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★ HARDWARE Manual ★ VDP2 User's Manual ★ Chapter 11 Priority Function
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VDP2 User's Manual/Chapter 11 Priority Function

■11.2 Special priority function

The special priority function is a function that changes the lowest bit of the 3-bit priority number corresponding to each scroll screen for each character or dot. By using this function and changing the priority of only certain parts of the scroll screen, you can make a single scroll screen appear to have multiple pages. Note that the special priority function changes only the lowest bit of the priority number; the register value is used as is for the upper two bits. The special priority function has three modes:

  1. The least significant bit of the priority number is specified for each screen.
  2. The least significant bit of the priority number is specified for each character.
  3. The least significant bit of the priority number is specified for each dot.

When specifying the least significant bit of the priority number for each screen, the value of the priority number register corresponding to each scroll screen is used as is.

When specified for each character, the value of the special priority bit in the pattern name data is used as the least significant bit of the priority number. For special priority bits, see " 4.6 Pattern Name Table (Page) ".

When specifying for each dot, in character patterns for which the special priority bit in the pattern name data is specified as 1, the lowest bit of the priority number is set to 1 only for dots that match the dot color code specified in the special function code. otherwise fix the least significant bit to 0. Do not set this mode when the scroll screen color format is RGB format. For dot color codes, see 10.3 Special Function Codes .

When the value of the 3-bit priority number obtained by the special priority function is 0H, that screen, character, or dot is considered transparent.
Table 11.2 shows the special priority functions depending on the mode.

Table 11.2 Special priority functions by mode
 special priority
 mode
 special priority
 Specified content
 color format
 Lowest bit value of priority number
mode 0
 Specify for each screen
 pallet format
 or RGB format
 The least significant bit value of the priority number register
mode 1
 By character
 specified in
 pallet format
 or RGB format
 Special priority bit value in pattern name data
mode 2
 Specify for each dot
 pallet format
 When the special priority bit in the pattern name data = 1,
 Dots that match the dot color code specified in the special function code
 becomes 1, otherwise becomes 0
RGB format Setting prohibited

If mode 1 or 2 is specified on a scroll screen where the display format is bitmap format, the special priority bits in the bitmap palette number register are used instead of the special priority bits in the pattern name data.

●Special priority mode register

The special priority mode register specifies the mode of the special priority function for each scroll screen. This is a 16-bit write-only register located at address 1800EA.
The value is cleared to 0 after power-on or reset, so be sure to set it.

SFPRMD 1800EAH
   15   
   14   
   13   
   12   
   11   
   10   
   09   
   08   
 -
 -
 -
 -
 -
 -
 R0SPRM1
 R0SPRM0

   07   
   06   
   05   
   04   
   03   
   02   
   01   
   00   
 N3SPRM1
 N3SPRM0
 N2SPRM1
 N2SPRM0
 N1SPRM1
 N1SPRM0
 N0SPRM1
 N0SPRM0

Special priority mode bit : Special priority mode bit
(N0SPRM1, N0SPRM0, N1SPRM1, N1SPRM0, N2SPRM1, N2SPRM0, N3SPRM1, N3SPRM0, R0SPRM1, R0SPRM0)
Specifies the mode of special priority function for each scroll screen.

N0SPRM1,N0SPRM0 1800EAH bit 1,0 For NBG0 (or for RBG1)
N1SPRM1,N1SPRM0 1800EAH bit 3,2 For NBG1 (or for EXBG)
N2SPRM1,N2SPRM0 1800EAH bit 5,4 For NBG2
N3SPRM1,N3SPRM0 1800EAH bit 7,6 For NBG3
R0SPRM1,R0SPRM0 1800EAH bit 9,8 For RBG0

xxSPRM1 xxSPRM0 mode process
 0
 0
 mode 0 The least significant bit of the priority number is specified for each screen.
 0
 1
 mode 1 The least significant bit of the priority number is specified for each character.
 1
 0
 mode 2 Specify the least significant bit of the priority number for each dot.
 1
 1
 − Setting prohibited

[Note] xx in the bit name is N0, N1, N2, N3, or R0.

If the color format of the scroll screen is RGB mode, do not set mode 2.
Do not set any mode other than 0 for EXBG.
A character or dot with a priority number of 0 is considered transparent.

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★ HARDWARE Manual ★ VDP2 User's Manual ★ Chapter 11 Priority Function
Copyright SEGA ENTERPRISES, LTD., 1997