS). It is activated upon startup. This run by the Main CPU, and decompressed into PRG-RAM for the Sub CPU to run.

Access - This is the operating in which data is read from a storage device, like a CD-ROM drive.

CD-ROM - Also known as a compact disc (read-only memory), this is basically the disc that holds all of the game data.

CDDA - Also known as a digital compact audio disc, this type of CD must use a frequency of 44.1 kHz with 16-bit quantization and 2 channels (stereo). A 12 centimeter CD-ROM with these standards can hold up to 75 minutes of music, just like CDDA. MegaCD discs can hold up to a maximum of 80 minutes, because of their own standards.

CD-G - The format of this CD type is almost identical to CDDA, except that the graphical data is stored in the tiny gaps between the tracks of the audio data. This type of CD was mainly used for CD karaoke machines.

CDD - Also known as the compact disc driver, this part of the hardware in which music and data are reproduced.

CDC - Also known as the CD data controller, this uses CD-ROM standards to correct errors in the CD data sent from the CDD.

Host data - This refers to the data corrected by the CDC.

Subcode - Refers to the data consisting of items such as CD timing information, CD-G, graphics data, etc.

Sampling - Refers to the process in which an original sound is divided into tiny slices and then the value for each slice is converted into PCM. The frequency in Hz indicates how small these slices are.

Stamp - This is the smallest unit of art data that can be processed by the ASIC. It can be 16x16 pixels, or 32x32 pixels in size.

Stamp map - A map consisting of stamp IDs arranged according to the screen structure.

Trace vector - This area of data is used to help the ASIC to convert the art data.

Image buffer - This is area in WordRAM in which the ASIC stores the converted art data. This data can be stored into VRAM.

Data cache - When a disc drive reads data, it's stored in a data dache, where hardware-based error detection and correction takes place. This is a function of the CD driver.

Fader - This performs the CD music volume adjustment. Fade ins and fade outs can be accomplished with this.

Communication - This is the process of either CPU transmitting data over for the other CPU to read.

LED - On a Model 1 MegaCD, there are 2 LED lights that display the state of the CD drive. This can be activated on an emulator.

In the next tutorial, I will explain the data mapping for the Main CPU and Sub CPU.