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Graphic VRAM

The structure of G-VRAM differs depending on the actual screen size and the number of colors. A total of 512 KB of VRAM is divided and allocated.

The pixel representation is a linear type and in any mode one pixel is represented by one word. In the 16/256 color display mode only the lower 4/8 bits are effective.

X = X coordinate in normal mode
Y = Y 〃
P = Plane number
ADR = 65536 color display mode address (X,Y)
BIT = 〃 bit position

Real screen 512×512 pixels、65536 color display

Valid bits Full 16 bits
1 Row 1024 bytes(512 words)
Page 0 $c00000~$c7ffff

Real screen 512×512 pixels、256 color display:

Valid bitsLower 8 bits
1 Row1024 bytes(512 words)
Page 0$c00000~$c7ffff
Page 1$c80000~$cfffff
ADR = $c00000+Y×1024+X×2
BIT = P×8 ~ P×8+7 for 8 bits

Real screen 512×512 pixels、16 colors

Valid bitsLower 4 bits
1 Row1024 bytes(512 words)
Page 0$c00000~$c7ffff
Page 1$c80000~$cfffff
Page 2$d00000~$d7ffff
Page 3$d80000~$dfffff
ADR = $c00000+Y×1024+X×2
BIT = P×4 ~ P×4+3 for 4 bits

Real screen 1024×1024 pixels、16 colors:

Valid bitsLower 4 bits
1 Row2048 bytes(1024 words)
Page 0$c00000~$dfffff
ADR = $c00000+(Y&511)×1024+(X&511)×2
BIT = (Y≧512)×8+(X≧512)×4 ~
      (Y≧512)×8+(X≧512)×4+3 for 4 bits		  


T-VRAM has a real screen size of 1024×1024 pixels fixed to 16 color display and 4 planes for a total of 512KB. Pixel representation is planar type, 1 word is 16 horizontal pixels. Each plane represents 1 bit of the 4-bit palette code.

1 Row128 bytes(64 words)
Page 0$e00000~$e1ffff
Page 1$e20000~$e3ffff
Page 2$e40000~$e5ffff
Page 3$e60000~$e7ffff
x68000/video_ram_basics.txt · Last modified: 2019/08/27 20:45 (external edit)