<13/86> VDP2 User's Manual / 3.3 VRAM access method during display period-2

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VDP2 User's Manual / Chapter 3 RAM / ■ 3.3 How to access VRAM during the display period

● Access to image data

In order to display the normal scroll screen (NBG0 to NBG3), the necessary image data must be read from VRAM. The image data required when the display format is cell format is pattern name data and character pattern data, and the image data required when the display format is bitmap format is bitmap pattern data. The number of VRAM accesses required in one cycle to obtain these image data is determined by the conditions.

Pattern data read access during one cycle is set to a maximum of 2 banks, either for VRAM-A0 or VRAM-B0, and for VRAM-A1 or VRAM-B1. Do not do it. If VRAM is not divided into two, the register for VRAM-A0 is used for VRAM-A and the register for VRAM-B0 is used for VRAM-B, so set it to only one of them. not. The access timing can be specified at any timing as long as it is within the register valid range in each TV screen mode. The number of accesses must be specified in just proportion to the number determined by the conditions, but it is not necessary to specify it at consecutive timings.
Table 3.2 shows the number of pattern name data read access, and Figure 3.3 shows the pattern name data read access specification limit.

Table 3.2 Number of pattern name table data accesses required during one cycle

item	NBG0 ~ NBG3
Reduction setting	1x	1/2 times	1/4 times
Required during one cycle VRAM access count	1	2	Four

Figure 3.3 Restrictions on access specification for pattern table data

As a general rule, character pattern data read access during one cycle can be specified at any timing of the four banks. However, the timing that can be specified is limited by the pattern data access timing. However, only when the pattern data access of NBG0 and NBG1 is specified in T0, each character pattern data read access can be specified at any timing of the four banks, and there is no limitation. .. The number of accesses must be specified in just proportion to the number determined by the conditions, but it is not necessary to specify it at consecutive timings.
The number of character pattern data read access is shown in Table 3.3, and the character pattern data read access specification limit is shown in Table 3.4.

Table 3.3 Character pattern data (bitmap pattern data) Read access count

item	NBG0 ~ NBG3
Number of character colors	16			256		2048	32768	16.77 million
Reduction setting	1x	1/2 times	1/4 times	1x	1/2 times	1x	1x	1x
Required during one cycle VRAM access count	1	2	Four	2	Four	Four	Four	8

Table 3.4 Character pattern data read access specification restrictions

item	TV screen mode	Character size	Access timing of pattern name table data
item	TV screen mode	Character size	T0	T1	T2	T3	T4	T5	T6	T7
Timing when character pattern table data access can be specified	normal	1 horizontal cell x 1 vertical cell 2 horizontal cells x 2 vertical cells	T0 to T2, T4 ~ T7	T0 to T3, T5 ~ T7	T0 to T3, T6 ~ T7	T0 to T3, T7	T0 to T3	T1 ~ T3	T2, T3	T3
	High resolution, Dedicated monitor	1 horizontal cell x 1 vertical cell	T0 to T2	T1 ~ T3	T0, T2, T3	T0, T1, T3	−	−	−	−
	High resolution, Dedicated monitor	2 horizontal cells x 2 vertical cells	T0 to T2	T1 ~ T3	T2, T3	T3	−	−	−	−

If the reduction setting is 1x and character pattern data read access is required more than once, all character pattern data read access must comply with the specified limits. If the reduction setting is 1/2 or 1/4 times, the number of accesses required when the reduction setting is 1 (1 time when the number of character colors is 16 colors, 2 times when the number of character colors is 256 colors) , It is necessary to comply with the specification limit by one pattern name data read access. For example, when the number of character colors is 256 and the reduction setting is 1/2 times, the character pattern data read access specification limit when pattern name data read access is specified for T1 and T3 is as shown in Fig. 3.4. increase.

Figure 3.4 Example of character pattern data read access specification

[Note] Character pattern data read access must be specified twice for each specifiable range.

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