struct rpul_payload_header uint32_t magic; // 0x5250554C ("RPUL") uint32_t version; // 0x00010000 for v1.0 uint32_t entry_point; // ARM physical address uint32_t load_addr; // Where to copy payload uint32_t payload_size; // In bytes uint32_t checksum; // CRC32 of payload uint8_t flags; // Boot mode, encryption, etc. uint8_t reserved[3]; ; Understanding this header is non-negotiable for anyone writing a custom payload loader. 3.1 Bare-Metal “Hello World” without an OS Using the RPUL PDF, you can write a simple ARM assembly program that lights an LED or outputs over UART — without Linux or bare-metal SD card images.
Introduction In the world of embedded systems and single-board computing, low-level boot processes remain one of the most misunderstood yet critical areas. For Raspberry Pi enthusiasts and embedded engineers alike, the Raspberry Pi Universal Payload Loader (RPUL) represents a powerful, flexible interface for loading custom payloads onto ARM-based systems. However, the key to unlocking its full potential lies in understanding the RPUL PDF documentation — a comprehensive technical manual that demystifies boot modes, payload formats, and hardware initialization. rpul pdf
The PDF’s Chapter 7 provides a logic analyzer waveform showing exactly what a valid RPUL transaction looks like — invaluable for debugging with a Saleae or DSLogic. | Bootloader | Protocol | Encryption | Multi-core | GPU Aware | Primary Use | |------------|----------|------------|------------|-----------|--------------| | RPUL | Custom UART/SPI | Optional AES | Yes | Yes | Research/recovery | | U-Boot | TFTP, MMC, USB | No | Partial | No | General Linux boot | | Barebox | Similar to U-Boot | No | Partial | No | Embedded Linux | | Raspberry Pi bootloader | Proprietary | Yes | Yes | Yes | Official OS boot | Introduction In the world of embedded systems and
Last updated: April 2026
| Problem | Likely Cause | Solution from PDF | |---------|--------------|--------------------| | No handshake response | Wrong baud rate | Set 115200 8N1, no flow control | | Payload crashes after 1 second | Missing stack pointer init | Set SP to 0x8000 - 16 | | Checksum mismatch | Binary includes BSS section | Use --only-section=.text | | Pi reboots in a loop | Payload too large (>64KB) | Split into multiple stages | The PDF’s Chapter 7 provides a logic analyzer
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