AMBA
Arm's open family of bus protocols for wiring processor cores, memories and peripherals together inside a chip.
The Advanced Microcontroller Bus Architecture arrived in 1996 and is royalty-free, which made it a de facto standard well beyond Arm's own cores. AMBA 2 in 1999 introduced the pair still visible in every microcontroller block diagram: AHB, the fast bus for the CPU, flash and DMA, and APB, a slow, low-power bus for timers, UARTs and GPIO, with a bridge in between. AMBA 3 added AXI in 2003, with separate read and write channels, and AMBA 5 brought CHI in 2013 for coherent many-core systems.
Firmware developers mostly meet AMBA indirectly. A register write to an APB peripheral taking several cycles, two bus masters contending, a write still sitting in a buffer after the code has moved on — that is bus architecture, not a bug.