SEGA SATURN DEVELOPER'S MANUAL

Pseudo drawing continuation

If the amount of data to be drawn is large and drawing does not end within one frame, drawing is forcibly terminated halfway and pseudo drawing continuation is used. Draw in two or more parts.
Pseudo continuation is done by the following procedure. Forcibly terminate drawing, rewrite the top command to jump to the processing interruption table address at that time, and set the plot trigger mode to drawing start (PTM = 01B) at the time of writing.
Draw to the extent that can be drawn within the time in one frame, and at that point the drawing is forcibly terminated (suspended). Next, the jump destination in the first command table on the VRAM from the CPU is rewritten to the command table address at the time of forced termination, and drawing starts immediately (PTM = 01B) in the plot trigger mode. In this way, pseudo-drawing can be continued. However, drawing cannot be continued if the command table address at the time of forced termination is in a subroutine. In this case, the drawing time required to transfer each pixel data to the frame buffer is set to be shorter than the time until the frame end.
If the translucent color operation was performed when drawing was forcibly terminated, the semi-transparent process is performed twice (double color operation is performed) when the drawing connection is made by the above method. Can occur. It is necessary to forcibly terminate drawing when translucent color calculation is not performed.