Here the registers used by MSX VDPs.

VDP Control Registers

These registers can only be written. Reading is possible in BASIC because the system stores the last value written in system variables. VDP() instruction reads the corresponding system variables to return the requested value.

You can divide these registers into several categories:

• VDP Mode Registers (0, 1, 8, 9, 25)
• VDP Table Base Address Registers (2 to 6, 10, 11)
• VDP Color Registers (7, 12, 16, 20 to 22)
  • VDP Color Palette Registers.
• VDP Access Registers (14, 15, 17)
• VDP Display Registers (13, 18, 19, 23, 26, 27)

How to write directly in a VDP control register

To write in a VDP register you must send two bytes one after the other to the VDP port #1. The first byte must be the data to write and the second the register number with the bit 7 set to indicate that it is an access to a register.

To do it you must know the writing CPU port connected to the VDP port #1. The number of the writing CPU port can be found in the Main-ROM at the address 0007h.

In the BASIC environment, the Main-ROM is selected by default, so you only need to read the byte at address 0007h.

Example in assembler:

VDP_DW	equ	0007h	; VDP data write port
 
; Routine to write a byte to a VDP register.
 
; Entry: A = data, B = register number + 80h (to set the bit 7)
;
; Modify: BC 
 
WR2VDPREG:
	push	af
	ld	a,(VDP_DW)	; A = First writing port used to access the internal VDP
	ld	c,a
	inc	c		; C = CPU port connected to the VDP writing port #1
 
	di			; Interrupts must be disabled here
	pop	af
	out	(c),a		; Write the data
	out	(c),b		; Write the register number (with the bit 7 always set)
	ei			; Interrupts can be enabled here
	ret

VDP Command Registers

These registers can only be written.

For details about VDP Command Registers, see here.

VDP Status Registers

These registers can only be read.

For details about VDP status registers, see here.