• Normally, two frame buffers use one for the front and the other for the back. Display the front side and draw on the back side. After one frame switching time has elapsed, the front and back sides are toggled. The front side that was being displayed becomes the back side to be drawn, and the back side that was being drawn becomes the front side to be displayed.
  
• The frame buffer can be accessed only on the drawing side. It cannot be displayed.
  
• There is a limit to the number of characters that can be drawn in one frame, so if you want to draw many characters, set to manual mode and control erase and change from the CPU.
  
• When any data is written from the CPU to the FBCR, the values ​​of FCM and FCT are updated inside VDP1 at the time of field change.
  
• In manual mode, the FCT bit setting is valid only for the next frame. The contents of FCT are updated internally when a frame change occurs.
  
• Set FCM and FCT immediately after the V blank OUT interrupt. Access is prohibited from the first H blank IN interrupt after the V blank OUT interrupt to the next H blank IN interrupt.

 switching mode  

 1-cycle mode  

 Setting prohibited  

 Manual mode (erase)  

 Manual mode (change)  

 Setting prohibited  

 Setting prohibited  

 Setting prohibited  

 Manual mode 
 (Erase & Change)  

VBE	FCM	FCT	switching time
0	0	0	changes every 1/60 seconds
0	0	1	−
0	1	0	Erase next field
0	1	1	Change in next field
1	0	0	−
1	0	1	−
1	1	0	−
1	1	1	Erase with V blank, Change in next field