● Frame buffer

• It consists of two 2Mbit DRAMs, which are divided into a display frame buffer and a drawing frame buffer.

• The function of the display frame buffer and the drawing frame buffer is switched immediately before the screen display period (DISP). After the power is turned on or reset, frame buffer 0 becomes the drawing frame buffer and frame buffer 1 becomes the display frame buffer.

• Read / write access from the syscon (system controller IC) to the frame buffer is made only to the drawing frame buffer. The display framebuffer is a back bank and cannot be accessed.

• Drawing is synchronized with the CPU operating clock. The CPU operating clock is 28MHz, and 1 pixel of data is drawn in synchronization with this.

• Read / write access from the syscon to the framebuffer and drawing access from VDP1 are prioritized.

• Framebuffer access priority is always
  System controller ＞ Drawing

  The order is.

  If you access from the syscon during drawing, drawing will be interrupted and weight will be applied, so avoid it as much as possible.

  To prevent drawing and CPU access at the same time, control the drawing start by using a method such as manual start.

  Access is word access. Byte access is possible when the display is 8bit / pixel (high resolution or rotation 8). Do not access bytes at 16bit / pixel.

  By reading the frame buffer diagonally, the entire frame buffer can be rotated and displayed. You can also zoom in and out on the entire framebuffer plane by skipping or repeating the read address. You can rotate and zoom in and out at the same time.

  The rotation is indicated by VDP2. Refer to the VDP2 manual for details on rotation instructions.

• Bit configuration (color RAM address) when going through color RAM
  

  16 color mode (4bit / pixel)
  MSB LSB
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ 15 │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └─────────────────────────────────┘└─────────┘　
  Color bank character data　
  
  64 color mode (6bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ 15 │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └───────────────────────────┘ └───────────────┘　
  Color bank character data　
  
  128 color mode (7bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ 15 │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └────────────────────────┘└──────────────────┘　
  Color bank character data　
  　
  256 color mode (8bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ 15 │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └─────────────────────┘└─────────────────────┘　
  Color bank character data　
  
  [Note] b15 = 0 when RGB data is mixed, optional when not mixed

• Bit configuration (RGB data) without going through color RAM
  

  MSB LSB
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ * │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  ↑ └────────────┘ └────────────┘└────────────┘　
  "1" BLUE data GREEN data RED data　
  

• Bit configuration (color RAM address) when pixel data is 8 bits

  16 color mode (4bit / pixel)
  MSB LSB
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └─────────┘└─────────┘　
  Color bank character data　
  
  64 color mode (6bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └───┘ └───────────────┘　
  Color bank character data　
  
  128 color mode (7bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └┘└──────────────────┘　
  Color bank character data　
  
  256 color mode (6bit / pixel)
  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ −− │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘
  └─────────────────────┘　
  Character data