VDP1

The VDP1 (Video Display Processor 1) is one of the two graphics processors for the Sega Saturn. Its used for drawing sprites, polygons, and lines. Priority and most other effects are actually handled via the VDP2.

Memory Map

VDP1 VRAM	0x25c00000	512 kilobytes
VDP1 Frame buffer	0x25c80000	256 kilobytes
VDP1 I/O register address	0x25d00000

Abbrev.	Description	Address	Access
TVMR	TV mode selection.	0x25d00000	Write only
FBCR	Controls the frame buffer. bit 0: Unused when bit 1 is set to 0, but when bit 1 is set to 1, it tells the VDP1 to erase(if this bit is 0) or change(if this bit is set to 1) at the next v-sync. bit 1: Toggles between auto erase/change and manual.	0x25d00002	Write only
PTMR	Plot trigger.	0x25d00004	Write only
EWDR	Erase/write color	0x25d00006	Write only
EWLR	Erase/write upper-left position.	0x25d00008	Write only
EWRR	Erase/write lower-right position.	0x25d0000a	Write only
ENDR	Force draw end.	0x25d0000c	Write only
EDSR	Transfer end status	0x25d0000e	Read only
LOPR	Return the last command table processed	0x25d00010	Read only
COPR	Return the current command table processed	0x25d00012	Read only
MODR	Mode status	0x25d00014	Read only

Initializing the VDP1

In order to initialize the VDP1, the VDP2 display must be enabled before anything can be displayed. The code below should initialize the VDP1 properly:

void Init_VDP1(void)
{
    /* Interrupts should be disabled. */
    TVMR = 0;
    PTMR = 2; /* Immediately start drawing. */
    FBCR = 0;
    EWDR = 0; /* Set the frame buffer to black. */

    /* Erase the entire screen using a rectangle ranging from (0,0) to (320,224) */
    EWLR = 0;
    EWRR = ((320 >> 3) << 9) | 224;

    /* Enable interrupts. */
}

Frame buffer

The VDP1 contains two frame buffers, both of which are 2 Mbit each. At any given time one frame buffer is designated as front(and displayed on the screen) and the other is designated as back(gets mapped to the 0x25C80000 area for rendering). Any time anything is drawn from the command tables, it always is drawn to the back frame buffer. When doing automatic and manual erase, it's always the front frame buffer that gets erased. When manually changing the buffers, it appears to take approximately 256 hblank's worth of time before the new back buffer can be accessed, regardless of when it's done.

Command tables

In order to display something onto the frame buffer, a 32 byte command table should be set up and written to the VDP1 VRAM. The command is set up like so:

typedef struct VDP1_CommandTable {
    unsigned short  cmdctrl;
    unsigned short  cmdlink;
    unsigned short  cmdpmod;
    unsigned short  cmdcolr;
    unsigned short  cmdsrca;
    unsigned short  cmdsize;
    signed short    cmdxa;
    signed short    cmdya;
    signed short    cmdxb;
    signed short    cmdyb;
    signed short    cmdxc;
    signed short    cmdyc;
    signed short    cmdxd;
    signed short    cmdyd;
    unsigned short  cmdgrda;
    unsigned short  dummy;
} VDP1_CommandTable;

The VDP1 supports the following command tables:

Command Table	Description	Command ID
Draw normal sprite	Draws a regular unscaled, unrotated sprite	0x0
Draw scaled sprite	Draws a scaled sprite	0x1
Draw distorted sprite	Draws a sprite with any four corner coordinates creating a distorted looking sprite	0x2
Draw Polygon	Draws a regular untextured quad	0x4
Draw Polylines	Draws four lines	0x5
Draw Line	Draws a single line	0x6
User clipping	An adjustable clipping rectangle. You can adjust this multiple times per frame	0x8
System clipping	The main clipping rectangle for all future drawing to the screen.	0x9
Local coordinates	Offsets all future draw commands' coordinates	0xA
Draw End	Causes VDP1 to stop parsing command tables and ends drawing	0xC, 0xD, 0xE, 0xF