Hardware bugs and quirks

From NESdev Wiki
Revision as of 16:22, 29 October 2015 by Furrykef (talk | contribs) (Created page with "== CPU == The NES uses the 6502 as its CPU core, which has a few bugs and quirks of its own. The only behavior different from...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigationJump to search

CPU

The NES uses the 6502 as its CPU core, which has a few bugs and quirks of its own. The only behavior different from a standard 6502 is the NES's decimal flag is inactive; it will behave as though it is always unset.

Some Famiclones may use a 65C02, which does have a working decimal flag and does not have the 6502's undocumented instructions. Official hardware never uses a 65C02.

PPU

  • Setting bit 6 of PPUCTRL ($2000) on a stock NES can potentially damage the PPU.
  • Setting PPUADDR ($2006) may corrupt the scroll registers. Be sure to set PPUCTRL ($2000) and PPUSCROLL ($2005) after you're done with PPUADDR.
  • Setting OAMADDR ($2003) corrupts OAM. Usually you'll just want to set it to $00 and follow it with OAMDMA in your vblank routine, which will help with other OAM bugs as well.
  • If OAMADDR is greater than 7 at the end of vblank, OAM may be corrupted.
  • Blanking the display mid-frame can corrupt OAM.
  • The sprite overflow flag can have false positives and false negatives.

APU

  • Playing DMC samples can corrupt controller reads. Usually circumvented by re-reading the controller until two consecutive reads match.
  • Writing to $4003 or $4007 will reset the phase and envelope of the channel. This means changing the high bits of a note's frequency will retrigger the note, which can cause popping noises during software sweeps. Similarly, writing to $400B will reload the triangle's linear counter.
  • A negative sweep on pulse channel 1 will subtract the shifted period value minus 1. This does not happen on pulse channel 2.