User:Ulfalizer: Difference between revisions
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CPU: [ Low ][ High ] -> 0 | CPU: [ Low ][ High ] -> 0 | ||
CPU: [ Low ][ High ] -> 0 | CPU: [ Low ][ High ] -> 0 | ||
CPU: [ Low ][ High ] -> | CPU: [ Low ][ High ] -> 1 (?) | ||
CPU: [ Low ][ High ] -> 1 | CPU: [ Low ][ High ] -> 1 | ||
CPU: [ Low ][ High ] -> 1 | CPU: [ Low ][ High ] -> 1 | ||
Revision as of 23:37, 5 July 2013
Misc. timing stuff brought together in one place
Reads and writes
Read (LDA $1234, NOP)
Tick-by-tick from http://nesdev.org/6502_cpu.txt interleaved with steps from Visual 2A03:
# address R/W description
--- ------- --- -------------------------------------------------
1 PC R fetch opcode, increment PC
ab db rw pc phi2
0000 ad 1 0000 0
0000 ad 1 0000 1
2 PC R fetch low byte of address, increment PC
ab db rw pc phi2
0001 34 1 0001 0
0001 34 1 0001 1
3 PC R fetch high byte of address, increment PC
ab db rw pc phi2
0002 12 1 0002 0
0002 12 1 0002 1
4 address R read from effective address
ab db rw pc phi2
1234 00 1 0003 0
1234 00 1 0003 1
Write (LDA #$AB, STA $1234, NOP)
Tick-by-tick from http://nesdev.org/6502_cpu.txt interleaved with steps from Visual 2A03:
# address R/W description
--- ------- --- ------------------------------------------
1 PC R fetch opcode, increment PC
ab db rw pc phi2
0002 8d 1 0002 0
0002 8d 1 0002 1
2 PC R fetch low byte of address, increment PC
ab db rw pc phi2
0003 34 1 0003 0
0003 34 1 0003 1
3 PC R fetch high byte of address, increment PC
ab db rw pc phi2
0004 12 1 0004 0
0004 12 1 0004 1
4 address W write register to effective address
ab db rw pc phi2
1234 12 0 0005 0
1234 ab 0 0005 1
Read/write observations
- Address bus and rw changes right away (during φ1).
- Values appear to be read during φ2 (dbx pins buffered on cclk), and db changes during φ2 for writes too.
- Clocks section of tutorial.
Interrupts
Sampled on the falling edge of φ2. IRQ detection depends on nodes nnT2BR (branch-related) and 646 (maybe a "sampling points" signal) being low.
Detection intervals for various instructions
Smallest IRQ assertion interval that will trigger an IRQ for LDA #FF: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=58A9FF1890FE&irq0=5&irq1=6&steps=20
Ditto for LSR $AB: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=5846AB1890FE&irq0=11&irq1=12&steps=20
Ditto for LSR $AB with NMI instead of IRQ: http://visual6502.org/JSSim/expert.html?logmore=Execute,nmi,~NMIG,State&a=0&d=58461890FE&nmi0=11&nmi1=12&steps=20
IRQ triggered by assertion during first cycle of BCC - no page crossing, offset 0, pad with SEDs: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=58189000F8F8&irq0=9&irq1=10&steps=30
IRQ triggered by assertion during first cycle of BCC - page crossing, offset E0 (backwards jump), pad with SEDs: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=581890E0&a=ffe4&d=F8F8&irq0=9&irq1=10&steps=30
IRQ triggered by assertion before fixup cycle of BCC - page crossing, offset E0 (backwards jump), pad with SEDs: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=581890E0&a=ffe4&d=F8F8&irq0=13&irq1=14&steps=30
IRQ during JMP Absolute (to after JMP instruction): http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=584C0400F8F8&irq0=7&irq1=8&steps=30
IRQ during JMP Relative (address stored after JMP instruction, to next location after that): http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=586C04000600F8F8&irq0=11&irq1=12&steps=30
RTI - interrupt flag changes *before* poll location: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=40&irq0=9&irq1=10&steps=30
CLI+SEI - interrupt flag changes *after* poll location for both: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=5878&irq0=5&irq1=6&steps=30
PLP - interrupt flag changes *after* poll location: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,State,nnT2BR,646&a=0&d=28F8F8&irq0=5&irq1=6&steps=30
Interrupt hijacking
http://en.wikipedia.org/wiki/Interrupts_in_65xx_processors
BRK hijacked by NMI: http://visual6502.org/JSSim/expert.html?logmore=Execute,nmi,~NMIG,State,nnT2BR,646&a=0&d=5800&nmi0=11&nmi1=12&steps=30
IRQ hijacked by NMI: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq,IRQP,nmi,~NMIG,State,nnT2BR,646&a=0&d=58F8F8&irq0=5&irq1=6&nmi0=15&nmi1=16&steps=30
Reset
Seems to take 1+7 cycles, where the last 7 is the ordinary interrupt sequence.
M2 duty cycle
The M2 duty cycle is 5/8 - forum post and CPU pin-out page. Low for 9/8 PPU cycles, high for 15/8 PPU cycles. Low for 4.5 master cycles, high for 7.5 master cycles. Low for ~210 ns, high for ~349 ns.
PPU interface
Input map:
Binding | Interpretation ----------------+-------------------------------------- 0 -> 15 | 0 -> /Enable second demultiplexer a15 -> 13 | a15 -> A_1b a14 -> 3 | a14 -> A_1a a13 -> 2 | a13 -> A_0a M2 -> 14 | M2 -> A_0b 11 -> 1 | /O_1b -> /Enable first demultiplexer 9 -> /ROM SEL | /O_3b -> /ROM SEL 5 -> /DBE | /O_1a -> /DBE 4 -> RAM CS | /O_0a -> RAM CS
Input/output map:
Inputs | Outputs ---------+---------------- E A0 A1 | O_0 O_1 O_2 O_3 ---------+---------------- 1 x x | 1 1 1 1 0 0 0 | 0 1 1 1 0 1 0 | 1 0 1 1 0 0 1 | 1 1 0 1 0 1 1 | 1 1 1 0
Hence:
ROM SEL = M2 && a15 <Enable first demultiplexer> = M2 && !a15 DBE = <Enable first demultiplexer> && !a14 && a13 RAM CS = <Enable first demultiplexer> && !a14 && !a13
Summary:
When M2 high,
A15-13 | Signal -------+-------- 1xx | ROM SEL 001 | DBE 000 | RAM CS
- rw signal together with ab lines directly generates signals like /r2002.
- _io_ce follows inverted M2.
- vbl_flag is set via a vpos was 241 and vpos is not 241 comparison. Goes high at hpos = 1 during at pclk0.
- The set_vbl_flag signal is high during pclk0 of vpos=241/hpos=1.
- spr0_hit goes high at tick x+2, during pclk1. Reading uses same _io_ce behavior as vbl_flag.
- spr_overflow reading behavior just like spr0_hit. TODO: set timing.
- $2005/$2006 and $2000 nametable bits writes use the same timing.
Timing charts
Around VBL flag setting:
set_vbl_flag signal active during this interval
|
[++]
Master: 0101010101010101010101010101010101010101010101010101010101010101010101010
PPU: [p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1]
CPU: [ Low ][ High ] -> 0, NMI (-8)
CPU: [ Low ][ High ] -> 0, NMI (-7)
CPU: [ Low ][ High ] -> 0, no NMI (-6)
CPU: [ Low ][ High ] -> 0, no NMI (-5)
CPU: [ Low ][ High ] -> 0, no NMI (-4)
CPU: [ Low ][ High ] -> 0, no NMI (-3)
CPU: [ Low ][ High ] -> 0, no NMI (-2)
CPU: [ Low ][ High ] -> 0, no NMI (-1)
CPU: [ Low ][ High ] -> 1, no NMI (0)
CPU: [ Low ][ High ] -> 1, no NMI (1)
CPU: [ Low ][ High ] -> 1, no NMI (2)
CPU: [ Low ][ High ] -> 1, no NMI (3)
CPU: [ Low ][ High ] -> 1, no NMI (4)
CPU: [ Low ][ High ] -> 1, no NMI (5)
CPU: [ Low ][ High ] -> 1, no NMI (6)
CPU: [ Low ][ High ] -> 1, NMI (7)
CPU: [ Low ][ High ] -> 1, NMI (8)
(NOTE: Read buffer latches the data, so value when high phase starts matters. Value at end probably matters for the rest.)
Around VBL flag clearing: cleared during this interval
[+++++++++++...
Master: 0101010101010101010101010101010101010101010101010101010101010101010101010
PPU: [p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1]
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 0 (?)
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
Sprite zero/overflow flag clearing: cleared during this interval
[+++++++++++....
Master: 0101010101010101010101010101010101010101010101010101010101010101010101010
PPU: [p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1]
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 0 (?)
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
Pixel location Flag goes high
Sprite 0 setting: -------- -----------------...
Master: 0101010101010101010101010101010101010101010101010101010101010101010101010
PPU: [p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1][p0][p1]
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 0
CPU: [ Low ][ High ] -> 1 (?)
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
CPU: [ Low ][ High ] -> 1
