Chapter 11 Priority Function

VDP2 compares the priority number values of the sprite and each scroll screen to determine the display priority up to the third from the top. At that time, the priority number of the sprite can be selected for each character from a maximum of 8 values, and the priority number of each scroll screen can be changed for each character or dot by using the special priority function. can.

A line color screen can be inserted in the second position below the specified screen when it has the highest priority.

■11.1 Priority function

The priority (display priority order) of the sprite and each scroll screen is determined by comparing the size of the priority number value of the non-transparent screen dot by dot located at the same TV screen coordinates. The screen priority increases as the priority number increases, and the image made up of dots with the highest priority is called the top image 061 third image. Also, the back screen is displayed where all dots on the sprite and each scroll screen are transparent. The priority function is shown in Figure 11.1.

Figure 11.1 Priority function

●Priority number

The scrolling screens have one 3-bit priority number register per screen. This priority number is normally used for the entire surface, but special priority modes allow you to change the value of the least significant bit on a character-by-character or dot-by-dot basis. sprite priority number

The bar can select one of eight 3-bit priority number registers for each character. For information on selecting a sprite's priority number register, see " Priority number selection " in " 9.2 Priority and color operations ."

The higher the priority number, the higher the screen priority, but if they are equal, the priority is determined according to the order shown in Table 11.1. When the priority number value is 0H, it is considered transparent.

Table 11.1 Priority when priority numbers are equal

priority	usually	External screen data When entering	rotating scroll screen When displaying on two sides	rotating scroll screen External screen on 2 sides When entering
high ↑ │ │ │ │ │ │ │ │ ↓ low	sprite	sprite	sprite	sprite
	RBG0	RBG0	RBG0	RBG0
	NBG0	NBG0	RBG1	RBG1
	NBG1	EXBG	−	EXBG
	NBG2	NBG2	−	−
	NBG3	NBG3	−	−

●Priority number register

The priority number register specifies the priority number. This is a 16-bit write-only register located at addresses 1800F8H to 1800FCH. The value is cleared to 0 after power-on or reset, so be sure to set it.

PRINA 1800F8H

15	14	13	12	11	10	09	08
-	-	-	-	-	N1PRIN2	N1PRIN1	N1PRIN0

07	06	05	04	03	02	01	00
-	-	-	-	-	N0PRIN2	N0PRIN1	N0PRIN0

PRINB 1800FAH

15	14	13	12	11	10	09	08
-	-	-	-	-	N3PRIN2	N3PRIN1	N3PRIN0

07	06	05	04	03	02	01	00
-	-	-	-	-	N2PRIN2	N2PRIN1	N2PRIN0

PRIR 1800FCH

15	14	13	12	11	10	09	08
-	-	-	-	-	-	-	-

07	06	05	04	03	02	01	00
-	-	-	-	-	R0PRIN2	R0PRIN1	R0PRIN0

Priority number bit (for scroll screen) : Priority number bit

(N0PRIN2~N0PRIN0, N1PRIN2~N1PRIN0, N2PRIN2~N2PRIN0, N3PRIN2~N3PRIN0, R0PRIN2~R0PRIN0)
Specify the priority number for each scroll screen.

N0PRIN2～N0PRIN0	1800F8H	bits 2-0	For NBG0 (or for RBG1)
N1PRIN2～N1PRIN0	1800F8H	bits 10-8	For NBG1 (or for EXBG)
N2PRIN2～N2PRIN0	1800FAH	bits 2-0	For NBG2
N3PRIN2～N3PRIN0	1800FAH	bits 10-8	For NBG3
R0PRIN2～R0PRIN0	1800FCH	bits 2-0	For RBG0

The higher the priority number, the higher the display priority. Also, when the priority number value is 0, it is considered transparent and is not displayed.
For information on sprite priority number registers, see " ●Priority number register " in " 9.2 Priority and color operations ".