Nestech.txt
This is an updated version of "Nintendo Entertainment System Documentation" by Jeremy Chadwick, aka "koitsu". It provides a brief overview of the NES hardware and may prove easier to understand, particularly for programmers new to the NES, than some of the other, more technical documentation on this wiki.
Introduction
Disclaimer
I am in no way held responsible for the results of this information. This is public-domain information. I am not trying to hinder anyone financially: if you wish to do development on Nintendo's current platforms, contact Nintendo via developer.nintendo.com.
All titles of cartridges and console systems are trademarks of their respective owners. (I just don't deem it necessary to list every single one by hand.)
Why?
At the time this document was created, there was only one piece of literature covering the internals to the NES: Marat Fayzullin's documentation, otherwise known as "NES.DOC".
While Fayzullin's documentation was lacking in many areas, it provided a strong base for the basics, and in itself truly stated how complex the little grey box was.
I took the opportunity to expand on "NES.DOC," basing other people's findings, as well as my own, on experience; experience which has helped make this document what it has become today. The beginning stages of this document looked almost like a replica of Fayzullin's documentation, with both minor and severe changes. Marat Fayzullin himself later picked up a copy of my documentation, and later began referring people to it.
Keep in mind, without Marat's "NES.DOC" document, I would have never had any incentive to write this one.
Mission
The goal of this document is simplistic: to continue providing accurate, understandable, and up-to-date information regarding the Nintendo Entertainment System and its Famicom counterpart.
Dedications
I'd like to dedicate this document to Alex Krasivsky. Alex has been a great friend, and in my eyes, truly started the ball of emulation rolling. During the good times, and the bad times, Alex was there. Spasibo, Alex; umnyj Russki...
"Thank You"s
I'd like to take the time and thank all the people who helped make this document what it is today. I couldn't have done it without all of you.
Alex Krasivsky
Andrew Davie
Avatar Z
Barubary
Bluefoot
CiXeL
Chi-Wen Yang
Chris Hickman
D
Dan Boris
David de Regt
Donald Moore
Fredrik Olsson
Icer Addis
Jon Merkel
Kevin Horton
Loopy
Marat Fayzullin
Mark Knibbs
Martin Nielsen
Matt Conte
Matthew Richey
Memblers
MiKael Iushin
Mike Perry
Morgan Johansson
Neill Corlett
Pat Mccomack
Patrik Alexandersson
Paul Robson
Ryan Auge
Stumble
Tennessee Carmel-Veilleux
Thomas Steen
Tony Young
Vince Indriolo
\FireBug\
Special thanks goes out to Stumble, for providing a myriad of information over IRC, while avoiding sleep to do so.
Acronyms
Internals
- CPU
- Central Processing Unit: Self-explanatory. The NES uses a standard 6502 (NMOS).
- PPU
- Picture Processing Unit: Used to control graphics, sprites, and other video-oriented features.
- pAPU
- pseudo-Audio Processing Unit: Native to the CPU; generates waveforms via (5) audio channels:: four (4) analogue, and one (1) digital. There is no separate IC for audio processing nor generation inside the NES.
- MMC
- Multi-Memory Controller or Memory Management Controller: Integrated circuits used in NES games to access memory beyond the 6502 64Kbyte boundary. They can also be used to access extra CHR-ROM, and may be used for "special effects" such as forcing an IRQ, and other things.
- VRAM
- Video RAM: The RAM which is internal to the PPU. There is 16kbits of VRAM installed in the NES.
- SPR-RAM
- Sprite RAM: 256 bytes of RAM which is used for sprites. It is not part of VRAM nor ROM, though it's local to the PPU.
- OAM
- Object Attribute Memory: Synonymous with SPR-RAM.
- PRG-ROM
- Program ROM: The actual program-code area of memory. Also can be used to describe areas of code which are external to the actual code area and are swapped in via an MMC.
- PRG-RAM
- Program RAM: Synonymous with PRG-ROM, except that it's RAM.
- CHR-ROM
- Character ROM: The VRAM data which is kept external to the PPU, swapped in and out via an MMC, or "loaded" into VRAM during the power-on sequence.
- VROM
- Synonymous with CHR-ROM.
- SRAM
- Save RAM: RAM which is commonly used in RPGs such as Crystalis, the Final Fantasy series, and The Legend of Zelda.
- WRAM
- Work RAM: Synonymous with SRAM.
- DMC
- Delta Modulation Channel: The channel of the APU which handles digital data. Commonly referred to as the PCM (Pulse Code Modulation) channel.
- EX-RAM
- Expansion RAM: This is the memory used within Nintendo's MMC5, allowing games to extend the capabilities of VRAM.
Hardware
- NES
- Nintendo Entertainment System: Self-explanatory.
- Famicom
- Family Computer: Synonymous with the NES, except for also supporting external audio synthesizers in the cartridge.
- FDS
- Famicom Disk System: Unit which sits atop the Famicom, supporting the use of 3" double-sided floppy disks for games.
- Dendy
- Third-party clone of the Famicom for PAL TV systems, distributed in Russia and elsewhere in the East.
CPU
General Information
The NES uses a customized NMOS 6502 CPU, engineered and produced by Ricoh. Its primary customization adds audio.
Different regional variants of the NES run the CPU different clock speeds.
- The Famicom and NTSC NES runs at 1.789773 MHz.
- The PAL NES runs at 1.662607 MHz.
- The Dendy and other PAL famiclones run at 1.773448 MHz.
Memory Map
+---------+-------+-------+-----------------------+
| Address | Size | Flags | Description |
+---------+-------+-------+-----------------------+
| $0000 | $800 | | RAM |
| $0800 | $800 | M | RAM |
| $1000 | $800 | M | RAM |
| $1800 | $800 | M | RAM |
| $2000 | 8 | | Registers |
| $2008 | $1FF8 | R | Registers |
| $4000 | $20 | | Registers |
| $4020 | $1FE0 | | Expansion I/O |
| $6000 | $2000 | | SRAM |
| $8000 | $4000 | | PRG-ROM |
| $C000 | $4000 | | PRG-ROM |
+---------+-------+-------+-----------------------+
Flag Legend: M = Mirror of $0000
R = Mirror of $2000-2008 every 8 bytes
(e.g. $2008=$2000, $2018=$2000, etc.)
Interrupts
The 6502 has three (3) interrupts: IRQ/BRK, NMI, and RESET.
Each interrupt has a vector. A vector is a 16-bit address which specifies a location to "jump to" when the interrupt is triggered.
IRQ/BRK is triggered under two circumstances, hence it's split name: when a software IRQ is executed (the BRK instruction), or when a hardware IRQ is executed (via the IRQ line).
RESET is triggered on power-up. The ROM is loaded into memory, and the 6502 jumps to the address specified in the RESET vector. No registers are modified, and no memory is cleared; these only occur during power-up.
NMI stands for Non-Maskable Interrupt, and is generated by each refresh (VBlank), which occurs at different intervals depending upon the system used (PAL/NTSC).
NMI is updated 60 times/sec. on NTSC consoles, and 50 times/sec on PAL. Interrupt latency on the 6502 is seven (7) cycles; this means it takes seven (7) cycles to move in and out of an interrupt.
Most interrupts should return using the RTI instruction. Some NES carts do not use this method, such as SquareSoft's Final Fantasy title. These carts return from interrupts in a very odd fashion: by manipulating the stack by hand, and then doing an RTS. This is technically valid, but morally shunned.
The aforementioned interrupts have the following vector addresses, mapped to areas of ROM:
- $FFFA = NMI
- $FFFC = RESET
- $FFFE = IRQ/BRK
Interrupt priorities are as follows:
- RESET (highest)
- NMI
- IRQ/BRK (lowest)
NES-specific Customizations
The NES's 6502 does not contain support for decimal mode. Both the CLD and SED opcodes function normally, but the 'd' bit of P is unused in both ADC and SBC. It is common practice for games to CLD prior to code execution, as the status of 'd' is unknown on power-on and on reset.
Audio registers are mapped internal to the CPU; all waveform generation is done internal to the CPU as well.
Notes
Please note the two separate 16K PRG-ROM segments; they may be linear, but they play separate roles depending upon the size of the cartridge. Some games only hold one (1) 16K bank of PRG-ROM, which should be loaded into both $C000 and $8000.
Most games load themselves into $8000, using 32K of PRG-ROM space. The first game to use this method is _Super Mario Bros._ However, almost all games with more than one (1) 16K bank of PRG-ROM load themselves into $8000 as well. These games use Memory Mappers to swap in and out PRG-ROM data, as well as CHR-ROM. A few Memory Mappers, especially on later games, allow swapping in half of a 16K segment.
When a BRK is encountered, the NES's 6502 pushes the CPU status flag onto the stack with the 'b' CPU bit set. On an IRQ or NMI, the CPU pushes the status flag onto the stack with the 'b' bit clear. This is done because of the fact that a hardware IRQ (IRQ) and a software IRQ (BRK) both share the same vector. For example, one could use the following code to distinguish the difference between the two:
C134: PLA ; Copy CPU status flag
C135: PHA ; Return status flag to stack
C136: AND #$10 ; Check D4 ('b' CPU bit)
C138: BNE is_BRK_opcode ; If set then it is a software IRQ (BRK)
Executing BRK inside of an interrupt handler (IRQ or NMI) will result in the pushed 'b' bit being set.
The 6502 has a bug in opcode $6C (jump absolute indirect). The CPU does not correctly calculate the effective address if the low-byte is $FF. Example:
C100: EF
C1FF: 00
C200: C3
..
D000: 6C FF C1 - JMP ($C1FF)
Logically, this will jump to address $C300. However, due to the fact that the high-byte of the calculate address is *NOT* increased on a page-wrap, this will actually jump to $EF00.
It should be noted that page wrapping does *NOT* occur in indexed-indirect addressing modes. Due to limitations of zero-page, all indexed-indirect read/writes should apply a logical AND #$FF to the effective address after calculation. Example:
C000: LDX #3 ; Reads indirect address from $0002+$0003,
C002: LDA ($FF,X) ; not $0102+$0103.
PPU
General Information
Mirroring (also referred to as "shadowing") is the process of mapping particular addresses or address ranges to other addresses/ranges via hardware.
Memory Map
Included here are two (2) memory maps. The first is a "RAM Memory Map," which despite being less verbose describes the actual regions which point to physical memory in the NES Control Deck and most Game Paks. The second is a "Programmer Memory Map" which is quite verbose and describes the entire memory region of the NES and how it's used/manipulated.
RAM Memory Map
+---------+-------+----------------+
| Address | Size | Description |
+---------+-------+----------------+
| $0000 | $2000 | Pattern Tables |
| $2000 | $800 | Name Tables |
| $3F00 | $20 | Palettes |
+---------+-------+----------------+
Programmer Memory Map
+---------+-------+-------+--------------------+
| Address | Size | Flags | Description |
+---------+-------+-------+--------------------+
| $0000 | $1000 | C | Pattern Table #0 |
| $1000 | $1000 | C | Pattern Table #1 |
| $2000 | $3C0 | | Name Table #0 |
| $23C0 | $40 | N | Attribute Table #0 |
| $2400 | $3C0 | N | Name Table #1 |
| $27C0 | $40 | N | Attribute Table #1 |
| $2800 | $3C0 | N | Name Table #2 |
| $2BC0 | $40 | N | Attribute Table #2 |
| $2C00 | $3C0 | N | Name Table #3 |
| $2FC0 | $40 | N | Attribute Table #3 |
| $3000 | $F00 | R | |
| $3F00 | $10 | | Image Palette #1 |
| $3F10 | $10 | | Sprite Palette #1 |
| $3F20 | $E0 | P | |
| $4000 | $C000 | F | |
+---------+-------+-------+--------------------+
C = Either CHR-ROM or CHR-RAM
N = Mirrored (see Subsection G)
P = Mirrored (see Subsection H)
R = Mirror of $2000-2EFF (VRAM)
F = Mirror of $0000-3FFF (VRAM)
Name Tables
The NES displays graphics using a matrix of "tiles"; this grid is called a Name Table. Tiles themselves are 8x8 pixels. The entire Name Table itself is 32x30 tiles (256x240 pixels). Keep in mind that the displayed resolution differs between NTSC and PAL units.
The Name Tables holds the tile number of the data kept in the Pattern Table (continue on).
Pattern Tables
The Pattern Table contains the actual 8x8 tiles which the Name Table refers to. It also holds the lower two (2) bits of the 4-bit colour matrix needed to access all 16 colours of the NES palette. Example:
VRAM Contents of Colour
Addr Pattern Table Result
------ --------------- --------
$0000: %00010000 = $10 --+ ...1.... Periods are used to
.. %00000000 = $00 | ..2.2... represent colour 0.
.. %01000100 = $44 | .3...3.. Numbers represent
.. %00000000 = $00 +-- Bit 0 2.....2. the actual palette
.. %11111110 = $FE | 1111111. colour #.
.. %00000000 = $00 | 2.....2.
.. %10000010 = $82 | 3.....3.
$0007: %00000000 = $00 --+ ........
$0008: %00000000 = $00 --+
.. %00101000 = $28 |
.. %01000100 = $44 |
.. %10000010 = $82 +-- Bit 1
.. %00000000 = $00 |
.. %10000010 = $82 |
.. %10000010 = $82 |
$000F: %00000000 = $00 --+
The result of the above Pattern Table is the glyph for the letter 'A', as shown in the "Colour Result" section in the upper right.
Attribute Tables
Each byte in an Attribute Table represents a 4x4 group of tiles on the screen. There are multiple ways to describe what the function of one (1) byte in the Attribute Table is:
- Holds the upper two (2) bits of a 32x32 pixel grid, per 16x16 pixels.
- Holds the upper two (2) bits of sixteen (16) 8x8 tiles.
- Holds the upper two (2) bits of four (4) 4x4 tile grids.
It's quite confusing; two graphical diagrams may help:
+------------+------------+
| Square 0 | Square 1 | #0-F represents an 8x8 tile
| #0 #1 | #4 #5 |
| #2 #3 | #6 #7 | Square [x] represents four (4) 8x8 tiles
+------------+------------+ (i.e. a 16x16 pixel grid)
| Square 2 | Square 3 |
| #8 #9 | #C #D |
| #A #B | #E #F |
+------------+------------+
The actual format of the attribute byte is the following (and corres ponds to the above example):
Attribute Byte
(Square #)
----------------
33221100
||||||++-- Upper two (2) colour bits for Square 0 (Tiles #0,1,2,3)
||||++---- Upper two (2) colour bits for Square 1 (Tiles #4,5,6,7)
||++------ Upper two (2) colour bits for Square 2 (Tiles #8,9,A,B)
++-------- Upper two (2) colour bits for Square 3 (Tiles #C,D,E,F)
Palettes
The NES has two 16-colour "palettes": the Image Palette and the Sprite Palette. These palettes are more of a "lookup table" than an actual palette, since they do not hold physical RGB values. They're actually closer to HSV, with lightness in D5-D4 and hue in D3-D0.
D7-D6 of bytes written to $3F00-3FFF are ignored.
For details see PPU palettes and NTSC video.
Name Table Mirroring
One should keep in mind that there are many forms of mirroring when understanding the NES. Some methods even use CHR-ROM-mapped Name Tables (mapper-specific).
The NES itself only contains 2048 ($800) bytes of RAM used for Name Tables. However, as shown in Subsection B, the NES has the capability of addressing up to four (4) Name Tables.
By default, many carts come with "horizontal" and "vertical" mirroring, allowing you to change where the Name Tables point into the NES's PPU RAM. This form of mirroring affects two (2) Name Tables simultaneously; you cannot switch Name Tables independently.
The following chart should assist in understanding all the types of mirroring encountered on the NES. Please note that the addresses shown (12-bit in size) refer to VRAM. Mirroring determines how raw addresses in VRAM are translated into addresses that the PPU sees in the "$2xxx" region.
Name NT#0 NT#1 NT#2 NT#3 Flags
+--------------------------+------+------+------+------+-------+
| Horizontal | $000 | $000 | $400 | $400 | |
| Vertical | $000 | $400 | $000 | $400 | |
| Four-screen | $000 | $400 | $800 | $C00 | F |
| Single-screen | | | | | S |
| CHR-ROM mirroring | | | | | C |
+--------------------------+------+------+------+------+-------+
F = Four-screen mirroring relies on an extra 2048 ($800) of RAM
(kept on the cart), resulting in four (4) physical independent
Name Tables.
S = Single-screen games have mappers which allow you to select
which PPU RAM area you want to use ($000, $400, $800, or
$C00); all the NTs point to the same PPU RAM address.
C = Mapper #68 (Afterburner 2) allows you to map CHR-ROM to the
Name Table region of the NES's PPU RAM area. Naturally this
makes the Name Table ROM-based, and one cannot write to it.
However, this feature can be controlled via the mapper itself,
allowing you to enable or disable this feature.
Palette Mirroring
Mirroring occurs between the Background Palette and the Sprite Palette on addresses that are multiples of 4. Any data which is written to $3F00 is mirrored to $3F10. Any data written to $3F04 is mirrored to $3F14, etc. etc...
Colour #0 in the upper three (3) palettes of both the Image and Sprite palette defines transparency (the actual colour stored there is not drawn on-screen).
In most cases, the PPU uses the value in $3F00 to define background colour.
For a more verbose explanation, assume the following:
- $0D has been written to $3F00 (mirrored to $3F10)
- $03 has been written to $3F08 (mirrored to $3F18)
- $1A has been written to $3F18
- $3F08 is read into the accumulator
The PPU will use $0D as the background colour, despite $3F08 holding a value of $03 (since colour #0 in all the palette entries defines transparency, it is not drawn). Finally, the accumulator will hold a value of $1A, which is mirrored from $3F18. Again, the value of $1A is not drawn, since colour #0 defines transparency.
The entire Background and Sprite Palettes are both mirrored to other areas of VRAM as well; $3F20-3FFF are mirrors of both palettes, respectively.
Again, D7-D6 of bytes written to $3F00-3FFF are ignored.
Background Scrolling
The NES can scroll the background (pre-rendered Name Table + Pattern Table + Attribute Table) independently of the sprites which are overlaid on top of it. The background can be scrolled horizontally and vertically.
Scrolling works as follows:
Horizontal Scrolling Vertical Scrolling
0 512
+-----+-----+ +-----+ 0
| | | | |
| A | B | | A |
| | | | |
+-----+-----+ +-----+
| |
| B |
| |
+-----+ 480
Name Table "A" is specified via Bits D1-D0 in register $2000, and "B" is the Name Table after (due to mirroring, this is dynamic). Games which use Horizontal & Vertical scrolling simultaneously need special attention paid to mirroring.
The background will span across multiple Name Tables, as shown here:
+---------------+---------------+
| Name Table #0 | Name Table #1 |
| ($2000) | ($2400) |
+---------------+---------------+
| Name Table #2 | Name Table #3 |
| ($2800) | ($2C00) |
+---------------+---------------+
But depending on mirroring, some of them will be repeated:
Horizontal Mirroring
+---------------+---------------+
| Name Table #0 | Duplicate #0 |
| ($2000) | ($2400) |
+---------------+---------------+
| Name Table #2 | Duplicate #2 |
| ($2800) | ($2C00) |
+---------------+---------------+
Vertical Mirroring
+---------------+---------------+
| Name Table #0 | Name Table #1 |
| ($2000) | ($2400) |
+---------------+---------------+
| Duplicate #0 | Duplicate #1 |
| ($2800) | ($2C00) |
+---------------+---------------+
Writes to the Horizontal Scroll value in $2005 range from 0 to 255. Writes to the Vertical Scroll value range from 0-239. Vertical Scroll values above 239 are considered negative (e.g. a write of 248 is really -8). This is because values above 239 cause the PPU to interpret Attribute Table data as Name Table data.
Screen and Sprite Layering
There is a particular order in which the NES draws its contents:
FRONT BACK
+----+-----------+----+-----------+-----+
| CI | OBJs 0-63 | BG | OBJs 0-63 | EXT |
+----+-----------+----+-----------+-----+
| SPR-RAM | | SPR-RAM |
| BGPRI==0 | | BGPRI==1 |
+-----------+ +-----------+
CI stands for 'Colour Intensity', which is synonmous with D7-D5 of $2001. BG is the BackGround, and EXT is for the EXTension port video signal.
'BGPRI' represents the 'Background Priority' bit in SPR-RAM, on a per-sprite basis (D5, Byte 2).
OBJ numbers represent actual Sprite numbers, not Tile Index values.
FRONT is considered what is seen atop all other layers (drawn last), and BACK is deemed what is below most other layers (drawn first).
If a sprite with BGPRI==1 has a lower index in SPR-RAM than a sprite with BGPRI==0, the front-to-back ordering is more complicated. See PPU sprite priority for full details.
Sprites and OAM
The NES supports 64 sprites, which can be either 8x8 or 8x16 pixels in size. The sprite data is kept within the Pattern Table region of VRAM.
Sprite attributes such as flipping and priority, are stored in SPR-RAM, which is a separate 256 byte area of memory, independent of ROM and VRAM. The format of SPR-RAM is as follows:
+-----------+-----------+-----+------------+
| Sprite #0 | Sprite #1 | ... | Sprite #63 |
+-+------+--+-----------+-----+------------+
| |
+------+----------+--------------------------------------+
+ Byte | Bits | Description |
+------+----------+--------------------------------------+
| 0 | YYYYYYYY | Y Coordinate - 1. Consider the coor- |
| | | dinate the upper-left corner of the |
| | | sprite itself. |
| 1 | IIIIIIII | Tile Index # |
| 2 | vhp000cc | Attributes |
| | | v = Vertical Flip (1=Flip) |
| | | h = Horizontal Flip (1=Flip) |
| | | p = Background Priority |
| | | 0 = In front |
| | | 1 = Behind |
| | | c = Upper two (2) bits of colour |
| 3 | XXXXXXXX | X Coordinate (upper-left corner) |
+------+----------+--------------------------------------+
The Tile Index # is obtained the same way as Name Table data.
Sprites which are 8x16 in size function a little bit differently. An 8x16 sprite which has an even-numbered Tile Index # use the Pattern Table at $0000 in VRAM; odd-numbered Tile Index #s use $1000.
- NOTE*: Register $2000 D3 has no effect on 8x16 sprites.
All 64 sprites contain an internal priority; sprite #0 is of a higher priority than sprites #63 (sprite #0 should be drawn last, etc.).
Only eight (8) sprites can be displayed per scan-line. Each entry in SPR-RAM is checked to see if it's in a horizontal range with the other sprites. Remember, this is done on a per scan-line basis, not on a per sprite basis (e.g. done 256 times, not 256/8 or 256/16 times).
NOTE: On a real NES unit, if both background and sprites are disabled (D4-D3 of $2001 is 00) for a long period of time, SPR-RAM will gradually degrade. Photographs of a decapped PPU have confirmed that SPR-RAM is actually DRAM, and rendering controls the DRAM refresh cycle.
Sprite #0 Hit Flag
The PPU is capable of figuring out where Sprite #0 is, and stores its findings in D6 of $2002. The way this works is as follows:
As the PPU draws each line, it scans for the first actual non-transparent "sprite pixel" and the first non-transparent "background pixel." A "background pixel" is a tile which is in use by the Name Table. Remember that colour #0 defines transparency.
The pixel which causes D6 to be set *IS* drawn.
The following example should help. The following are two tiles. Transparent colours (colour #0) are defined via the underscore ('_') character. An asterisk ('*') represents when D6 will be set.
Sprite BG Result
------ -- ------
__1111__ ________ __1111__
_111111_ _______2 _1111112
11222211 ______21 11222211
112__211 + _____211 = 112__*11 '*' will be drawn as colour #2
112__211 ____2111 112_2211
11222211 ___21111 11222211
_111111_ __211111 _1111111
__1111__ _2111111 _2111111
This also applies to sprites that are underneath the BG (via the 'Background Priority' SPR-RAM bit), though the above example would be 'BG+Sprite'.
However, D6 does not get set if the overlap is at the far right of the screen (X==255).
Also, D6 is cleared (set to 0) at the end of each VBlank.
Horizontal and Vertical Blanking
The NES, like every console, has a refresh: where the display device relocates the electron gun to display visible data. The most common display device is a television set. The refresh occurs 60 times a second on an NTSC device, and 50 on a PAL device.
The gun itself draws pixels left to right: this process results in one (1) horizontal scanline being drawn. After the gun is done drawing the entire scanline, the gun must return to the left side of the display device, becoming ready to draw the next scanline. The process of the gun returning to the left side of the display is the Horizontal Blank period (HBlank).
When the gun has completed drawing all of the scanlines, it must return to the top of the display device. The time it takes for the gun to re-position itself atop the device is called the Vertical Blank period (VBlank).
As you can see from the below diagram, the gun more or less works in a zig-zag pattern until VBlank is reached, then the process repeats:
+-----------+
+--->|***********| <-- Scanline 0
| | ___---~~~ | <-- HBlank
V |***********| <-- Scanline 1
B | ___---~~~ | <-- HBlank
l | ... | ...
a | ... | ...
n |***********| <-- Scanline 239
k +-----+-----+
| |
+--VBlank--+
The screen is 256 pixels wide, and the HBlank time that follows each line adds an additional 85 for a total of 341.
An NTSC NES has the following refresh and screen layout:
0 256 340
+--------+ | 0 --+
| | | |
| | H | |
| Screen | B | +-- (0-239) 256x240 on-screen results
| | l | |
| | | |
+--------+ | 239 --+
+--------+ 240 --+-- (240) Post-render
+--------+ 241 --+
| | |
| VBlank | +-- (241-260) VBlank
| | |
+--------+ 260 --+
+--------+ 261 --+-- (261) Pre-render
The Vertical Blank (VBlank) flag is contained in D7 of $2002. It indicates whether PPU is in VBlank or not. A program can reset D7 by reading $2002. Reading $2002 at the very start of VBlank will reset $2002 even though D7 appears false.
$2005/$2006 Magic
Games can split the screen by changing the video memory address using both the $2005 and $2006 registers. This is often used to draw a status bar at the top or bottom of the screen. For detailed information pertaining to splits, refer to Loopy's $2005/2006 document. His document provides entirely accurate information regarding how these registers work. See PPU scrolling for more information
Reading VRAM
The first byte read from VRAM is invalid. Due to this aspect, the NES will returned pseudo-buffered values from VRAM rather than linear as expected. See the below example:
- VRAM $2000 contains $AA $BB $CC $DD.
- VRAM incrementation value is 1.
- The result of execution is printed in the comment field.
LDA #$20
STA $2006
LDA #$00
STA $2006 ; VRAM address now set at $2000
LDA $2007 ; A=?? VRAM Buffer=$AA
LDA $2007 ; A=$AA VRAM Buffer=$BB
LDA $2007 ; A=$BB VRAM Buffer=$CC
LDA #$20
STA $2006
LDA #$00
STA $2006 ; VRAM address now set at $2000
LDA $2007 ; A=$CC VRAM Buffer=$AA
LDA $2007 ; A=$AA VRAM Buffer=$BB
As shown, the PPU will post-increment it's internal address data after the first read is performed. This only applies to VRAM $0000-3EFF (e.g. Palette data and their respective mirrors do not suffer from this phenomenon).
Notes
The PPU will auto-increment the VRAM address by 1 or 32 (based on D2 of $2000) after accessing $2007.
While rendering is off ($2001 D4-D3 set to 00), and the current video memory address is at $3F00-$3FFF, the PPU outputs the palette entry at that address. To avoid streaks of rainbow colours, make sure to update the palette during VBlank.
pAPU
This section was not completed in time for Nestech.txt version 2.00. See APU for accurate information.
Joypads, paddles, expansion ports
General Information
The NES supports several different input devices, including joypads, Zapper (light guns), and four-player devices.
Joypad #1 and #2 are read via $4016 and $4017, respectively.
The joypads are reset via a strobing-method: writing 1, then 0, to $4016. This address controls the strobe on both joypads. See "Expansion ports" for information regarding "half-strobing."
After a full strobe, the joypad's button status will be returned in a single-bit stream (D0). Multiple reads need to be made to read all the information about the controller.
The standard controller can be read 8 times, once for each button:
1 = A
2 = B
3 = SELECT
4 = START
5 = UP
6 = DOWN
7 = LEFT
8 = RIGHT
Reads after the eighth return a 1 bit on most controllers. A few return 0 instead.
The Zapper
The Zapper (otherwise known as the "Light Gun") uses bits within $4016 and $4017, described in Section 8. D4 and D3 are connected to the trigger and light sensor respectively. The light sensor needs to be read throughout the frame, as it returns a light signal for only about 2000 cycles.
It is possible to have two Zapper units connected to both joypad ports simultaneously. The unlicensed games Chiller and Zap Ruder use this.
Four-player devices
Some NES games allow the use of a four-player adapter, extending the number of usable joypads from two (2) to four (4). Carts which use the NES Four Score or NES Satellite are Mindscape/Tengen's Gauntlet II and Nintendo/Rare's R.C. Pro-Am 2.
All four (4) controllers read their status-bits from D0 of $4016 or $4017, as Subsection A states.
For register $4016, reads #1-8 control joypad #1, reads #9-16 control joypad #3, and reads #17-24 return a signature. For $4017, it is respective for joypad #2 and #4.
The following is a list of read #s and their results.
1 = A 9 = A 17 = +--+
2 = B 10 = B 18 = +-- Signature
3 = SELECT 11 = SELECT 19 = |
4 = START 12 = START 20 = +--+
5 = UP 13 = UP 21 = 0
6 = DOWN 14 = DOWN 22 = 0
7 = LEFT 15 = LEFT 23 = 0
8 = RIGHT 16 = RIGHT 24 = 0
The Famicom has a different four-player adapter. On that system (not the NES), $4016 reads controllers 1 and 3 at the same time, with controller 1 in D0 and controller 3 in D1. Likewise, $4016 returns controller 2 in D0 and controller 4 in D1. In Famicom games that do not use four players, the player expects to be able to use controllers 3 and 4 as if they were controllers 1 and 2.
Paddles
Taito's Arkanoid uses a paddle as it's primary controller.
The paddle position on the Famicom version is read via D1 of $4017; the read data is inverted (0=1, 1=0). The first value read is the MSB, and the 8th value read is (obviously) the LSB. The valid value range is about 160 units wide. One unit was measured with a range 98 to 242, where 98 represents the paddle being turned completely counter-clockwise.
For example, if %01101011 is read, the value would be NOT'd, making %10010100 which is 146.
The paddle also contains one button, which is read via D1 of $4016. A value of 1 specifies that the button is being pressed.
On the NES version of the Arkanoid, D3 and D4 of $4017 are used instead. See Arkanoid controller for full information.
Signatures
A signature allows the programmer to detect if a device is connected to one of the four (4) ports or not, and if so, what type of device it is.
The NES Four Score and NES Satellite have a signature in bits 17-24:
- %0001 0000 = Joypad ($4016 only)
- %0010 0000 = Joypad ($4017 only)
Other controllers may or may not have a signature.
Expansion ports
The joypad strobing process requires dual writes: 1, then 0. Some specialised controllers, especially for the Famicom's front expansion port, use a non-standard order for the strobing process.
For example, reading a Super NES Mouse while it is half strobed changes its sensitivity. A program would execute the following code:
LDA #%00000001 ; Begin half strobe
STA $4016
LDA $4017 ; Send one clock while half strobed
LDA #%00000000 ; End half strobe
STA $4016
Memory Mapping Hardware
Due to the hundreds of different mappers in use, both discrete logic and integrated MMCs, this document cannot describe them all. See Mapper for complete information.
Registers
Programmers communicate with the PPU and pAPU via registers, which are nothing more than pre-set memory locations which allow the coder to make changes to the NES. Without registers, programs would have no way to communicate with the outside world.
Each register is a 16-bit address. Each register has a statistics field in parentheses located immediately after its description. The legend:
R = Readable W = Writable 2 = Double-write register 16 = 16-bit register
Notes:
- 16-bit registers actually consist of two linear 8-bit registers, which can (and will be) *INDEPENDENTLY* assigned. The reason for specifying them as 16-bit is for ease of documentation. For instance, "$4002+$4003" would mean that D15-D8 would be in $4003, and D7-D0 would be in $4002.
- Bits not listed are to be considered unused.
+---------+----------------------------------------------------------+
| Address | Description |
+---------+----------------------------------------------------------+
| $2000 | PPU Control Register #1 (W) |
| | |
| | D7: Execute NMI on VBlank |
| | 0 = Disabled |
| | 1 = Enabled |
| | D6: PPU Master/Slave Selection --+ Always write 0 |
| | 0 = Receive EXTBG +-- in unmodified |
| | 1 = Send EXTBG --+ Control Deck |
| | D5: Sprite Size |
| | 0 = 8x8 |
| | 1 = 8x16 |
| | D4: Background Pattern Table Address |
| | 0 = $0000 (VRAM) |
| | 1 = $1000 (VRAM) |
| | D3: Sprite Pattern Table Address |
| | 0 = $0000 (VRAM) |
| | 1 = $1000 (VRAM) |
| | D2: PPU Address Increment |
| | 0 = Increment by 1 |
| | 1 = Increment by 32 |
| | D1-D0: Name Table Address |
| | 00 = $2000 (VRAM) |
| | 01 = $2400 (VRAM) |
| | 10 = $2800 (VRAM) |
| | 11 = $2C00 (VRAM) |
+---------+----------------------------------------------------------+
| $2001 | PPU Control Register #2 (W) |
| | |
| | D7-D5: Colour Intensity |
| | 000 = None +--+ NOTE: Some TVs don't |
| | 001 = Intensify red | sync well if more |
| | 010 = Intensify green | than one type used; |
| | 100 = Intensify blue +--+ PAL swaps D6/D5 |
| | D4: Sprite Visibility |
| | 0 = Sprites not displayed |
| | 1 = Sprites visible |
| | D3: Background Visibility |
| | 0 = Background not displayed |
| | 1 = Background visible |
| | D2: Sprite Clipping |
| | 0 = Sprites invisible in left 8-pixel column |
| | 1 = No clipping |
| | D1: Background Clipping |
| | 0 = BG invisible in left 8-pixel column |
| | 1 = No clipping |
| | D0: Display Type |
| | 0 = Colour display |
| | 1 = Monochrome display (all palette values |
| | ANDed with $30) |
+---------+----------------------------------------------------------+
| $2002 | PPU Status Register (R) |
| | |
| | D7: VBlank Occurance |
| | 0 = Not occuring |
| | 1 = In VBlank |
| | D6: Sprite #0 Hit |
| | 0 = Sprite #0 not found |
| | 1 = PPU has hit Sprite #0 since end of VBlank |
| | D5: Scanline Sprite Count |
| | 0 = No scanline with more than eight (8) |
| | sprites |
| | 1 = At least one line with more than 8 sprites |
| | since end of VBlank |
| | |
| | NOTE: D7 is set to 0 after read occurs. |
| | NOTE: After a read occurs, $2005 is reset, hence the |
| | next write to $2005 will be Horizontal. |
| | NOTE: After a read occurs, $2006 is reset, hence the |
| | next write to $2006 will be the high byte portion. |
| | NOTE: D4-D0 return the last value written to any PPU |
| | register. |
| | |
| | For detailed information regarding D6, see Section 4, |
| | Subsection L. |
+---------+----------------------------------------------------------+
| $2003 | SPR-RAM Address Register (W) |
| | |
| | D7-D0: 8-bit address in SPR-RAM to access via $2004. |
| | |
| | NOTE: The SPR-RAM DRAM controller is very touchy. |
| | Write $00 here. |
+---------+----------------------------------------------------------+
| $2004 | SPR-RAM I/O Register (W) |
| | |
| | D7-D0: 8-bit data written to SPR-RAM. |
| | |
| | NOTE: It is strongly recommended to use $4014 instead. |
+---------+----------------------------------------------------------+
| $2005 | VRAM Address Register #1 (W2) |
| | |
| | Commonly used used to "pan/scroll" the screen (sprites |
| | excluded) horizontally and vertically. However, there |
| | is no actual panning hardware inside the NES. This |
| | register controls VRAM addressing lines. |
| | |
| | Refer to Section 4, Subsection N, for more information. |
+---------+----------------------------------------------------------+
| $2006 | VRAM Address Register #2 (W2) |
| | |
| | Commonly used to specify the 16-bit address in VRAM to |
| | access via $2007. However, this register controls VRAM |
| | addressing bits, and therefore should be used with |
| | knowledge of how it works, and when it works. |
| | |
| | Refer to Section 4, Subsection N, for more information. |
+---------+----------------------------------------------------------+
| $2007 | VRAM I/O Register (RW) |
| | |
| | D7-D0: 8-bit data read/written from/to VRAM. |
+---------+----------------------------------------------------------+
| $4000 | pAPU Pulse #1 Control Register (W) |
| $4001 | pAPU Pulse #1 Ramp Control Register (W) |
| $4002 | pAPU Pulse #1 Fine Tune (FT) Register (W) |
| $4003 | pAPU Pulse #1 Coarse Tune (CT)/Length Register (W) |
| $4004 | pAPU Pulse #2 Control Register (W) |
| $4005 | pAPU Pulse #2 Ramp Control Register (W) |
| $4006 | pAPU Pulse #2 Fine Tune Register (W) |
| $4007 | pAPU Pulse #2 Coarse Tune/Length Register (W) |
| $4008 | pAPU Triangle Linear Counter Register #1 (W) |
| $4009 | Unused |
| $400A | pAPU Triangle Fine Tune Register Register #1 (W) |
| $400B | pAPU Triangle Coarse Tune/Length Register #2 (W) |
| $400C | pAPU Noise Control Register #1 (W) |
| $400D | Unused |
| $400E | pAPU Noise Frequency Register (W) |
| $400F | pAPU Noise Length Register (W) |
| $4010 | pAPU Delta Modulation Control Register (W) |
| $4011 | pAPU Delta Modulation D/A Register (W) |
| $4012 | pAPU Delta Modulation Address Register (W) |
| $4013 | pAPU Delta Modulation Data Length Register (W) |
+---------+----------------------------------------------------------+
| $4014 | Sprite DMA Register (W) |
| | |
| | Transfers 256 bytes of memory into SPR-RAM. The address |
| | read from is $100*N, where N is the value written. |
+---------+----------------------------------------------------------+
| $4015 | pAPU Sound/APU Frame Counter Register (R) |
| | |
| | D6: APU Frame IRQ Availability |
| | 0 = One (1) frame occuring, hence IRQ cannot |
| | occur |
| | 1 = One (1) frame is being interrupted via IRQ |
| | D4: Delta Modulation |
| | D3: Noise |
| | D2: Triangle |
| | D1: Pulse #2 |
| | D0: Pulse #1 |
| | 0 = Not in use |
| | 1 = In use |
| +----------------------------------------------------------+
| | pAPU Channel Control (W) |
| | |
| | D4: Delta Modulation |
| | D3: Noise |
| | D2: Triangle |
| | D1: Pulse #2 |
| | D0: Pulse #1 |
| | 0 = Channel disabled |
| | 1 = Channel enabled |
+---------+----------------------------------------------------------+
| $4016 | Joypad #1 (RW) |
| | |
| | READING: |
| | D4: Zapper Trigger |
| | 0 = Pulled or released |
| | 1 = Half pulled |
| | D3: Zapper Light Detection |
| | 0 = Light in front of barrel |
| | 1 = Dark in front of barrel |
| | D1: Famicom Expansion Joypad Data |
| | D0: Joypad Data |
| +----------------------------------------------------------+
| | WRITING: |
| | Joypad Strobe (W) |
| | |
| | D0: Joypad Strobe |
| | 0 = Finish polling both joypads |
| | 1 = Poll both joypads |
+---------+----------------------------------------------------------+
| $4017 | Joypad #2/SOFTCLK (RW) |
| | |
| | READING: |
| | D4: Zapper Trigger |
| | 0 = Pulled or released |
| | 1 = Half pulled |
| | D3: Zapper Light Detection |
| | 0 = Light in front of barrel |
| | 1 = Dark in front of barrel |
| | D1: Famicom Expansion Joypad Data |
| | D0: Joypad Data |
+---------+----------------------------------------------------------+
File Formats
iNES Format (.NES)
Most common ROMs will be found in this format:
+--------+------+------------------------------------------+
| Offset | Size | Content(s) |
+--------+------+------------------------------------------+
| 0 | 3 | 'NES' |
| 3 | 1 | $1A |
| 4 | 1 | 16K PRG-ROM page count |
| 5 | 1 | 8K CHR-ROM page count |
| 6 | 1 | ROM Control Byte #1 |
| | | %####vTsM |
| | | | ||||+- 0=Horizontal mirroring |
| | | | |||| 1=Vertical mirroring |
| | | | |||+-- 1=SRAM enabled |
| | | | ||+--- 1=512-byte trainer present |
| | | | |+---- 1=Four-screen mirroring |
| | | | | |
| | | +--+----- Mapper # (lower 4-bits) |
| 7 | 1 | ROM Control Byte #2 |
| | | %####0000 |
| | | | | |
| | | +--+----- Mapper # (upper 4-bits) |
| 8-15 | 8 | $00 |
| 16-.. | | Actual 16K PRG-ROM pages (in linear |
| ... | | order). If a trainer exists, it precedes |
| ... | | the first PRG-ROM page. |
| ..-EOF | | CHR-ROM pages (in ascending order). |
+--------+------+------------------------------------------+
See iNES for full details.
Some ROMs using more obscure mappers require an extension to the iNES format called "NES 2.0", first proposed by Kevin Horton. See NES 2.0 for full details.
Programming the NES
General Information
None.
CPU Notes
None. See "CPU Notes" in Section 11 "Emulation" for more possible information.
PPU Notes
Reading and writing to VRAM consists of a multi-step process:
Writing to VRAM Reading from VRAM
--------------- -----------------
1) Wait for VBlank 1) Wait for VBlank
2) Write upper VRAM address 2) Write upper VRAM address
byte into $2006 byte into $2006
3) Write lower VRAM address 3) Write lower VRAM address
byte into $2006 byte into $2006
4) Write one or more bytes 4) Read $2007 (invalid data once)
to $2007 5) Read one or more bytes
from $2007
NOTE: Step #4 when reading VRAM is only necessary when reading VRAM data not in the $3F00-3FFF range.
NOTE: Accessing VRAM should only be performed during VBlank, or with rendering turned off ($2001 D4-D3 cleared to 0). Attempts to access VRAM outside of VBlank will usually result in garbage showing up on the screen. See Section 4, Subsection N for more information regarding why this occurs.
Waiting for VBlank is quite simple:
8000: LDA $2002
BPL $8000
Reading $2002 will result in all bits being returned; however, D7 will be reset to 0 after the read is performed. To ensure the PPU is warmed up, this should be done twice before any access to $2003-$2007.
Waiting for VBlank during a game should be done by waiting for the NMI handler to change a variable:
8000: INC $FF
RTI
8003: LDA $FF
8004: CMP $FF
BNE $8003
The actual on-screen palette used by the NES, as stated prior, is not RGB. However, a near-exact replica can be found in common NES emulators today. See PPU palettes to obtain a valid RGB palette.
You will often encounter a situation where a palette fade or a VRAM update will cause the screen "to be trashed" (squares on the screen, or what seem to be graphical "glitches"). The reason for this is that your code took longer than a VBlank. When the VBlank ends, the PPU begins to refresh the screen again, taking whatever value is in the internal VRAM address and uses that as the starting base for Name Table #0. The solution is to fix your code to take no more than 2273 cycles, which is 20 scanlines times 341 pixels per scanline divided by 3 pixels per cycle.
In addition, after you update video memory through $2006 and $2007, the scroll position becomes corrupt, and the PPU again begins at the internal VRAM address. To solve this, make sure to reset the scroll position through $2005 and $2000 before the next frame begins. Such code would be:
LDA #$00
STA $2005
STA $2005
LDA #$88
STA $2000
If your game scrolls, use different values for $2005 and for $2000 D1-D0.
Emulation
General Information
If you're going to be programming an emulator in C or C++, please be familiar with pointers. Being familiar with pointers will help you out severely when it comes to handling mirroring and VRAM addressing. For you assembly buffs out there, obviously pointers are nothing more than indirect addressing -- it's easier to change a 32-bit value than to swap in and out an entire 64K of data.
When SRAM ($6000-7FFF) is disabled, writes to the memory area should be ignored. Reads will possibly return data previously left on the bus, and therefore when emulated should return the previous bus value. This will usually (but not always) be the high byte of the address. See Open bus for more details.
RAM-based memory areas ($0000-07FF) should *NOT* be zeroed on RESET; they should be set to some value on power on. Please make sure that a cold boot and a warm boot do different things. (Technically, the RAM is not zeroed on power on either: the RAM will slowly dissipate over time when the unit is off.)
CPU Notes
The NES uses the NMOS 6502, not a CMOS 6502 or any other variant.
There are 154 valid opcodes (out of 256 total) on the NES. Please support the option of trapping opcodes which are bad. Sometimes a ROM contains bad opcodes, due to dirty connectors on the cartridge during the extraction process (or other reasons). But other games out there, such as Puzznic and Super Cars, contain bad opcodes even in a correct dump.
PPU Notes
The formulae to calculate the base address of a Name Table tile number is:
(TILENUM * 16) + PATTERNTABLE
Where TILENUM is the tile number in the Name Table, and PATTERNTABLE is the Pattern Table Address defined via register $2000.
Most emulators do not limit the number of sprites which can be displayed per scanline, while the actual NES will show flicker as a result of more than eight (8). One supports 15 sprites per line by changing the order of sprite-related Pattern Table fetches during HBlank.
Emulators should _NOT_ mask out unused bits within registers; doing so may result in a cart not working.
Reference Material
Many of these no longer exist:
Mailing Lists
There is a NES Development Mailing List in existence. Contact Mark Knibbs for more information. This list is for anyone who wishes to discuss technical issues about the NES; it is not a list for picking up the latest and greatest information about NES emulators or what not.
However, most users have since migrated to a phpBB forum at https://forums.nesdev.org/
WWW Sites
The following are a list of WWW sites which contain NES-oriented material. If you encounter errors, bad links, or other anomalies while visiting these sites, contact the site authors/owners, NOT me. Thanks.
- https://nesdev.org/
- Contains a verbose amount of documentation regarding anything NES-oriented, including hard-to-find mapper documentation. Seems to be a decent NES information depository.
- http://www.ameth.org/~veilleux/NES_info.html
- Currently only contains hardware-oriented material, such as overviews of cart and unit ASICs, mappers, and MMCs. Many pinout diagrams for mappers and NES units are available here. Also provides documentation on NES repair, modifying your NES to give fake stereo output, applying stereo mixing to your NES, and much much more.
Hardware Information
The following security bits may be purchased from MCM Electronics (http://www.mcmelectronics.com/):
- For NES, Game Boy, Super NES, and Nintendo 64 Game Paks: 22-1145 (3.8mm security bit)
- For NES, Super NES, and Nintendo 64 Control Deck: 22-1150 (4.5mm security bit)
Other sellers may offer them under the "Line" or "GameBit".
An alternate method is to cut a notch in a standard flat head screwdriver that grips two of the six indentations on the screw.
See also
- Nestech.txt errata - Errata from version 2.00 used to produce this updated document
