Technical specifications
The 16-bit design of the SNES incorporates graphics and sound co-processors that perform tiling and simulated 3D effects, a palette of 32,768 colors, and 8-channel ADPCM audio. These base platform features, plus the ability to dramatically extend them all through substantial chip upgrades inside of each cartridge, represent a leap over the 8-bit NES generation and some significant advantages over 16-bit competitors such as the Genesis.
CPU and RAM
| CPU reference | |
| Processor | 16-bit Custom WDC 65C816 core |
| Clock rates (NTSC) | Input: 21.47727 MHzBus: 3.58 MHz, 2.68 MHz, or 1.79 MHz |
| Clock rates (PAL) | Input: 21.28137 MHzBus: 3.55 MHz, 2.66 MHz, or 1.77 MHz |
| Buses | 24-bit and 8-bit address buses, 8-bit data bus |
| Additional features | DMA and HDMATimed IRQParallel I/O processingHardware multiplication and division |
The CPU is a Ricoh 5A22, which is a derivative of the 16-bit WDC 65C816 microprocessor. In NTSC regions, its nominal clock speed is 3.58 MHz but the CPU will slow down to either 2.68 MHz or 1.79 MHz when accessing some slower peripherals.
This CPU has an 8-bit data bus and two address buses. The 24-bit “Bus A” is designated for general accesses, and the 8-bit “Bus B” can access support chip registers such as the video and audio co-processors.
The WDC 65C816 also supports an 8-channel DMA unit; an 8-bit parallel I/O port a controller port interface circuits allowing serial and parallel access to controller data; a 16-bit multiplication and division unit; and circuitry for generating non-maskable interrupts on V-blank and IRQ interrupts on calculated screen positions.
Early revisions of the 5A22 used in SHVC boards are prone to spontaneous failure; this can produce a variety of symptoms including graphics glitches during Mode 7 operation, a black screen on power-on, or inability to read the controllers properly. The first revision 5A22 also had a fatal bug in the DMA controller that could cause games to crash when running; this was corrected in subsequent revisions.
The console contains 128 KB of general-purpose RAM, which is separate from the RAM dedicated to the video and audio subsystems.
Video
| Video reference | |
| Resolutions | Progressive: 256×224 (8:7), 512×224 (16:7), 256×239 (256:239), 512×239 (512:239)Interlaced: 512×448 (8:7), 512×478 (256:239) |
| Pixel depth | 2, 4, 7, or 8 bpp indexed; 8 or 11 bpp direct |
| Total colors | 32768 (15-bit) |
| Sprites | 128, 32 max per line; up to 64 × 64 pixels |
| Backgrounds | Up to 4 planes; each up to 1024 × 1024 pixels |
| Effects | Pixelization (mosaic) per backgroundColor addition and subtractionClipping windows (per background, affecting color, math, or both)Scrolling per 8 × 8 tileMode 7 matrix operations |
The Picture Processing Unit (PPU) consists of two separate but closely tied IC packages. It contains 64 KB of SRAM for storing video data, 544 bytes of object attribute memory (OAM) for storing sprite data, and 256 × 15 bits of color generator RAM (CGRAM) for storing palette data. This CGRAM allows the console to display up to 256 colors, chosen from the 15-bit RGB color space, for a total of 32,768 possible colors. The PPU is clocked by the same signal as the CPU and generates a pixel every two or four cycles. Eight video modes are available to the programmer:
- Mode 0: 4 layers, all using 4-color palettes. Each BG uses its section of the SNES palette. Up to 96 colors can be displayed on the backgrounds, 24 colors per layer.
- Mode 1: 3 layers, two using 16-color palettes and one using 4-color palettes. Up to 120 colors can be displayed by the first two layers and 24 colors by the third layer.
- Mode 2: 2 layers, both using 16-color palettes. Each tile can be individually scrolled. Up to 120 colors can be displayed on the screen.
- Mode 3: 2 layers, one using the full 256-color palette and one using 16-color palettes. The 256-color layer can also directly specify colors from an 11-bit (RGB443) colorspace. Up to 256 colors are displayed by the first layer and 120 colors by the second layer.
- Mode 4: 2 layers, one using the full 256-color palette and one using 4-color palettes. The 256-color layer can directly specify colors, and each tile can be individually scrolled. Up to 256 colors are displayed by the first layer and 24 colors by the second layer.
- Mode 5: 2 layers, one using 16-color palettes and one using 4-color palettes. Tile decoding is altered to facilitate the use of the 512-width and interlaced resolutions. Up to 120 colors are displayed by the first layer and 24 colors by the second layer.
- Mode 6: 1 layer, using 16-color palettes. Tile decoding is as in Mode 5, and each tile can be individually scrolled. Up to 120 colors can be displayed on screen.
- Mode 7: 1 layer of 128×128 tiles of size 8×8 from a set of 256, which may be interpreted as a 256-color one-plane layer or a 128-color two-plane layer. The layer may be rotated and scaled using matrix transformations. A programming technique called HDMA can be used to change the matrix parameters for each scanline to generate perspective effects.
Audio
| Audio reference | |
| Processors | Nintendo S-SMP |
| Clock rates | Input: 24.576 MHzSPC700: 1.024 MHz |
| Output | 8 channels, stereo |
| Effects | ADSR envelope controlFrequency scaling and modulation using Gaussian interpolationEcho: 8-tap FIR filter, with up to .24s delayNoise generation |
The audio subsystem is called the S-SMP, which is a dedicated single chip consisting of an 8-bit CPU, a 16-bit DSP, and 64 KB of SRAM. It is designed and produced by Sony and is completely independent from the rest of the system. It is clocked at a nominal 24.576 MHz in both NTSC and PAL systems. It is capable of producing stereo sound, composed from 8 voices generated using 8 bit audio samples and various effects such as echo.
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Originally posted on December 29, 2020 @ 12:41 pm