GTROM
| Company | Membler Industries |
| Boards | GTROM |
| PRG ROM capacity | 512K |
| PRG ROM window | 32K |
| PRG RAM capacity | 8K |
| PRG RAM window | 4K |
| CHR capacity | 16K |
| CHR window | 8K |
| Nametable mirroring | 4 fixed |
| Bus conflicts | No |
| IRQ | No |
| Audio | No |
| iNES mappers | 111 |
Cheapocabra or GTROM is a low-end, self-flashable cartridge board intended for homebrew releases with its own unique mapper assigned to iNES Mapper 111. The board and mapper were designed by Membler Industries which also manufatures the cartridges.
Overview
The Cheapocabra board and mapper were developed in 2015 by Memblers to meet the needs of the lower end of the homebrew cartridge market. This includes games that would have historically used an NROM board as well as other simpler boards which provide some kind of bank switching but lacked more advanced features like IRQs. The underlying philosophy is that stocking larger numbers of a single board design allows for lower costs by economies of scale when compared to stocking smaller individual quantities of a suite of lower end boards all with similar capabilities. This is especially true when it is considered that using smaller flash chips and removing mapper logic does not significantly reduce the cost of manufacturing a board.
The name Cheapocabra is a play on Chupacabra, a creature from American folklore, where the similar sounding first syllable is replaced with the word "cheap" to indicate the intended low cost of the hardware. The GTROM moniker, meaning "Goat ROM," is derived directly from this as Chupacabra translates literally as "goat-sucker" and the Chupacabras of legend are known for attacking goats and other livestock.
Banks
Registers
It is only well-defined for 512KiB PRG, 32 KiB CHR-RAM, and 4-screen nametables.
Mask: $D000
$5000: [grnc pppp]
|||| ||||
|||| ++++- Select 32 KiB PRG at CPU $8000-$FFFF
|||+------ Select 8 KiB CHR RAM at PPU $0000-$1FFF
||+------- Select 8 KiB nametable and scratch RAM at PPU $2000-$3EFF
|+-------- 0: red LED is lit, 1: off
+--------- 0: green LED is lit, 1: off
Hardware
Like iNES Mapper 030, the specific flash used on this board is the SST39SF040. Games are expected to use the chip's self-erase and self-write abilities for game saves.
The specific hardware on this board is a 74377, a 7402, and a 7410. The exact connectivity is not known, but the functions evaluated by the logic must include:
- CHRRAMA14 = if (PPUA13==1) { n } else { c }
- PRGROM/OE = NOT(R/W)
- Latch on M2, /ROMSEL, A14, A12 high, and maybe on R/W low
See also
- Membler Industries in 2015 - forum post describing the board.
