VDP Ports

Data port

The Data port is at 0xC00000 or $C00002 and is used for reading/writing to the VRAM,CRAM or VSRAM.

You first have to select the address and RAM using the control port.

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	D15-D0

Control Port

The control port is at $C00004 or $C00006 (only for writing).

Read

Reading it will get the status register.

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	Nothing						0 : 1 : FIFO empty	0 : 1 : FIFO full	0 : 1 : V interrupt happened	0 : No sprites overflow 1 : Sprites overflow	0 : No collision 1 : Collision between two sprites	0 : Even frame 1 : Odd frame (in interlace mode)	0 : Not in VBLANK 1 : In VBLANK	0 : Not in HBLANK 1 : In HBLANK	0 : DMA not busy 1 : DMA busy	0 : NTSC mode 1 : PAL mode

Write

Writing to it can be done for setting registers or setting RAM address for further read/write.

Write 1 - Setting Register

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	always 1	always 0		Register number					Register value

Write 2 - Setting RAM address

First word

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	CD1-CD0		A13-A0

Second word

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	0								CD5-CD2				0		A15-A14

CD0-CD5 select the address mode.

Access mode	CD5	CD4	CD3	CD2	CD1	CD0
VRAM Write	0	0	0	0	0	1
CRAM Write	0	0	0	0	1	1
VSRAM Write	0	0	0	1	0	1
VRAM Read	0	0	0	0	0	0
CRAM Read	0	0	1	0	0	0
VSRAM Read	0	0	0	1	0	0

CD4 and CD5 are used while doing DMA access.

DMA Mode	CD5	CD4
Memory to VRAM	1	0
VRAM Fill	1	0
VRAM Copy	1	1

HV Counter

The HV counter port is at $C00008 and is used to read the current line/frame drawn by the VDP.

Non interlace mode

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	Frame number								Line number

Interlace mode

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	VC7	VC6	VC5	VC4	VC3	VC2	VC1	VC8	Line number

In active display the vcounter goes from 0 to 223 (0xDF) in V28 CELL mode and from 0 to 239 (0xEF) in V30 CELL mode.

In active display the hcounter goes from 0 to 127 (0x7F) in H32 CELL mode and from 0 to 0x159 (0x9F) in H40 CELL mode.

In HBLANK in H30 CELL mode when the counter reach 132 (0x84) the vcounter value will increment. It will then increment normally till 148 (0x94) and then jump to a value around 233 (0xE9). I think it's when the beam restart at the new line.

In VBLANK in V28 CELL mode the counter will continue to increment normally from 224 (0xE0) to 234 (0xEA). Then the counter will reset itself to 229 (0xE5) and then continue normally till 255 (0xFF). I think it's when the beam restart at the beginning of the screen.