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Saturn VDP1 hardware notes (2003-05-17)

From Sega Retro

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This is a copy of an "unofficial" document containing original research, for use as a source on Sega Retro. This page likely exists for historical purposes - the contents should ideally be copy-edited and wikified to make better use of Sega Retro's software.
Original source: https://emu-docs.org/Saturn/vdp1tech.txt

 Saturn VDP1 hardware notes
 by Charles MacDonald
 WWW: http://cgfm2.emuviews.com

 Here are some findings about how the VDP1 operates that I've discovered
 through testing. That said, I can't guarantee that my interpretation of
 the results are correct.

 What's new:

 [05/17/03]
 - Added information about the erase/write feature.
 - Added information about the VDP1 drawing end interrupt.
 [05/12/03]
 - Added information about color calculation modes
 [05/07/03]
 - Added information about undocumented command types
 - Added information about the jump mode settings of CMDCTRL
 - Added details about CMDLINK

 Table of contents:

 Command types
 Jump mode and list processing
 Scaled sprite zoom point settings
 Color calculation modes
 Color modes
 Command table elements
 Erase/write
 Credits
 Disclaimer

 ----------------------------------------------------------------------------
 Command types
 ----------------------------------------------------------------------------

 Bits 3-0 of the CMDCTRL word define the type of command to carry out:

 0000 - Normal sprite                   1000 - Set user clip
 0001 - Scaled sprite                   1001 - Set system clip
 0010 - Distorted sprite                1010 - Set local coordinates
 0011 - Distorted sprite *              1011 - Set user clip *

 0100 - Polygon draw                    1100 - Abort *
 0101 - Polyline draw                   1101 - Abort *
 0110 - Line draw                       1110 - Abort *
 0111 - Polyline draw *                 1111 - Abort *

 Commands marked with '*' are undocumented. Some appear to be aliases of
 other commands, but it's possible they function differently.

 The abort command makes the VDP stop rendering. CEF of EDSR will be set to
 zero because the list end bit is never fetched. LOPR and COPR will point to
 the address of the command table with the abort command. No drawing end
 interrupt is generated.

 ----------------------------------------------------------------------------
 Jump mode and list processing
 ----------------------------------------------------------------------------

 Command tables are stored in VDP1 VRAM, starting from address zero onwards.
 When the plot trigger is set, the VDP1 goes through the list of command
 tables one at a time. It checks bits 15-12 to see how it should handle the
 current command and what to do next.

 If bit 15 is set, the VDP ignores the current command stored in the command
 table and stops rendering. It will set CEF of EDSR to show that the
 end-of-list bit has been fetched and drawing terminated normally, and also
 attempt to trigger the drawing end interrupt which the SCU can mask out.

 However, in the following situations CEF will not be set and no interrupt
 is generated:

 - An Abort command type is found.

 - The VDP1 runs out of time to render before the end of list bit has been
   found. (e.g. when drawing too many objects or a really big sprite)

 - The ENDR register is written to, which stops rendering prematurely.

 If bit 14 is set, the VDP ignores the current command and examines the jump
 mode field (bits 13-12) to determine where the next command table is at. If
 bit 14 is cleared, it processes the command first (e.g. draws a sprite), and
 then looks at the jump mode.

 There are four possible jump mode settings:

 D13 D12    Mode    Description

  0   0     NEXT    The VDP1 will advance to the next command table stored
                    after the current one and process it.

  0   1     JUMP    The VDP1 will process the command table who's address is
                    stored in the CMDLINK word.

  1   0     CALL    The VDP1 pushes the address of the next command table
                    stored after the current one to a single-entry stack, and
                    then processes the command table who's address is stored
                    in the CMDLINK word.

  1   1     RETURN  The VDP pops a previously pushed address off the stack and
                    processes the command table at that address.

 The VDP1 remembers when CALL is used, and will prevent any subsequent CALLs
 from overwriting the originally pushed return address.

 For example, in a sequence of CALL/CALL/CALL/RETURN, the address popped off
 the stack is for the command table after the first CALL, not the third one.

 The VDP1 will treat RETURN as NEXT under the following circumstances:

 - You have not used CALL previously.

 - You have already returned from a CALL and are trying to RETURN again.

 It is possible to create an infinite loop using the jump modes, for instance
 where a command table jumps to itself. If this happens, the VDP will continue
 to process the same command repeatedly until the drawing period is over.
 Because the end-of-list bit was never read, CEF is set to 0.

 ----------------------------------------------------------------------------
 Scaled sprite zoom point settings
 ----------------------------------------------------------------------------

 The 4-bit zoom point field in the CMDCTRL word specifies the origin point
 within a sprite where zooming takes place, and which sets of coordinates
 determine the zoomed width/height.

 A sprite appears as a rectangle with the following points:

    Left    Center   Right
    +------------------+
    |E        F       G|        Top
    |H        I       J|        Center
    |K        L       O|        Bottom
    +------------------+

 The point names are arbitrary, I made them up for illustrative purposes.

 CMDXA/CMDYA pick a screen location to draw a sprite. The zoom point says
 which one of the above nine points CMDXA/CMDYA reference:

 0,1,2,3 - CMDXA/CMDYA reference the top (left/left/center/right) points.
 4,5,6,7 - CMDXA/CMDYA reference the top (left/left/center/right) points.
 8,9,A,B - CMDXA/CMDYA reference the center (left/left/center/right) points.
 C,D,E,F - CMDXA/CMDYA reference the bottom (left/left/center/right) points.

 As you can see bits 0,1 of the zoom point pick the point within a row, and
 bits 3,2 select the row.

 In addition, the zoom point says which coordinate pair (CMDXB/CMDYB or
 CMDXC/CMDYC, or both partially) define the width and height:

 CMDYC is the height for 0,1,2,3
 CMDYB is the height for 4,5,6,7,8,9,A,B,C,D,E,F
 CMDXC is the width for 0,4,8,C
 CMDXB is the width for 1,2,3,5,6,7,9,A,B,D,E,F

 Sega only documents the zoom points which use CMDXB/CMDYB or CMDXC/CMDYC
 exclusively, not the ones that use a mixture of both. Also note that
 some settings (e.g. 0 and 5) function identically but just use a different
 coordinate pair.

 ----------------------------------------------------------------------------
 Color calculation modes
 ----------------------------------------------------------------------------

 When the VDP1 is rendering a sprite or primitive, it can apply effects such
 as Gouraud shading, partial transparency, etc. Sega refers to this as
 'color calculation'. Bits 2-0 of CMDPMOD pick the color calculation type:

 D2 D1 D0   Type
  0  0  0   Replace 
  0  0  1   Shadow
  0  1  0   Half-luminance
  0  1  1   Half-transparency
  1  0  0   Gouraud + Replace
  1  0  1   Invalid
  1  1  0   Gouraud + Half-luminance
  1  1  1   Gouraud + Half-transparency

 Replace

 The object is rendered to the framebuffer normally.

 Shadow

 For each pixel to be rendered, the VDP1 reads the framebuffer to retrieve
 the underlying pixel that will be overwritten. If bit 15 of this word is
 set, it is assumed to be an RGB pixel. The RGB values are divided in half
 and written back to the framebuffer. If bit 15 is cleared, it is assumed
 to be a palette index and is not modified, in this case nothing is written
 back to the framebuffer and processing continues with the next pixel.

 For lines, polylines, and polygons, every pixel of the shape being drawn
 is affected by this mode.

 For all sprite types, only opaque pixels are affected. Transparent pixels
 or pixels not shown due to end codes are ignored as usual, though this is
 subject to the ECD and SPD bits in CMDPMOD.

 Half-luminance

 Each pixel is treated as RGB data. The RGB values are divided in half and
 written to the framebuffer.

 Half-transparency

 For each pixel to be rendered, the VDP1 reads the framebuffer to retrieve
 the underlying pixel that will be overwritten. If bit 15 of this word is
 set, it is assumed to be an RGB pixel. The RGB values of the framebuffer
 pixel and source pixel are divided in half and added together, then written
 back to the framebuffer. If bit 15 is cleared, it is assumed to be a palette
 index. In this case the result is the same as Replace, meaning the
 underlying pixel is overwritten.

 For lines, polylines, and polygons, every pixel of the shape being drawn
 is affected by this mode.

 For all sprite types, only opaque pixels are affected. Transparent pixels
 or pixels not shown due to end codes are ignored as usual, though this is
 subject to the ECD and SPD bits in CMDPMOD.

 Gouraud + Replace

 The object being drawn has Gouraud shading applied, and is written to the
 framebuffer.

 Invalid

 Logically, this mode corresponds to what should be Gouraud + Shadow. It
 reads the underlying framebuffer pixels for each pixel rendered, and takes
 a little extra time just like the Gouraud shading modes do. However, it
 always writes zero to the framebuffer, and does not examine bit 15 of the
 framebuffer data.

 For lines, polylines, and polygons, every pixel of the shape being drawn
 is affected by this mode.

 For all sprite types, only opaque pixels are affected. Transparent pixels
 or pixels not shown due to end codes are ignored as usual, though this is
 subject to the ECD and SPD bits in CMDPMOD.

 Gouraud + Half-luminance

 The object being drawn has Gouraud shading applied. The resulting pixel has
 it's RGB values divided in half, and is written to the framebuffer.

 Gouraud + Half-transparency

 For each pixel to be rendered, the VDP1 reads the framebuffer to retrieve
 the underlying pixel that will be overwritten. If bit 15 of this word is
 set, it is assumed to be an RGB pixel. The RGB values of the framebuffer
 pixel and source pixel are divided in half and added together, Gouraud
 shading is applied, and the result is then written back to the framebuffer.
 If bit 15 is cleared, it is assumed to be a palette index. In this case,
 the result is the same as Gouraud + Replace, meaning the underlying pixel
 is overwritten.

 For lines, polylines, and polygons, every pixel of the shape being drawn
 is affected by this mode.

 For all sprite types, only opaque pixels are affected. Transparent pixels
 or pixels not shown due to end codes are ignored as usual, though this is
 subject to the ECD and SPD bits in CMDPMOD.

 Timing

 The Replace, Gouraud, and Half-luminance modes all take the same amount
 of time. Gouraud takes slightly more, but only by a few cycles - this is
 most likely due to the shading table being read, and not from any
 per-pixel processing which would have a larger impact on timing.

 The Shadow, Invalid, and Half-transparency modes all take the same amount
 of time. They apparently read framebuffer data even for pixels where the
 source data is transparent. The only way you can cut down on excessive
 framebuffer reads when there are large transparent areas is to use end codes.

 The mesh feature does not affect timing at all. You'd think rendering would
 be faster due to every other pixel being skipped, but there is no change
 in how fast an object is rendered with mesh processing turned on or off.

 Other details

 The Gouraud, half-luminance, and half-transparency modes are intended to be
 used where the source data is in RGB format, meaning lines, polylines, or
 polygons that specify an RGB color in the CMDCOLR word, or sprites using
 color mode 1 with RGB codes in the lookup table, or color mode 5.

 If you use palette data instead, the palette index will be come mangled as
 the VDP1 will still treat the data as RGB values. But this can be useful
 in a limited way if you set up the palette to map meaningful colors to the
 resulting pixel data. Think of the VDP1 has being 'hard-wired' in performing
 RGB processing for some of the color calculation modes.

 For Shadow and Half-transparent processing, the VDP will re-draw pixels
 to fill in gaps when the vertices of a sprite or primitive are positioned
 in certain ways. This causes the shadow or transparency effect to be
 applied twice.

 It seems that this would significantly reduce the usefulness of shadow or
 transparency processing on polygons/distorted sprites, and is probably why
 so few Saturn games use transparency, instead faking the effect by using
 the mesh feature.

 ----------------------------------------------------------------------------
 Color modes
 ----------------------------------------------------------------------------

 The VDP1 supports several different types of texture map data for sprites.
 This is mainly to give flexibility in palette selection and the amount of
 VRAM used for texture map storage.

 Note that the color mode has no effect on lines/polylines/polygons. Sega
 advises setting the color mode to zero.

 There are eight color modes, but only six are of any real use:

 0 - 4-bit texture, 16 colors, color bank
 1 - 4-bit texture, 16 colors, lookup table
 2 - 8-bit texture, 64 colors, color bank
 3 - 8-bit texture, 128 colors, color bank
 4 - 8-bit texture, 256 colors, color bank
 5 - 16-bit texture, 32K colors, RGB
 6 - Invalid
 7 - Invalid

 The framebuffer is scanned by the VDP2, which controls how the contents
 are displayed. This brings up two important issues:

 1. The VDP2 seems to always handle a framebuffer word of $0000 as being
    transparent. This means that the underlying VDP2 background layers are
    visible through zero-value pixels.

    For example, if you draw a scaled sprite with the transparent pixel
    disable bit set, and it uses palette 0, then zero value pixels will
    not show the color from entry 0 of palette 0. Instead, you'll get
    whatever the VDP2 has been set up to show, such as the back color
    screen.

    I haven't checked, but there may be a way to disable this feature.
    The VDP2 layers have a transparent pixel disable bit, and perhaps
    the sprite layer does too. (this would be handled on the VDP2 side)

 2. The VDP2 controls how the framebuffer words should be used. Normally
    it looks at bit 15 to determine how the data is formatted. If bit 15
    is cleared, the remaining bits are an index into CRAM to show a color.
    If bit 15 is set, the remaining bits are a 5:5:5 BGR color:

    MSB          LSB
    0????ccccccccccc    Palette entry (?= ignored, c= CRAM index)
    1bbbbbgggggrrrrr    RGB color (r,g,b = Red, Green, Blue components)

    Note that in a CRAM color mode with less colors (modes 1,2,3) that the
    index into CRAM would only be 10 bits and not 11. The high-order bits
    are simply ignored by the VDP2 as shown.

    The VDP can override this feature through the SPCLMD bit of SPCTL, which
    can force it to ignore bit 15 and treat all pixels as being palette
    indicies instead of RGB values.

    Having pixel data formatted as an RGB value is really only intended for
    modes 1 and 5 (color bank and RGB). If you set bit 15 of CMDCOLR for
    the other modes (0,2,3,4) then the texture data supplies values for
    the red and green elements depending on the pixel size, and the rest
    of the bits come from CMDCOLR. For example:

    MSB          LSB
    1bbbbbgggggrrrrr    Format of RGB word
    ccccccccccccdddd    Mode 0 (c= CMDCOLR bits, d= Texture map bits)
    ccccccccccdddddd    Mode 2 (c= CMDCOLR bits, d= Texture map bits)
    cccccccccddddddd    Mode 3 (c= CMDCOLR bits, d= Texture map bits)
    ccccccccdddddddd    Mode 4 (c= CMDCOLR bits, d= Texture map bits)

    So you can draw sprites like this, but it's not very useful due to
    the limited way in which colors are selected.

 Here is a list of the various color modes:

 0 - 4-bit texture, 16 colors, color bank

     A 16-bit value is made from bits 15-4 of the CMDCOLR word and bits 3-0
     of the texture map data. Note that bits 3-0 of CMDCOLR are replaced
     with the texture map data.

 1 - 4-bit texture, 16 colors, lookup table

     The texture map data is used as an index into a 16-word table stored
     in VRAM. You can mix palette indicies and RGB codes within the same
     table.

     The CMDCOLR word holds the address of the lookup table / 8.
     The VDP1 will ignore bits 1,0 of CMDCOLR, so the table always has to
     start on a 32-byte boundary.

 2 - 8-bit texture, 64 colors, color bank

     A 16-bit value is made from bits 15-6 of the CMDCOLR word and bits 5-0
     of the texture map data. Note that bits 5-0 of CMDCOLR are replaced
     with the texture map data.

 3 - 8-bit texture, 128 colors, color bank

     A 16-bit value is made from bits 15-7 of the CMDCOLR word and bits 6-0
     of the texture map data. Note that bits 6-0 of CMDCOLR are replaced
     with the texture map data.

 4 - 8-bit texture, 256 colors, color bank

     A 16-bit value is made from bits 15-8 of the CMDCOLR word and bits 7-0
     of the texture map data. Note that bits 7-0 of CMDCOLR are replaced
     with the texture map data.

     %00000000 signifies a transparent pixel if enabled.
     $11111111 signifies an end code if enabled.

 5 - 16-bit texture, 32K colors, RGB

     The texture map data is drawn directly to the framebuffer.

     Trasparent pixel disable turned off =

     $0000-$7FFE : VDP doesn't draw anything to the framebuffer (transparent)
           $7FFF : As above, and functions as an end code if the end code
                   disable bit is turned off.
     $8000-$FFFF : RGB values from black to white.

     Trasparent pixel disable turned on =

     $0000-$7FFE : Value is written to the framebuffer (e.g. palette index)
           $7FFF : Same as above, and functions as an end code if the
                   end code disable bit is turned off.
     $8000-$FFFF : RGB values from black to white.

 6 - Invalid
 7 - Invalid

     Both of these color modes function identically. Word zero from VRAM
     is written to each pixel location in the framebuffer. The CMDCOLR and
     CMDSRCA have no effect on the color or source address, which is always
     fixed at VRAM address zero.

     I do not know if the VDP will check the value written to observe
     transparency or end code detection. I haven't attempted to use it as
     an RGB code as setting bit 15 would prevent the object from being
     drawn. I've only tried using it as a CRAM index, which seems to work
     as expected.

 ----------------------------------------------------------------------------
 Command table elements
 ----------------------------------------------------------------------------

 CMDLINK
 15:30, 20 October 2016 (CDT)~~
 When using the JUMP or CALL jump modes, CMDLINK holds the address of the
 next command table to process / 8. The VDP1 ignores bits 1,0 of CMDLINK
 which are assumed to be zero, meaning command tables must start on 32-byte
 boundaries.

 CMDSRCA
 15:30, 20 October 2016 (CDT)~~
 The VDP1 manual advises that texture map data in VRAM should always start
 on 32-byte boundaries, and therefore bits 1,0 of CMDSRCA should be set to
 zero.

 However, it doesn't appear this this is a real restriction of the hardware.
 CMDSRCA specifies the start address of the texture map in units of 8 bytes,
 and showing sprites at offsets of 8, 16, or 24 pixels doesn't result in any
 unusual behavior.

 CMDSIZE
 15:30, 20 October 2016 (CDT)~~
 If the width in CMDSIZE is set to zero, the first pixel in the texture map
 is shown for every pixel of every line.

 If the height in CMDSIZE is set to zero, it works the same as a height of
 one. (e.g. the first line of the texture map is shown for all lines of
 the sprite)

 The VDP1 manual says both settings of zero are prohibited.

 Coordinates (CMDXA through CMDYD)
 15:30, 20 October 2016 (CDT)15:30, 20 October 2016 (CDT)15:30, 20 October 2016 (CDT)15:30, 20 October 2016 (CDT)15:30, 20 October 2016 (CDT)15:30, 20 October 2016 (CDT)[[User:Over9000|Over9000]] ([[User talk:Over9000|talk]])
 Coordinate values have the following layout:

 MSB          LSB
 ???svnnnnnnnnnnn

 ? = Value is ignored
 s = Sign bit
 v = Distorted coordinates

 This divides up the available ranges like so:

 $0000-$07FF : Positive coordinates
 $0800-$0FFF : Positive coordinates (distorted)
 $1000-$17FF : Negative coordinates (distorted)
 $1800-$1FFF : Negative coordinates

 I tested this by controlling the lower-left point of a scaled sprite using
 a zoom point of zero. Moving this point around so that the X/Y values
 were within $07FF scales the sprite positively. Moving the point around so
 that the X/Y values were within $1800-$1FFF scales the sprite negatively.

 When ranges of $0800-0FFF or $1000-$17FF are used, the VDP1 doesn't read
 the texture map correctly and heavily distorts the image. Too many pixels
 are skipped which shrinks parts (skipping rows/columns) and in other places
 no pixels are read so the same color is copied across multiple rows/columns.

 I think normally people would use values of $0000-$07FF and $F800-$FFFF
 when programming (e.g. -2047 to 2048, the regular range of positive/negative
 values) so the invalid ranges would be hard to reach. And the sprite wasn't
 even visible until getting close to the edge of the ranges, such as
 $0E00-$0FFF and $1000-$11FF, so it would probably appear invisible most of
 the time.

 ----------------------------------------------------------------------------
 Erase/write
 ----------------------------------------------------------------------------

 The VDP1 has a high-speed memory fill feature used to clear out the
 framebuffer prior to rendering called erase/write.

 I've only tested the default framebuffer configuration (512x256, 16-bit, no
 rotation) with a non-interlaced display and can't comment on how erase/write
 works with other settings.

 EWLR and EWRR define the upper left and lower right points, respectively,
 of a rectangle within the framebuffer to be erased. They both have the
 following layout:

 MSB          LSB
 xxxxxxxyyyyyyyyy : EWLR and EWRR

 x = X coordinate ($00-$7F)
 y = Y coordinate ($000-$1FF)

 EWDR holds a 16-bit value which is written to the framebuffer. This can
 be a palette index or RGB code. Normally this would be zero, but you can
 use it to fill the framebuffer with a particular color.

 The X coordinate is in units of eight pixels, while the Y coordinate is
 in units of single lines.

 If both X coordinates are the same, the VDP1 will fill a single pixel for
 each line within the Y coordinates.

 If both Y coordinates are the same, the VDP1 will flil a single line for
 each pixel within the X coordinates.

 When the EWLR X coordinate is bigger then or equal to the EWRR X coordinate,
 then a single dot is written at the X position indicated by EWLR.

 When the EWLR Y coordinate is bigger than or equal to the EWRR Y coordinate,
 then a single line is written at the Y position indicated by EWLR.

 Even though the framebuffer is 512x256 in it's default configuration, the
 VDP1 limits the area you can erase, regardless of the rectangle size:

 - For a 224 or 240 line display, you can clear 224 or 240 lines.

 - For a 320 or 352 pixel display, you can clear 400 or 428 pixels.
   Note that 428 is not a multiple of eight, only the first four pixels
   of the last eight pixel group have the erase/write data written to them.

 For example, a 352x240 display would have a 428x240 pixel area erased if
 EWLR = $0000 and EWRR = $FFFF (which logically corresponds to an erase
 area from 0,0 to 1023,511).

 It also seems that when the erase rectangle is bigger than the clipping
 rectangle the VDP1 defines, it doesn't do any extra work and so the
 erase/write sequence takes the same amount of time.

 ----------------------------------------------------------------------------
 Credits
 ----------------------------------------------------------------------------

 - Chris MacDonald for testing my software
 - Bart Trzynadlowski for the startup script and VDP2 setup code
 - Stefano for help getting the sprites working and testing ideas.

 ----------------------------------------------------------------------------
 Disclaimer
 ----------------------------------------------------------------------------

 If you use any information from this document, please credit me
 (Charles MacDonald) and optionally provide a link to my webpage
 (http://cgfm2.emuviews.com/) so interested parties can access it.

 The credit text should be present in the accompanying documentation of
 whatever project which used the information, or even in the program
 itself (e.g. an about box)

 Regarding distribution, you cannot put this document on another
 website, nor link directly to it.