Format of S3 Streams Processor memory-mapped registers

Offset  Size    Description     (Table M097)
8180h  DWORD   primary stream control (write-only)
       bits 30-28: ??? (set to 7 for 32bpp, 5 for 5-6-5, 3 for 5-5-5,
           0 for 8bpp)

       Note: the primary stream is the output from the display RAM
8184h  DWORD   chroma key control (write-only)
       bits 31-29: ???
       bit 28: =1 strict chroma-key
           =0 relaxed chroma-key
       bit 27: ???
       bits 26-24: ???
       bits 23-0: chroma-key color value

       Note: Win95 drivers write this register twice
8188h  DWORD   unused??? (high word seems to echo 8184h, low word 8180h)
818Ch  DWORD   unused??? (high word seems to echo 8184h, low word 8180h)
8190h  DWORD   secondary stream control (write-only)
       bits 11-0: ??? (set to 2*inwidth-outwidth-1)
       bits 27-12: ???
       bits 29-28: scaling factor
           (ratio between inwidth & outwidth:
             03 = greater than 4
             02 = 2 .. 4
             01 = 1 .. 2
             00 = other )
       bit 30: ???
       bit 31: ???

       Note: the secondary stream is typically live video
8194h  DWORD   chroma key upper bound (write-only)
       (actually, only bits 28-31 are write-only)
8198h  DWORD   secondary stream stretch (write-only)
       (actually, only bits 27-31 are write-only)
       bits 26-16: input width - output width
       bit 11: ???
       bits 10-0: input width - 1
       (the above two fields together determine the actual stretch/
         shrink factor)
819Ch  DWORD   ???
81A0h  DWORD   blend control (write-only) (see #M104)
81A4h 3 DWORDs unused??? (reads as FFFFFFFFh)
81B0h 4 DWORDs ???
81C0h  DWORD   primary frame buffer address 0
81C4h  DWORD   primary frame buffer address 1
81C8h  DWORD   primary stream stride (bits 11-0 only)
81CCh  DWORD   double buffer control (bit flags)
       bit 0: ??? (if set, display messed up)
       bit 1: secondary stream buffer select
           =0 use frame buffer address 0 (81D0h)
           =1 use frame buffer address 1 (81D4h)
       bit 2: primary stream buffer select???
       bit 3: ???
       bit 4: ??? (if set, causes direct writes to video memory)
       bits 6-5: ???
       bits 31-7: ???
81D0h  DWORD   secondary frame buffer address 0
81D4h  DWORD   secondary frame buffer address 1
81D8h  DWORD   secondary stream stride (bits 11-0 only)
81DCh  DWORD   opaque overlay control (some bits seem to be read-only)
       bit 31: ???
81E0h  DWORD   K1 -- vertical stretch (lines in)
       (bits 10-0 only)  set to one less than # lines in
81E4h  DWORD   K2 -- vertical stretch (stretch factor)
       (bits 10-0 only)  set to (#lines_in - #lines_out)
81E8h  DWORD   "DDA_vert" (bits 11-0 only) (lines out)
       set to 1-(#lines_out)
81ECh  DWORD   streams FIFO
       bit 2: ???
       bit 3: ???
       bit 6: ???
       bit 7: ???
       bit 12: ???  (if clear, DOS-box scrolls cause snow)
       bit 13: ???
       bit 14: ???
       bits 18-15: ???
       bit 19: ??? (set by Win95 driver when using ISA bus)
       bits 22-20: ???
       bits 31-23: read-only 0000
81F0h  DWORD   primary start coordinate
       bits 26-16: X (column)
       bits 10-0: Y (row)
       other bits appear to be read-only
81F4h  DWORD   primary window size
       bits 26-16: width
       bits 10-0: height
       other bits appear to be read-only
81F8h  DWORD   secondary start coordinate
       bits 26-16: X (column)
       bits 10-0: Y (row)
       other bits appear to be read-only
81FCh  DWORD   secondary window size
       bits 26-16: width
       bits 10-0: height
       other bits appear to be read-only

See Also: #M095,#M100,#M101
Bitfields for S3 Streams Processor blend control:
Bit(s) Description (Table M104)
 31-27 unused???
 26-24 blend type
   000 show secondary stream (video) unconditionally
   001 disable secondary stream
   010 blend pri/sec. streams (secondary at full intensity)
   011 blend pri/sec. streams
   100 blank display
   101 show secondary stream only where chroma-key color present
   110 show secondary stream (video) unconditionally
   111 blank display
 23-13 unused???
 12-8  primary stream intensity (04h-1Fh)
   (values of 00h-03h force secondary stream to black)
 4-0   secondary stream intensity (00h-1Fh)
   (ignored for blend type 010)

See Also: #M097
--------V-MA0008200--------------------------
MEM A000h:8200h - S3 - MEMORY-MAPPED MEMORY-PORT CONTROL REGISTERS
Size:  40 BYTEs
Access:    Write-Only ???

Note:  the S3 graphics processor registers can be mapped at either
     linear 000A0000h or at offset 16M from the start of the linear
     frame buffer