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Espressif ESP8266

The ESP8266 is Espressif's low-cost WiFi microcontroller — the chip that launched the hobbyist WiFi boom — built around a single 32-bit Tensilica Xtensa LX106 at up to 80MHz (boostable to 160). The LX106 is a cut-down Xtensa: variable-length 24-bit and 16-bit (density) instructions, a shift-amount register, and PS / EPC1 / EXCCAUSE for exceptions, but no register windows — code uses the plain CALL0 ABI (a0-a15, call0/callx0/ret). It integrates SRAM, cache-mapped external SPI flash, GPIO, UART and a WiFi radio. Here it runs real bare-metal firmware on a from-scratch Xtensa LX106 interpreter, sharing the Xtensa core with the ESP32.