Register Overview

Address Bus B Registers

TODO: note on fast access time

Register	Address	Name	Style	Access	Timing
Screen Display Register	$2100	INIDISP	single	write	any time
Object Size and Character Size Register	$2101	OBSEL	single	write	f-blank, v-blank
OAM Address Registers (Low)	$2102	OAMADDL	single	write	f-blank, v-blank
OAM Address Registers (High)	$2103	OAMADDH	single	write	f-blank, v-blank
OAM Data Write Register	$2104	OAMDATA	single	write	f-blank, v-blank
BG Mode and Character Size Register	$2105	BGMODE	single	write	f-blank, v-blank, h-blank
Mosaic Register	$2106	MOSAIC	single	write	f-blank, v-blank, h-blank
BG Tilemap Address Registers (BG1)	$2107	BG1SC	single	write	f-blank, v-blank
BG Tilemap Address Registers (BG2)	$2108	BG2SC	single	write	f-blank, v-blank
BG Tilemap Address Registers (BG3)	$2109	BG3SC	single	write	f-blank, v-blank
BG Tilemap Address Registers (BG4)	$210A	BG3SC	single	write	f-blank, v-blank
BG Character Address Registers (BG1&2)	$210B	BG12NBA	single	write	f-blank, v-blank
BG Character Address Registers (BG3&4)	$210C	BG34NBA	single	write	f-blank, v-blank
BG Scroll Registers (BG1)	$210D	BG1HOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG1)	$210E	BG1VOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG2)	$210F	BG2HOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG2)	$2110	BG2VOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG3)	$2111	BG3HOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG3)	$2112	BG3VOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG4)	$2113	BG4HOFS	dual	write	f-blank, v-blank, h-blank
BG Scroll Registers (BG4)	$2114	BG4VOFS	dual	write	f-blank, v-blank, h-blank
Video Port Control Register	$2115	VMAIN	single	write	f-blank, v-blank
VRAM Address Registers (Low)	$2116	VMADDL	single	write	f-blank, v-blank
VRAM Address Registers (High)	$2117	VMADDH	single	write	f-blank, v-blank
VRAM Data Write Registers (Low)	$2118	VMDATAL	single	write	f-blank, v-blank
VRAM Data Write Registers (High)	$2119	VMDATAH	single	write	f-blank, v-blank
Mode 7 Settings Register	$211A	M7SEL	single	write	f-blank, v-blank
Mode 7 Matrix Registers	$211B	M7A	dual	write	f-blank, v-blank, h-blank
Mode 7 Matrix Registers	$211C	M7B	dual	write	f-blank, v-blank, h-blank
Mode 7 Matrix Registers	$211D	M7C	dual	write	f-blank, v-blank, h-blank
Mode 7 Matrix Registers	$211E	M7D	dual	write	f-blank, v-blank, h-blank
Mode 7 Matrix Registers	$211F	M7X	dual	write	f-blank, v-blank, h-blank
Mode 7 Matrix Registers	$2120	M7Y	dual	write	f-blank, v-blank, h-blank
CGRAM Address Register	$2121	CGADD	single	write	f-blank, v-blank, h-blank
CGRAM Data Write Register	$2122	CGDATA	dual	write	f-blank, v-blank, h-blank
Window Mask Settings Registers	$2123	W12SEL	single	write	f-blank, v-blank, h-blank
Window Mask Settings Registers	$2124	W34SEL	single	write	f-blank, v-blank, h-blank
Window Mask Settings Registers	$2125	WOBJSEL	single	write	f-blank, v-blank, h-blank
Window Position Registers (WH0)	$2126	WH0	single	write	f-blank, v-blank, h-blank
Window Position Registers (WH1)	$2127	WH1	single	write	f-blank, v-blank, h-blank
Window Position Registers (WH2)	$2128	WH2	single	write	f-blank, v-blank, h-blank
Window Position Registers (WH3)	$2129	WH3	single	write	f-blank, v-blank, h-blank
Window Mask Logic registers (BG)	$212A	WBGLOG	single	write	f-blank, v-blank, h-blank
Window Mask Logic registers (OBJ)	$212B	WOBJLOG	single	write	f-blank, v-blank, h-blank
Screen Destination Registers	$212C	TM	single	write	f-blank, v-blank, h-blank
Screen Destination Registers	$212D	TS	single	write	f-blank, v-blank, h-blank
Window Mask Destination Registers	$212E	TMW	single	write	f-blank, v-blank, h-blank
Window Mask Destination Registers	$212F	TSW	single	write	f-blank, v-blank, h-blank
Color Math Registers	$2130	CGWSEL	single	write	f-blank, v-blank, h-blank
Color Math Registers	$2131	CGADSUB	single	write	f-blank, v-blank, h-blank
Color Math Registers	$2132	COLDATA	single	write	f-blank, v-blank, h-blank
Screen Mode Select Register	$2133	SETINI	single	write	f-blank, v-blank, h-blank
Multiplication Result Registers	$2134	MPYL	single	read	f-blank, v-blank, h-blank
Multiplication Result Registers	$2135	MPYM	single	read	f-blank, v-blank, h-blank
Multiplication Result Registers	$2136	MPYH	single	read	f-blank, v-blank, h-blank
Software Latch Register	$2137	SLHV	single		any time
OAM Data Read Register	$2138	OAMDATAREAD	dual	read	f-blank, v-blank
VRAM Data Read Register (Low)	$2139	VMDATALREAD	single	read	f-blank, v-blank
VRAM Data Read Register (High)	$213A	VMDATAHREAD	single	read	f-blank, v-blank
CGRAM Data Read Register	$213B	CGDATAREAD	dual	read	f-blank, v-blank
Scanline Location Registers (Horizontal)	$213C	OPHCT	dual	read	any time
Scanline Location Registers (Vertical)	$213D	OPVCT	dual	read	any time
PPU Status Register	$213E	STAT77	single	read	any time
PPU Status Register	$213F	STAT78	single	read	any time
APU IO Registers	$2140	APUIO0	single	both	any time
APU IO Registers	$2141	APUIO1	single	both	any time
APU IO Registers	$2142	APUIO2	single	both	any time
APU IO Registers	$2143	APUIO3	single	both	any time
WRAM Data Register	$2180	WMDATA	single	both	any time
WRAM Address Registers	$2181	WMADDL	single	write	any time
WRAM Address Registers	$2182	WMADDM	single	write	any time
WRAM Address Registers	$2183	WMADDH	single	write	any time

Old Style Joypad Registers

TODO: note on extra slow access time

Register	Address	Name	Style	Access	Timing
Old Style Joypad Registers	$4016	JOYSER0	single (write)	read/write	any time that is not auto-joypad
Old Style Joypad Registers	$4017	JOYSER1	many (read)	read	any time that is not auto-joypad

Internal CPU Registers

TODO: note on fast access time

Register	Address	Name	Style	Access	Timing
Interrupt Enable Register	$4200	NMITIMEN	single	write	any time
IO Port Write Register	$4201	WRIO	single	write	any time
Multiplicand Registers	$4202	WRMPYA	single	write	any time
Multiplicand Registers	$4203	WRMPYB	single	write	any time
Divisor & Dividend Registers	$4204	WRDIVL	single	write	any time
Divisor & Dividend Registers	$4205	WRDIVH	single	write	any time
Divisor & Dividend Registers	$4206	WRDIVB	single	write	any time
IRQ Timer Registers (Horizontal - Low)	$4207	HTIMEL	single	write	any time
IRQ Timer Registers (Horizontal - High)	$4208	HTIMEH	single	write	any time
IRQ Timer Registers (Vertical - Low)	$4209	VTIMEL	single	write	any time
IRQ Timer Registers (Vertical - High)	$420A	VTIMEH	single	write	any time
DMA Enable Register	$420B	MDMAEN	single	write	any time
HDMA Enable Register	$420C	HDMAEN	single	write	any time
ROM Speed Register	$420D	MEMSEL	single	write	any time
Interrupt Flag Registers	$4210	RDNMI	single	read	any time
Interrupt Flag Registers	$4211	TIMEUP	single	read	any time
PPU Status Register	$4212	HVBJOY	single	read	any time
IO Port Read Register	$4213	RDIO	single	read	any time
Multiplication Or Divide Result Registers (Low)	$4214	RDDIVL	single	read	any time
Multiplication Or Divide Result Registers (High)	$4215	RDDIVH	single	read	any time
Multiplication Or Divide Result Registers (Low)	$4216	RDMPYL	single	read	any time
Multiplication Or Divide Result Registers (High)	$4217	RDMPYH	single	read	any time
Controller Port Data Registers (Pad 1 - Low)	$4218	JOY1L	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 1 - High)	$4219	JOY1H	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 2 - Low)	$421A	JOY2L	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 2 - High)	$421B	JOY2H	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 3 - Low)	$421C	JOY3L	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 3 - High)	$421D	JOY3H	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 4 - Low)	$421E	JOY4L	single	read	any time that is not auto-joypad
Controller Port Data Registers (Pad 4 - High)	$421F	JOY4H	single	read	any time that is not auto-joypad

DMA Registers

These registers can be read or written at any time. TODO: write something about fast access time. TODO: write something about x here.

Register	Address	Name
DMA Control Register	$43x0	DMAPx
DMA Destination Register	$43x1	BBADx
DMA Source Address Registers	$43x2	A1TxL
DMA Source Address Registers	$43x3	A1TxH
DMA Source Address Registers	$43x4	A1Bx
DMA Size Registers (Low)	$43x5	DASxL
DMA Size Registers (High)	$43x6	DASxH

HDMA Registers

These registers can be read or written at any time. TODO: write something about fast access time. TODO: write something about x here.

Register	Address	Name
HDMA Control Register	$43x0	DMAPx
HDMA Destination Register	$43x1	BBADx
HDMA Table Address Registers	$43x2	A1TxL
HDMA Table Address Registers	$43x3	A1TxH
HDMA Table Address Registers	$43x4	A1Bx
HDMA Indirect Address Registers	$43x5	DASxL
HDMA Indirect Address Registers	$43x6	DASxH
HDMA Indirect Address Registers	$43x7	DASBx
HDMA Mid Frame Table Address Registers (Low)	$43x8	A2AxL
HDMA Mid Frame Table Address Registers (High)	$43x9	A2AxH
HDMA Line Counter Register	$43xA	NTLRX

---

Register Explanations

The flags are:

rw?fvha
||||||+--> '+' if it can be read/written at any time, '-' otherwise
|||||+---> '+' if it can be read/written during H-Blank
||||+----> '+' if it can be read/written during V-Blank
|||+-----> '+' if it can be read/written during force-blank
||+------> Read/Write style: 'b'     => byte
||                           'h'/'l' => read/write high/low byte of a word
||                           'w'     => word read/write twice low then high
|+-------> 'w' if the register is writable for an effect
+--------> 'r' if the register is readable for a value or effect (i.e. not [open bus](/open-bus)).

To find the entry for a particular register, search for the register number (i.e. ‘2100’) at the very beginning of the line. Note that the DMA registers are combined, so e.g. to find $4300, $4310, $4320, $4330, $4340, $4350, $4360, or $4370 you’d search for ‘43x0’.

For most registers (and most undefined bits of readable registers), the returned value is Open Bus, that is the last value read over the main bus from the ROM (typically part of the opcode arguments or the indirect base address).

Registers matching $21x4-6 or $21x8-A (where x is 0-2) return the last value read from any of the PPU1 registers $2134-6, $2138-A, or $213E. This is known as PPU1 Open Bus. Similarly, PPU2 Open Bus involves reading registers $213B-D or $213F (NOT $21xB-D though).

Note that it may be possible to write registers anytime even if marked ‘-’, but until we have proof ‘-’ is a better guess.