# RTSS cold boot flow Note This boot flow is applicable to Qualcomm Dragonwing ^TM^ IQ-9075 and Qualcomm Dragonwing ^TM^ IQ-8275. The Real Time Subsystem (RTSS) is designed to deliver high real-time performance through an independent, highly integrated architecture that significantly enhances computing, networking, and safety support. RTSS embodies the “SoC-within-a-SoC” concept, consisting of two domains: > > > - APSS > - RTSS For IoT products, RTSS is employed for bootup, reset use cases, dump collection, low-power mode, real-time application development, and more. ## RTSS bootup The RTSS boots up in parallel with the APSS boot flow. The RTSS operates as a separate subsystem and supports the Free Real Time Operating System (FreeRTOS). In the current release, [FreeRTOS](https://github.com/FreeRTOS/FreeRTOS) is enabled as a binary-only release with functionality limited to booting up the device and initializing clocks and peripherals in IoT applications. The cold boot sequence in RTSS involves the following steps: 1. Reset state exit: The boot core exits the reset state and begins executing the RTSS PBL. 2. PBL execution: - Initializes the PLLs and clocks. - Loads and authenticates the RTSS image (boot code in the EL2 layer and FreeRTOS in the EL1 layer). - Jumps to the RTSS EL2. 3. EL2 image initialization: - Initializes core drivers such as clock, GPIO, temperature monitoring, voltage monitoring, UART, I^2^C, etc. - Initializes crash dump collection. - Hands over control to RTSS EL1. 4. EL1 image initialization: - Completes FreeRTOS initialization to schedule and start tasks. - Initializes dedicated peripherals like QUP (I ^2^C, SPI, UART). - Shares DDR memory between the main domain and RTSS. Ensure that the mailbox software drivers on both APSS and RTSS use the dedicated DDR shared memory space for communication between APSS and RTSS. For information about flashing RTSS, see: [Flash sail](https://docs.qualcomm.com/bundle/publicresource/topics/80-70020-254/flash_images.html#flash-sail). Last Published: Jun 25, 2025 [Previous Topic APSS cold boot flow](https://docs.qualcomm.com/bundle/publicresource/80-70020-4/topics/bootloader-architecture.md) [Next Topic Set up your development environment](https://docs.qualcomm.com/bundle/publicresource/80-70020-4/topics/getting-started.md)