<15/54> VDP1 User's Manual / 4.4 Erase Light

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VDP1 User's Manual / Chapter 4 System Registers

■ 4.4 Erase Light

Before starting drawing, the sprite IC erases the contents of the frame buffer. This erasure is called erase light, and the fill area to be erased and the fill data to be written to that area are specified. There are three registers related to erase light: the erase light data register , the erase light upper left coordinate register , and the erase light lower right coordinate register.

■ Erase light data register

The erase write data register (EWDR) defines the fill data for erase write. This is a write-only 16-bit register located at address 100006H. After turning on the power or resetting, the value will be undefined, so be sure to set the fill data.

At 16bit / pixel

EWDR 100006H (W) bit15 bit14 bit13 bit12 bit11 bit10 bit9 bit8 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 Erase light data

At 8bit / pixel

EWDR 100006H (W) bit15 bit14 bit13 bit12 bit11 bit10 bit9 bit8 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 Even X coordinate erase light data Odd X coordinate erase light data

Erase write data: erase write data, bit15 ~ 0

EWDR 100006H (W)	bit15	bit14	bit13	bit12	bit11	bit10	bit9	bit8	bit7	bit6	bit5	bit4	bit3	bit2	bit1	bit0
Erase light data

EWDR 100006H (W)	bit15	bit14	bit13	bit12	bit11	bit10	bit9	bit8	bit7	bit6	bit5	bit4	bit3	bit2	bit1	bit0
Even X coordinate erase light data	Odd X coordinate erase light data

When drawing a part in the framebuffer, VDP1 automatically clears the framebuffer. At the time of this erase (erase write), the 16-bit data set in the erase write data register is filled (filled) in the frame buffer.

When drawing to the frame buffer is 8bit / pixel (high resolution or rotation 8), it will be erased by 2 pixels.
The area to be erased is set by the upper left coordinate register of the erased light and the lower right coordinate register of the erased light.

■ Erase light upper left coordinate register

The Erase write left coordinate register (EWLR) sets the upper left coordinate of the erase light area. This is a write-only 16-bit register located at address 100008H. After turning on the power or resetting, the value will be undefined, so be sure to set the coordinates. Set the unused bit to 0.

EWLR 100008H (W) bit15 bit14 bit13 bit12 bit11 bit10 bit9 bit8 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 0 Upper left coordinate X1 Upper left coordinate Y1

Erase light upper left coordinates X1: bit14-9

Erase light upper left coordinates Y1: bit8 ~ 0

■ Erase light lower right coordinate register

EWLR 100008H (W)	bit15	bit14	bit13	bit12	bit11	bit10	bit9	bit8	bit7	bit6	bit5	bit4	bit3	bit2	bit1	bit0
0	Upper left coordinate X1	Upper left coordinate Y1

The Erase write right coordinate register (EWRR) sets the lower right coordinates of the erase light area. A write-only 16-bit register located at address 10000AH. After turning on the power or resetting, the value will be undefined, so be sure to set the coordinates.

EWRR 10000AH (W) bit15 bit14 bit13 bit12 bit11 bit10 bit9 bit8 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 Lower right coordinate X3 Lower right coordinate Y3

Erase light lower right coordinates X3: bit15-9

Erase light lower right coordinates Y3: bit8 ~ 0

Specifies the fill area of the frame buffer during erase write.
Set the X coordinate in 8 pixel units for 16bit / pixel (normal, rotation 16, HDTV) and in 16pixel units for 8bit / pixel (high resolution, rotation 8). When the value of the system register is 1, it becomes 8 or 16.
The actual lower right X coordinate is the register setting value multiplied by 8 or 16 and subtracted by 1.
The Y coordinate is set in units of one line. In double-dense interlacing, the Y coordinate register setting value is doubled, so set 1/2 of the actual coordinate value. For example, when the setting value is 223, the coordinate value is 447.
The actual X coordinate is expressed by the following formula.
At 16bit / pixel
Upper left coordinate X1 = register set value x 8
Lower right coordinate X3 = Register set value x 8-1
At 8bit / pixel
Upper left coordinate X1 = register set value x 16
Lower right coordinate X3 = Register set value x 16-1

EWRR 10000AH (W)	bit15	bit14	bit13	bit12	bit11	bit10	bit9	bit8	bit7	bit6	bit5	bit4	bit3	bit2	bit1	bit0
Lower right coordinate X3	Lower right coordinate Y3

Therefore, the possible values of the upper left coordinate X1 and the lower right coordinate X3 are as follows.

System register value	16bit / pixel		8bit / pixel
	16bit / pixel		High resolution		Rotation 8
	Upper left coordinate X1	Lower right coordinate X3	Upper left coordinate X1	Lower right coordinate X3	Upper left coordinate X1	Lower right coordinate X3
0	0	Setting prohibited	0	Setting prohibited	0	Setting prohibited
1	8	7	16	15	16	15
2	16	15	32	31	32	31
:	:	:	:	:	:	:
31	248	247	496	495	496	495
32	256	255	512	511	Setting prohibited	511
33	264	263	528	527	Setting prohibited	Setting prohibited
:	:	:	:	:	:	:
40	320	319	640	639	:	:
:	:	:	:	:	:	:
43	344	343	688	687	:	:
44	352	351	704	703	:	:
:	:	:	:	:	:	:
62	496	495	992	991	:	:
63	504	503	1008	1007	:	:
64	Setting prohibited	511	Setting prohibited	1023	:	:
65 ~	Setting prohibited	Setting prohibited	Setting prohibited	Setting prohibited	Setting prohibited	Setting prohibited

Since the coordinates of the erase light are not checked, set X1 <X3, Y1 ≤ Y3 in advance on the CPU.
Set the erase light range in each TV mode within the range of each memory map.
Figure 4.2 Erase light area

When X1 ≥ X3 or Y1> Y3 is set,
Normal, high resolution → 1 dot
Rotation / HDTV → 8 dots
Erase light will be done. In this case, erase light is performed assuming that the area is set to (X1, Y1), (X3 = X1 + 1, Y3 = Y1).
In normal and high resolution, the erase light when VBE = 0 runs out of the frame buffer during the display period and performs erase light. When the X3 coordinates are set beyond the display screen, the erase light continues at 4 pixels during normal and 8 pixels during high resolution after the final valid data (the fall of HTIM), and the erase light is displayed outside the other display screens. It will not be done.
If the erase light range is set in the display screen, no erase light outside the display range will be performed.
Erase lights are not affected by clipping.
During HDTV, frame buffer enlargement / reduction, and rotation display, frame buffer erase write cannot be performed during display. In this case, the erase light is performed during the vertical blanking interval (V_BLANK), so there is not enough time to erase the entire screen. If you want to erase light the entire screen, you need to fill the unerased part with polygons at the start of drawing.
The number of pixels required for V blank erase
(X3-X1 + 1) x (Y3-Y1 + 1) x 8
It is represented by. If this is within the number of pixels available for V-blank erase, the erase light is complete.
The number of pixels that can be used for V blank erase
{(Number of pixels in one raster)-200} x {(Number of rasters in one field)-(Number of display rasters)}
It will be. The respective values are as follows:

Table 4.4 Number of rasters and pixels

Screen mode	Number of horizontal pixels	Number of pixels in one raster	Number of rasters in one field
NTSC	320	1708	263
PAL	352	1820	313
31KC	−	852	525
HDTV	−	848	562

Table 4.5 V Number of pixels that can be used for blank erase (when non-interlaced)

Screen mode	Resolution (horizontal x vertical)	Number of pixels available
NTSC	320 x 224	58812
	320 x 240	34684
	352 x 224	63180
	352 x 240	37260
PAL	320 x 224	134212
	320 x 240	110084
	320 x 256	85956
	352 x 224	144180
	352 x 240	118260
	352 x 256	92340
31KC	320 x 480	29340
HDTV	352 x 480	53136

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