Chapter 3 RAM

A dedicated VRAM for defining pattern name tables, character patterns, etc. is connected to VDP2. VRAM is divided into two parts of equal capacity called VRAM-A and VRAM-B, respectively. VRAM-A and VRAM-B can be divided into two with the same capacity, called bank 0 and bank 1, respectively. The four banks with the same capacity divided in this way are called VRAM-A0, VRAM-A1, VRAM-B0, and VRAM-B1. Define the data shown in Table 3.1 in VRAM.
It also has built-in color RAM for defining color data for scroll screens and sprites.

Table 3.1 Data defined in VRAM

Always when the display format is cell format. Data that must be defined	Display format is bitmap For formats, be sure to define data that must be	Data defined as required
pattern name table data Character pattern data	bitmap pattern data	line scroll table data Vertical cell scroll table data Rotation parameter table data Coefficient table data Line color screen table data Back screen table data line window table data

■3.1 Address map

VDP2 can support two types of VRAM: 4Mbit and 8Mbit. A program written for a system using 4Mbits of VRAM can also be used on a system using 8Mbits of VRAM, but not vice versa.

The address map changes as shown in Figure 3.1 depending on the amount of VRAM used in the system.

Figure 3.1 VRAM address map by capacity

 ●VRAM capacity: 4M bits　
  000000H┌────────────┐
         │            │
         │   VRAM-A0  │
  01FFFFH│            │
  020000H├────────────┤
         │            │
         │   VRAM-A1  │
  03FFFFH│            │
  040000H├────────────┤
         │            │
         │   VRAM-B0  │
  05FFFFH│            │
  060000H├────────────┤
         │            │
         │   VRAM-B1  │
         │            │
  07FFFFH└────────────┘

 ●VRAM capacity: 8M bits　
  000000H┌────────────┐
         │            │
         │            │
         │   VRAM-A0  │
         │            │
  03FFFFH│            │
  040000H├────────────┤
         │            │
         │            │
         │   VRAM-A1  │
         │            │
  07FFFFH│            │
  080000H├────────────┤
         │            │
         │            │
         │   VRAM-B0  │
         │            │
  0BFFFFH│            │
  0C0000H├────────────┤
         │            │
         │            │
         │   VRAM-B1  │
         │            │
         │            │
  0FFFFFH└────────────┘

●VRAM size register

The VRAM size register specifies the amount of VRAM used in the system. It is a readable and writable 16-bit register located at address 180006H. However, bits 3-0 are read-only bits. The value of bit 15 (VRAMSZ) is cleared to 0 after power-on or reset, so be sure to set it.

VRSIZE 180006H

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

07	06	05	04	03	02	01	00
-	-	-	-	VER3	VER2	VER1	VER0

▲ VRAM size bit : VRAM size bit (VRAMSZ), bit 15

Specify the amount of VRAM used in your system.

VRAMSZ	VRAM capacity
0	4Mbit
1	8Mbit

This bit must be set before writing data to VRAM.

▲ Version number bit : Version number bit (VER3 to VER0), bits 3 to 0

Represents the version number of VDP2. Initially it is 0.