CPU power up state: Difference between revisions
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(Clarified irrelevant bits 5 and 4 of P per http://nesdev.parodius.com/bbs/viewtopic.php?p=86248#86248) |
(Koitsu raised a good point at http://nesdev.parodius.com/bbs/viewtopic.php?p=89675#89675 about how ESLers might not understand "inhibit") |
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| Line 2: | Line 2: | ||
At power-up | At power-up | ||
:P = $34 ( | :P = $34 (IRQ disabled)* | ||
:A, X, Y = 0 | :A, X, Y = 0 | ||
:S = $FD | :S = $FD | ||
:All internal memory ($0000-$07FF) was consistently set to $ff except | :All internal memory ($0000-$07FF) was consistently set to $ff except for a few bytes, which probably vary from console to console: | ||
:*$0008=$F7 | :*$0008=$F7 | ||
:*$0009=$EF | :*$0009=$EF | ||
| Line 18: | Line 18: | ||
:A, X, Y were not affected | :A, X, Y were not affected | ||
:S was decremented by 3 (but nothing was written to the stack) | :S was decremented by 3 (but nothing was written to the stack) | ||
:The I flag was set (status ORed with $04) | :The I (IRQ disable) flag was set to true (status ORed with $04) | ||
:The internal memory was unchanged | :The internal memory was unchanged | ||
:APU mode in $4017 was unchanged | :APU mode in $4017 was unchanged | ||
Revision as of 17:13, 7 February 2012
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 WRAM ($0000-$07FF).
At power-up
- P = $34 (IRQ disabled)*
- A, X, Y = 0
- S = $FD
- All internal memory ($0000-$07FF) was consistently set to $ff except for a few bytes, which probably vary from console to console:
- $0008=$F7
- $0009=$EF
- $000a=$DF
- $000f=$BF
- $4017 = $00 (frame irq enabled)
- $4015 = $00 (all channels disabled)
- $4000-$400F = $00 (not sure about $4010-$4013)
- IRQ was first asserted about 1/60 second after power-up, by APU
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)
Notes
- The golden log of nestest differs from this in the irrelevant bits 5 and 4 of P.
