MMIO registers

TODO

• $2100-213F PPU registers
• $2140-2413 APUIO
• $2180 WMDATA
• $2181-2183 WMADD
• $4016 Legacy Gamepad 1
• $4017 Legacy Gamepad 2
• $4200 NMITIMEN
• $4201 WRIO
• $4202-4203 WRMPY
• $4204-4206 WRDIV
• $4207-4208 HTIME
• $4209-420A VTIME
• $420B MDMAEN
• $420C HDMAEN
• $420D MEMSEL
• $420E-420F ?
• $4210 RDNMI
• $4211 TIMEUP
• $4212 HVBJOY
• $4213 RDIO
• $4214-4215 RDDIV
• $4216-4217 RDMPY
• $4218-421F STD CNTRL1-4
• $4300-437F DMA registers

Interrupts

NMITIMEN - Interrupts and Joypad reading ($4200 write)

---

7  bit  0
---- ----
N.VH ...J
| ||    |
| ||    +- Joypad auto-read enable
| ++------ H/V timer IRQ:
|           00 = Disable timer
|           01 = IRQ when H counter == HTIME
|           10 = IRQ when V counter == VTIME and H counter == 0
|           11 = IRQ when V counter == VTIME and H counter == HTIME
+--------- Vblank NMI enable

TODO: Details on the exact conditions for an NMI or IRQ.

Screen timer values

---

HTIMEL, HTIMEH - H timer target ($4207, $4208 write)

  HTIMEH      HTIMEL
  $4208       $4207
7  bit  0   7  bit  0
---- ----   ---- ----
.... ...H   HHHH HHHH
        |   |||| ||||
        +---++++-++++- H counter target for timer IRQ

Note that setting a value larger than the maximum H counter value of 339 will prevent the timer's H condition from being met.

VTIMEL, VTIMEH - V timer target ($4209, $420A write)

  VTIMEH      VTIMEL
  $420A       $4209
7  bit  0   7  bit  0
---- ----   ---- ----
.... ...V   VVVV VVVV
        |   |||| ||||
        +---++++-++++- V counter target for timer IRQ

Note that setting a value larger than the maximum V counter value will prevent the timer's V condition from being met. The maximum depends on the region (261 for NTSC, 311 for PAL) and interlacing (1 additional scanline every other frame).

Status

---

RDNMI - Vblank flag and CPU version ($4210 read)

7  bit  0
---- ----
Nxxx VVVV
|||| ||||
|||| ++++- CPU version
|+++------ Open bus
+--------- Vblank flag

On read: RDNMI D7 = 0

The vblank flag is set at the start of vblank and cleared at the end of vblank or on read.

TIMEUP - Timer flag ($4211 read)

7  bit  0
---- ----
Txxx xxxx
|||| ||||
|+++-++++- Open bus
+--------- Timer flag

On read: TIMEUP D7 = 0

The timer flag is set when the timer condition specified in NMITIMEN becomes true and is cleared on read.

HVBJOY - Screen and Joypad status ($4212 read)

7  bit  0
---- ----
VHxx xxxJ
|||| ||||
|||| |||+- Joypad auto-read in-progress flag
||++-+++-- Open bus
|+-------- Hblank flag
+--------- Vblank flag