User:Ulfalizer

Misc. timing stuff brought together in one place

Read (LDA $1234, NOP)

http://www.qmtpro.com/~nes/chipimages/visual2a03/?graphics=false&logmore=Execute,phi2&a=0&d=ad3412ea&steps=25

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)

http://www.qmtpro.com/~nes/chipimages/visual2a03/?graphics=false&logmore=Execute,phi2&a=0&d=a9ab8d3412ea&steps=29

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

• Values appear to be read during φ2 (dbx pins buffered on cclk).
• db changes during φ2 for writes too.

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.