CPU power up state

The following results are from a US (NTSC) NES, original front-loading design, RP2A03G CPU chip, NES-CPU-07 main board revision, manufactured in 1988. The memory values are probably slightly different for each individual NES console. Please note that you should NOT rely on the state of any registers after Power-UP and especially not the stack register and RAM ($0000-$07FF).

At power-up

P = $34 (IRQ disabled)

A, X, Y = 0

S = $FD

$4017 = $00 (frame irq enabled)

$4015 = $00 (all channels disabled)

$4000-$400F = $00 (not sure about $4010-$4013)

Internal memory ($0000-$07FF) has unreliable startup state. Some machines may have consistent RAM contents at power-on, but others do not.

• Emulators often implement a consistent RAM startup state (e.g. all $00 or $FF, or a particular pattern), and flash carts like the PowerPak may partially or fully initialize RAM before starting a program, so an NES programmer must be careful not to rely on the startup contents of RAM.

After reset

A, X, Y were not affected

S was decremented by 3 (but nothing was written to the stack)

The I (IRQ disable) flag was set to true (status ORed with $04)

The internal memory was unchanged

APU mode in $4017 was unchanged

APU was silenced ($4015 = 0)

See also

• Power-up state of PPU