The Nintendo S-SMP is the audio CPU of the SNES video game console.
The SNES contains a sound module called APU (Audio Processing Unit) which is almost completely separate from the rest of the system: it is clocked at a nominal 24.576 MHz in both NTSC and PAL systems, and can only communicate with the main board via 4 registers on Bus B. It was designed by Ken Kutaragi and manufactured by Sony, who subsequently entered the video game console industry with its PlayStation brand.
The S-SMP (which has an 8-bit Sony SPC700[1] core) is located on the left side of the sound module. It shares 64 KB of PSRAM with the S-DSP (which actually generates the sound) and runs at 2.048 MHz, divided by 12 off of the 24.576 MHz crystal. It has six internal registers, and can execute 256 opcodes. The SPC700 instruction set is quite similar to that of the 6502 CPU family, but includes additional instructions, including XCN (eXChange Nibble) which swaps the upper and lower 4-bit portions of the 8-bit accumulator, and an 8-by-8-to-16-bit multiply instruction.
The S-DSP is capable of producing and mixing 8 simultaneous voices at any relevant pitch and volume in 16-bit stereo at a sample rate of 32 kHz. It has support for voice panning, ADSR envelope control, echo with filtering (via a programmable 8-tap FIR), and using noise as sound source (useful for certain sound effects such as wind). S-DSP sound samples are stored in RAM in compressed (BRR) format. Communications between the S-SMP and the S-DSP is carried out via memory-mapped I/O.
The RAM is accessed at 3.072 MHz, with accesses multiplexed between the S-SMP (⅓) and the DSP (⅔). This RAM is used to store the S-SMP code and stack, the audio samples and pointer table, and the DSP's echo buffer.
The S-SMP operates in a somewhat unconventional manner for a sound chip. A boot ROM is running on the S-SMP upon power-up or reset, and the main SNES CPU uses it to transfer code blocks and sound samples to the RAM. The code is machine code developed specifically for the SPC700 instruction set in much the same way that programs are written for the main CPU; as such, the S-SMP can be considered as a coprocessor dedicated for sound on the SNES.
Since the module is mostly self-contained, the state of the APU can be saved as an .SPC file, and can be emulated in a stand-alone manner to play back all game music (except for a few games that constantly stream their samples from ROM). Custom cartridges or PC interfaces can be used to load .SPC files onto a real SNES SPC700 and DSP. The sound format name .SPC comes from the name of the audio CPU core.