VTxx refers to V.R. Technology's "NES-on-a-chip" Famiclones, some of which having greatly extended capabilities. These capabilities are described in separate articles:
- VT01 STN Palette
- VT02+ PRG-ROM Bankswitching
- VT02+ Video Modes
- VT02+ CHR Pattern Data Layout
- VT02+ CHR-ROM Bankswitching
- VT02+ MMC3 Compatibility Registers
- VT03+ Enhanced Palette
- VT02+ Sound
- VT02+ Registers
- VT02+ Cartridge Connector
VTxx-numbered Famiclones, with two digits after "VT", are backwards-compatible to lower-numbered models as well as the original NES/Famicom, with a few exceptions. VTxxx-numbered consoles, with three digits after "VT", drop compatibility to the original NES/Famicom.
CPU Memory Map
CPU $0000-$07FF: 2 KiB of internal RAM CPU $0800-$0FFF: VT16+: VT01-VT03: Mirror of CPU $0000-$07FF, VT16+: Further 2 KiB of internal RAM CPU $1000-$1FFF: Mirror of CPU $0000-$0FFF CPU $2000-$2007: RP2C02-compatible PPU registers CPU $2010-$201F: VT02+: New PPU registers CPU $4000-$401F: RP2A03-compatible APU, DMA and I/O registers CPU $4020-$403F: VT02+: New APU, DMA and I/O registers CPU $4100-$41FF: VT02+: New miscellaneous registers CPU $6000-$7FFF: Optional WRAM, if present on cartridge CPU $8000-$FFFF (write): VT02+: When forwarding is enabled (register $410B bit 3 "FWEN" =0): MMC3-compatible registers that are forwarded to $4101-$4108) CPU $8000-$FFFF (read): Four 8 KiB PRG-ROM banks.
PPU Memory Map
PPU $0000-$1FFF: CHR-ROM, all modes: CHR pattern data, bit planes 0 and 1 PPU $2000-$2FFF: NTRAM (2 KiB, selectable mirroring) PPU $3F00-$3F1F: CGRAM in 2bpp modes, entries $3F10/$3F14/$3F18/$3F01 mirrors of $3F00/$3F04/$3F08/$3F0C. VT03+: Also CGRAM in 4bpp modes, bits 0 to 5, of colors 00-1F; same mirroring of address bit 4 applies. PPU $3F20-$3F7F: VT03+: CGRAM in 4bpp modes, bits 0 to 5, of colors 20-7F; no mirroring of address bit 4. PPU $3F80-$3FFF: VT03+: CGRAM in 4bpp modes, bits 6 to 11, of colors 00-7F; no mirroring of address bit 4. PPU $4000-$5FFF: CHR-ROM, 4bpp modes: CHR pattern data, bit planes 2 and 3 (VT03+)
List of VTxx consoles
The first of the V.R. Technology Famiclones to add major enhancements:
- OneBus mode: PRG and CHR data come from the same address space. The cartridge connector's PPU Address lines are repurposed as higher-order Address lines.
- Integrated MMC3-compatible bankswitching by the console, active when in OneBus mode.
- Ability to use NTRAM as CHR-RAM.
- Choice of PPU A12 and HBLANK as a source for clocking the scanline interrupt.
- Automatic X/Y light gun position determination.
- Integrated RS232 interface.
- Direct NTSC/PAL and 50/60 Hz identification.
- Automatic per-attribute-tile background and per-sprite sprite bankswitching.
- DMA can be used to transfer data to PPU memory (via $2007) in addition to the normal transferring of data to OAM memory (via $2004).
- A second APU doubling the number of sound channels.
- Raw PCM output using full eight bits of resolution.
- DMA-driven raw PCM output.
- Data for DMA-driven DPCM/PCM output can be placed anywhere in CPU address space, not just within the $C000-$FFFF range.
The VT03 greatly enhances the graphical capabilities of the original RP2C02:
- Sprites can be 16 pixels wide.
- Graphics can use four bits-per-pixel, so that together with two attribute data bits, sixty-four rather than 16 colors each for background and sprites may be chosen.
- Enhanced color palette with twelve bits rather than six bits per color.
VT09, VT16, VT18
The VT16 is the first of the NES-compatible Famiclones after the VT03 for which a datasheet is publicly available. It is likely however that many of its enhancements were already included in the VT09 console.
- Internal CPU RAM is 4 KiB rather than just 2 KiB.
- The CPU can be set to run at the same speed (5.37 MHz) as the PPU.
- The CPU can be set to retrieve interrupt vectors from RAM ($0FFx) rather than ROM ($FFFx).
- DMA-driven DPCM/PCM output can be set to use its own banking registers independent of normal PRG banking.
- Hardware random number generator.
- Publicly-available datasheets on V.R. Technology's webiste