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

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VDP1 User's Manual/Chapter 4 System Registers

■4.4 Erase write

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

■Erase write data register

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

16bit/pixel

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

8bit/pixel

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

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

When drawing parts to the framebuffer, VDP1 automatically clears the framebuffer. During this erase (erase write), the frame buffer is filled with the 16-bit data set in the erase write data register.

When drawing to the frame buffer is 8bit/pixel (high resolution or rotation 8), erase/write will be done in 2pixel increments.
The area to be erased and written is set by the erase write upper left coordinate register and the erase write lower right coordinate register.

■Erase write upper left coordinate register

The erase write left coordinate register (EWLR) sets the upper left coordinate of the erase write area. This is a 16-bit write-only register located at address 100008H. After turning on the power or resetting, the values will be unstable, so be sure to set the coordinates. Set unused bits 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 write upper left coordinates X1: bit14~9

Erase write upper left coordinate Y1: bit8~0

■Erase write 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 coordinate of the erase write area. This is a 16-bit write-only register located at address 10000AH. After turning on the power or resetting, the values will be unstable, 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 write lower right coordinate X3: bit15~9

Erase write lower right coordinate Y3: bit8~0

Specifies the fill area of the frame buffer when writing an erase.

The X coordinate is set in 8 pixel units for 16 bit/pixel (normal, rotation 16, HDTV), and in 16 pixel units for 8 bit/pixel (high resolution, rotation 8). When the system register value is 1, it becomes 8 or 16.
The actual lower right X coordinate is the value obtained by multiplying the register setting value by 8 or 16 and subtracting 1.

Set the Y coordinate in units of one line. When using double-dense interlacing, the value will be twice the Y coordinate register setting value, so please set it to 1/2 of the actual coordinate value. For example, when the setting value is 223, the coordinate value will be 447.

The actual X coordinate is expressed by the following formula.

16bit/pixel
Upper left coordinate X1 = register setting value x 8
Lower right coordinate X3 = register setting value x 8−1

8bit/pixel
Upper left coordinate X1 = register setting value x 16
Lower right coordinate X3 = register setting value × 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 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 write are not checked, the CPU＜ Please set so that X3, Y1≦Y3.

Please set the erase write range for each TV mode within the range of each memory map.

Figure 4.2 Erase write area

X1≧X3, or Y1＞ If set to Y3,

Normal, high resolution → 1 dot rotation/HDTV → 8 dots

The erase write will be performed. In this case, the erase write is performed assuming that the area is set to (X1, Y1), (X3 = X1 + 1, Y3 = Y1).

In normal and high resolution mode, when VBE = 0, erase write is performed outside the frame buffer during the display period. If the X3 coordinate is set beyond the display screen, the erase write will continue at 4 pixels in normal mode and 8 pixels in high resolution after the last valid data (the falling edge of HTIM), and the erase write will continue outside the display screen. Not done.

If the erase write range is set within the display screen, no erase write will be performed outside the display range.

Erase writes are not affected by clipping.

When displaying HDTV, scaling or rotating the frame buffer, it is not possible to erase and write the frame buffer during display. In this case, the erase write is performed during the vertical retrace period (V_BLANK), so there is not enough time to erase the entire screen. If you want to erase write the entire screen, you need to fill in the unerased areas with polygons when you start drawing.

The number of pixels required for V blank erase is

(X3−X1+1)×(Y3−Y1+1)×8

It is expressed as If this is within the number of pixels that can be used for V blank erase, erase write is completed.

The number of pixels that can be used for V blank erase is

{(Number of pixels in 1 raster) − 200 } × {(Number of rasters in 1 field) − (Number of displayed rasters)}

It becomes. The values for each are:

Table 4.4 Number of rasters and number of pixels

screen mode	Number of horizontal pixels	in 1 raster number of pixels in	in 1 field Number of rasters in
NTSC	320	1708	263
PAL	352	1820	313
31KC	−	852	525
HDTV	−	848	562

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

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

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