AMBA / AXI Bus — ARM's standardized on-chip interconnect protocol family that defines how IP blocks (CPUs, GPUs, DMAs, peripherals) communicate inside an SoC.

AMBA Protocol Family

• AXI (Advanced eXtensible Interface): High-performance, high-bandwidth. Used for CPU↔memory, GPU, DMA. Supports out-of-order transactions, burst transfers
• AHB (Advanced High-Performance Bus): Medium performance. Used for on-chip RAM, flash controllers. Simpler than AXI
• APB (Advanced Peripheral Bus): Low-bandwidth, low-power. Used for configuration registers, UART, SPI, I2C. Simple request-response

AXI Key Features

• Separate read/write channels: 5 channels (read address, read data, write address, write data, write response)
• Outstanding transactions: Master can issue multiple requests without waiting for responses
• Burst transfers: Transfer 1–256 beats in a single transaction
• Out-of-order completion: Responses can return in different order from requests (tagged with ID)

Typical SoC Interconnect

<svg viewBox="0 0 578 112" xmlns="http://www.w3.org/2000/svg" style="max-width:100%;height:auto" role="img"><rect x="0" y="0" width="578" height="112" rx="12" fill="#0d1117"/><g font-family="ui-monospace,SFMono-Regular,Menlo,Consolas,&quot;Liberation Mono&quot;,monospace" font-size="14"><text xml:space="preserve" x="20" y="31.7"><tspan fill="#c9d1d9">CPU </tspan><tspan fill="#6e7681">──┐</tspan></text><text xml:space="preserve" x="20" y="50.7"><tspan fill="#c9d1d9">GPU </tspan><tspan fill="#6e7681">──┼──</tspan><tspan fill="#c9d1d9"> [AXI Interconnect/NoC] </tspan><tspan fill="#6e7681">──┬──</tspan><tspan fill="#c9d1d9"> DDR Controller</tspan></text><text xml:space="preserve" x="20" y="69.7"><tspan fill="#c9d1d9">DMA </tspan><tspan fill="#6e7681">──┘</tspan><tspan fill="#c9d1d9">                             </tspan><tspan fill="#6e7681">├──</tspan><tspan fill="#c9d1d9"> On-chip SRAM</tspan></text><text xml:space="preserve" x="20" y="88.7"><tspan fill="#c9d1d9">                                    </tspan><tspan fill="#6e7681">└──</tspan><tspan fill="#c9d1d9"> APB Bridge </tspan><tspan fill="#6e7681">→</tspan><tspan fill="#c9d1d9"> Peripherals</tspan></text></g></svg>

AMBA is the de-facto standard — virtually every ARM-based SoC (smartphones, IoT, automotive) uses AMBA protocols. Even non-ARM designs often adopt AXI for IP compatibility.