Open bus behavior

When the CPU attempts to read from an address which has no devices active, the result is open bus behavior. The value that results from a read in such a region is undefined, but on specific hardware predictable results may appear.

For example, a few games may make reads to the region $6000-7FFF, but have no WRAM present here. This can be a problem for emulators, as the original iNES file format had no way to specify a lack of WRAM, leaving the emulator to provide WRAM behavior in that region by default.

On a standard NES, reading open bus repeats the last value that was read from the bus before this read. For many instructions, this will be the high byte of the address being read, though for immediate instructions, it will simply be the last byte of the instruction. A DMC DMA may also alter the last value read if it interrupts an instruction.^{[verification needed]} An indexed read that crosses a 256-byte (page) boundary will return data from the previous page.

Open bus can be limited to part of a byte. The controller ports ($4016 and $4017) affect only bits 4-0. Bits 7-5 repeat the corresponding bits from the previous read, usually 010 from the high byte $40.

Open bus behavior is relatively rare. In general, where games rely on it, it seems to be unintentional due to a programming error. Known examples:

• Castlevania - Just after the introduction will read a few values from ~$7FFX before the first stage begins.
• Low G Man - WRAM instead of open bus behavior may cause a crash Chapter 1 Scene 3B a few seconds into the boss fight music, or graphical glitches when using the boomerang weapon.
• Battletoads & Double Dragon - At the end of the first level, reads a values from ~$600X when the Abobo boss finished destroying the wall. A value of $00 read here will crash the game.
• Paperboy - Relies on the controller port being exactly $41 for a button press to be recognized.

PPU open bus

The PPU has two data buses: the I/O bus, used to communicate with the CPU, and the video memory bus. Their open bus behavior differs.

Writes to any PPU port, including the nominally read-only status port at $2002, load a value onto the PPU's I/O bus. Reading a value from $2004 or $2007 loads a byte from OAM, video memory, or the palette onto this bus. Reading the PPU's status port loads a value onto bits 7-5 of the bus, leaving the rest unchanged. Reading any PPU port, including write-only ports $2000, $2001, $2003, $2005, $2006, returns the PPU I/O bus's value.

Open bus on the video memory bus behaves differently. Video memory's data bus is multiplexed with the low byte of the address bus on pins 31 through 38. Thus a read from an address with no memory connected will usually return the low byte of the address. This is used only for copy protection if at all.

Electrical basis

A data bus behaves as a dynamic latch. Once a low or high voltage is placed on each line of the bus, the capacitance of the long traces that make up the data bus holds it in place to be read again.

See also

• Bus conflict