<52/86>VDP2 User's Manual/7.2 Back screen

★ HARDWARE Manual ★ VDP2 User's Manual ★ Chapter 7 Line Screen

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VDP2 User's Manual/Chapter 7 Line Screen

■7.2 Back screen

The back screen (BACK) is a screen that is displayed only when no other screens are displayed, and you can choose to make the entire screen a single color or specify a color for each line. Specify the color data to be used for that line in 5 bits each of RGB.
In non-interlaced and double-dense interlaced modes, you can specify a color for each line, but in single-dense interlaced mode, you can only specify every two lines.
Figure 7.4 shows the configuration of the back screen table for each interlace mode, and Figure 7.5 shows the data configuration on the back screen table.

Figure 7.4 Back screen table configuration

●Non-interlaced and double-dense interlaced modes

	Bit 15 ← Back screen table (VRAM) → 0
+00H	1st line RGB data
+02H	2nd line RGB data
+04H	3rd line RGB data
+06H	4th line RGB data
+08H	5th line RGB data
+0AH	6th line RGB data
: :	: :

[note]

In the case of a single color, the RGB data of the first line is used for the entire line color.
For double-dense interlacing, store line color data for even and odd fields together

●Single dense interlace mode

	Bit 15 ← Back screen table (VRAM) → 0
+00H	RGB data of 1st and 2nd lines
+02H	RGB data for 3rd and 4th lines
+04H	RGB data for 5th and 6th lines
+06H	RGB data for 7th and 8th lines
+08H	RGB data for 9th and 10th lines
+0AH	RGB data for 11th and 12th lines
: :	: :

[note]

In the case of a single color, the RGB data of the 1st and 2nd lines will be used for the entire line color.

Figure 7.5 Bit configuration of back screen table data

15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

BLUE data 5 bits GREEN data 5 bits RED data 5 bits

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
	BLUE data 5 bits	GREEN data 5 bits	RED data 5 bits

part is ignored.

Also, add 0 to the lower 3 bits of each RGB to make it 8 bits.

●Back screen table address register

The back screen table address register specifies the back screen color mode and the start address of the table. This is a 32-bit write-only register located at addresses 1800ACH to 1800AEH. The value is cleared to 0 after power-on or reset, so be sure to set it.

BKTAU 1800AC

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

07
06
05
04
03
02
01
00

-
-
-
-
-
BKTA18
BKTA17
BKTA16

07	06	05	04	03	02	01	00
-	-	-	-	-	BKTA18	BKTA17	BKTA16

BKTAL 1800AE

BKTA15	BKTA14	BKTA13	BKTA12	BKTA11	BKTA10	BKTA9	BKTA8
15	14	13	12	11	10	09	08

07
06
05
04
03
02
01
00

BKTA7
BKTA6
BKTA5
BKTA4
BKTA3
BKTA2
BKTA1
BKTA0

Back screen color mode bit : BACK color mode bit (BKCLMD), bit 15

07	06	05	04	03	02	01	00
BKTA7	BKTA6	BKTA5	BKTA4	BKTA3	BKTA2	BKTA1	BKTA0

Specify the color mode of the back screen.

BKCLMD	back screen color
0	Make it monochrome
1	Specify for each line

Back screen table address bit : BACK color table address bit (BKTA18 to BKTA0)

Specify the start address of the back screen table on VRAM.

BKTA18～BKTA16	1800ACH	bits 2-0
BKTA15～BKTA0	1800AEH	bits 15-0

The actual first VRAM address is calculated using the following formula. If the VRAM capacity is 4Mbits, the most significant bit of the address is ignored.


(Back screen table start address)
             = (back screen table address register value 19 bits) x 2H

If the back screen color mode bit is set to "single color", the color data specified by the back screen table address bit will be used for the entire screen.

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★ HARDWARE Manual ★ VDP2 User's Manual ★ Chapter 7 Line Screen