CPU power up state: Difference between revisions
From NESdev Wiki
Jump to navigationJump to search
(→At power-up: PC and P aren't actually written to the stack, it just goes through the motions) |
(→At power-up: P is not 8 bits wide. Don't define the other two bits because they don't exist.) |
||
| (8 intermediate revisions by 3 users not shown) | |||
| Line 2: | Line 2: | ||
==At power-up== | ==At power-up== | ||
: | :Status: Carry, Zero, Decimal, Overflow, Negative clear. Interrupt Disable set. | ||
:A, X, Y = 0 | :A, X, Y = 0 | ||
:S = $FD<ref name="reset-stack-push">RESET uses the logic shared with NMI, IRQ, and BRK that would push PC and P. However, like [ | :S = $FD<ref name="reset-stack-push">RESET uses the logic shared with NMI, IRQ, and BRK that would push PC and P. However, like [[Visual6502wiki/6502 BRK and B bit#masking of the stack writes during RESET|some but not all 6502s]], the 2A03 prohibits writes during reset. [https://forums.nesdev.org/viewtopic.php?p=184247#p184247 This test] relies on open bus being precharged by these reads. See [https://www.youtube.com/watch?v=fWqBmmPQP40&t=41m45s 27c3: Reverse Engineering the MOS 6502 CPU (en)] from 41:45 onward for details</ref> | ||
:$4017 = $00 (frame irq enabled) | :$4017 = $00 (frame irq enabled) | ||
:$4015 = $00 (all channels disabled) | :$4015 = $00 (all channels disabled) | ||
:$4000-$400F = $00 | :$4000-$400F = $00 | ||
:$4010-$4013 = $00 <ref>[https://forums.nesdev.org/viewtopic.php?t=18278 Eliminator Boat Duel]</ref> | |||
:All 15 bits of noise channel LFSR = $0000<ref>[https://forums.nesdev.org/viewtopic.php?p=172797#p172797 Noise channel init log]</ref>. The first time the LFSR is clocked from the all-0s state, it will shift in a 1. | :All 15 bits of noise channel LFSR = $0000<ref>[https://forums.nesdev.org/viewtopic.php?p=172797#p172797 Noise channel init log]</ref>. The first time the LFSR is clocked from the all-0s state, it will shift in a 1. | ||
:APU Frame Counter: | |||
:: 2A03E, G, various clones: [[APU Frame Counter]] reset. | |||
:: 2A03letterless: APU frame counter powers up at a value equivalent to 15 | |||
:Internal memory ($0000-$07FF) has unreliable startup state. Some machines may have consistent RAM contents at power-on, but others do not. | :Internal memory ($0000-$07FF) has unreliable startup state. Some machines may have consistent RAM contents at power-on, but others do not. | ||
| Line 20: | Line 24: | ||
:APU mode in $4017 was unchanged | :APU mode in $4017 was unchanged | ||
:APU was silenced ($4015 = 0) | :APU was silenced ($4015 = 0) | ||
:APU triangle phase is reset to 0 (i.e. outputs a value of 15, the first step of its waveform) | |||
:APU DPCM output ANDed with 1 (upper 6 bits cleared) | |||
:: APU Frame Counter: | |||
:: 2A03E, G, various clones: [[APU Frame Counter]] reset. | |||
:: 2A03letterless: APU frame counter retains old value <ref>[https://forums.nesdev.org/viewtopic.php?p=214939#p214939 2A03letterless is missing transistor to set frame counter LFSR on reset]</ref> | |||
== See also == | == See also == | ||
*[[PPU power up state]] | *[[PPU power up state]] | ||
== Notes == | == Notes == | ||
Latest revision as of 08:41, 20 March 2024
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
- Status: Carry, Zero, Decimal, Overflow, Negative clear. Interrupt Disable set.
- A, X, Y = 0
- S = $FD[1]
- $4017 = $00 (frame irq enabled)
- $4015 = $00 (all channels disabled)
- $4000-$400F = $00
- $4010-$4013 = $00 [2]
- All 15 bits of noise channel LFSR = $0000[3]. The first time the LFSR is clocked from the all-0s state, it will shift in a 1.
- APU Frame Counter:
- 2A03E, G, various clones: APU Frame Counter reset.
- 2A03letterless: APU frame counter powers up at a value equivalent to 15
- 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)[1]
- 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)
- APU triangle phase is reset to 0 (i.e. outputs a value of 15, the first step of its waveform)
- APU DPCM output ANDed with 1 (upper 6 bits cleared)
- APU Frame Counter:
- 2A03E, G, various clones: APU Frame Counter reset.
- 2A03letterless: APU frame counter retains old value [4]
See also
Notes
- ↑ 1.0 1.1 RESET uses the logic shared with NMI, IRQ, and BRK that would push PC and P. However, like some but not all 6502s, the 2A03 prohibits writes during reset. This test relies on open bus being precharged by these reads. See 27c3: Reverse Engineering the MOS 6502 CPU (en) from 41:45 onward for details
- ↑ Eliminator Boat Duel
- ↑ Noise channel init log
- ↑ 2A03letterless is missing transistor to set frame counter LFSR on reset
