User talk:Fiskbit
I'll be staging information here before finding a place to move it to on the wiki.
CPU / APU
Behavior
- When the CPU comes out of reset, it is random whether it is aligned on the first or second half of an APU cycle (and thus random whether it begins on a GET or PUT cycle).
- The state of the CPU's OUT pins is only updated when transitioning from a GET to PUT cycle. Thus, changes to OUT state lasting only 1 cycle (such as when toggled with an INC instruction) may be missed by the joypad device. This can be used to determine CPU/APU alignment, but poses a problem for expansion port devices that use OUT bits to signal when to snoop the data bus.
- Readable APU registers ($4015 and, if modded, the test registers) drive the internal CPU bus, but not the external CPU bus, so they do not affect external open bus state. Therefore, a dummy read from one of these registers on the cycle before a read from external open bus (such as by using page crossing on an indexed read) will not change the external open bus value.
- The CPU clock divider is not affected by reset. Therefore, resetting the CPU preserves CPU/PPU sub-cycle alignment.
Revision differences
Letterless
- 4-cycle DMA glitches do not occur.
- When DMA collides with a joypad read, extra reads occur on each cycle the CPU is halted instead of just one extra read for the whole DMA.
PPU
Behavior
- Reads from palette RAM do not set bits 7-6, so these will return PPU open bus.
- Writes to OAM during rendering are ignored.
- The PPU clock divider is affected by reset. Therefore, resetting the PPU changes the CPU/PPU sub-cycle alignment.
Revision differences
2C02A
- Writes to $2001 disrupt rendering similar to toggling it off and on.
- PPU bus write timing differs from later revisions in a way that causes glitches on the Everdrive Pro. (Normal cartridges seem unaffected.)
TV-NET
MC-1200
$5000-57FF: CPU/PPU RAM $6000-6007: Registers (mirrored across $6000-6FFF) $7000-7FFF: Cartridge PRG-RAM $8000-FFFF: Modem/cartridge ROM
$6006 appears to control what is mapped into the $5000-57FF window. Observed writes are 4 before writing to what appears to be 1 KB of special graphics memory (MMC5-style ExRAM?) at $5000 and 3 after, and 5 to map in CHR-RAM, which is 32 KB and can be banked using $6002 in 1 KB amounts (unknown if bit 0 ignored).
When booted without a cartridge inserted, modem ROM is loaded at $8000-FFFF, with unique data at $8000-CFFF and the same 4 KB mirrored across $D000-FFFF. When booted with a cartridge, $8000-DFFF are replaced with cartridge ROM, and $7000-7FFF contains PRG-RAM. The JRA-PAT 3.00 cartridge (blue/green/yellow label) has been found to contain a 256 KB ROM and 32 KB battery-backed RAM. Bankswapping is not yet understood.
Contents of $8000-FFFF on letterless, A, and B revision modems found to be the same. Included is the string "ROM VERSION A1.0-06.15", suggesting the remaining ROM contents are likely also the same.
MC-1200 controller
Covered on the forums here. Japanese is naively translated with a dictionary. Uses 3 shift registers; every button can be pressed simultaneously.
0 - P/T switch (1 if T) 1 - 終了 (Shuuryou / End) 2 - F3 3 - (Always 1) 4 - F1 5 - F2 6 - F4 7 - F5 8 - 1 9 - 4 10 - 7 11 - (Always 1) 12 - 2 13 - 3 14 - 5 15 - 6 16 - * 17 - Left 18 - 実行 (Jikkou / Run) 19 - Right 20 - 8 21 - 9 22 - 0 23 - . 24+ - (Always 1)
MC-4800 controller
Included with the TV-NET Rank 2, this controller offers the same functionality as the base TV-NET controller plus 8 additional buttons.
0 - P/T switch (1 if T) 1 - • / 実行 (Jikkou / Run) 2 - 後退 (Koutai / Backspace) / F3 3 - (Always 1) 4 - F1 5 - 番組 (Bangumi / Program) / F2 6 - 印字 (Inji / Typing?) / F4 7 - 取消 (Toke / Cancel) / F5 8 - 1 9 - 4 10 - 7 11 - (Always 1) 12 - 2 13 - 3 14 - 5 15 - 6 16 - * 17 - Left 18 - # / 実行 (Jikkou / Run) 19 - Right 20 - 8 21 - 9 22 - 0 23 - , 24 - 入力 (Nyuuryoku / Input) 25 - Up 26 - Down 27 - 文字 (Moji / Character) 28 - 機能 (Kinou / Function) 29 - 切替 (Kika / Exchange) / F6 30 - 再送 (Saisou / Resend) / F7 31 - 停再 (Tomasai? / Stop again?) / F8
