●Frame buffer

• It consists of two 2Mbit DRAMs and is divided into two sides: a display frame buffer and a drawing frame buffer.

• The display frame buffer and drawing frame buffer functions are switched immediately before the screen display period (DISP). After power-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 system controller (system controller IC) to the frame buffer is performed only to the drawing frame buffer. The display frame buffer is in the back bank and cannot be accessed.

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

• Read/write access from the system controller to the frame buffer and drawing access from VDP1 are prioritized.

• Frame buffer access priority is always
  system controller＞ drawing

  This is the order.

  If you access from the system controller while drawing, the drawing will be interrupted and the process will take a long time, so please avoid this as much as possible.

  In order to prevent drawing and CPU access from occurring at the same time, use a method such as manual start of drawing to control it.

  Word access is possible. Byte access is possible when the display is 8bit/pixel (high resolution or rotation 8). Do not perform byte access when using 16bit/pixel.

  By reading the frame buffer diagonally, the entire frame buffer can be rotated and displayed. Additionally, the entire frame buffer plane can be expanded or contracted by skipping or repeating the read addresses. Rotation, enlargement, and reduction can be performed at the same time.

  Rotation is instructed using VDP2. Please refer to the VDP2 manual for detailed rotation instructions.

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

  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 when not going through color RAM (RGB data)
  

  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 when pixel data is 8 bits (color RAM address)

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