An Exposition on SAV in RSX-11M-PLUS V2.0

SAV (System Archive/Save) is a crucial utility in RSX-11M-PLUS V2.0 designed to create a "snapshot" of a running system by copying its main memory contents into a bootable system image file on disk. This process preserves the system's active state, including tasks and their contexts, allowing the system to be restored or booted later, potentially on different hardware, without the need for a full reinstallation from scratch.

The functionality of SAV is divided into two main phases:

1. Saving the System: SAV operates as a privileged task, first ensuring the system is in a "quiet" or inactive state. This involves numerous checks, such as verifying no active user tasks, no outstanding I/O operations, all non-system devices are offline, and accounting is disabled. It then embeds a specialized disk driver (like SAVDL or SAVCM) into a temporary memory location. This driver is used to write the system's entire memory image to a designated file on disk, updating the file's label block to make it bootable.
2. Booting/Restoring the System: During boot-up, the hardware bootstrap loads the system image file into main memory. Control is then passed to SAV, which takes over the restoration process. SAV reinitializes the CPU's context, memory management registers, and reconfigures system components. It "reinstalls" tasks by rebinding their Logical Unit Numbers (LUNs) and updating task headers, adapting them to the new system's hardware configuration.

Key aspects of SAV's design and operation include:

• Special Drivers: These compact, highly constrained drivers are critical for low-level disk I/O during both saving and booting. SAV dynamically modifies and embeds these drivers into the system's boot block, enabling the system image to be transportable across various disk types (e.g., RL02, RK06/07, UDA50) and controllers.
• System State Quiescing: SAV enforces strict conditions before saving, making it distinct from traditional backup tools. It must ensure the system is largely inactive to create a consistent snapshot.
• Hardware Adaptability: SAV includes mechanisms to adjust for differences in CPU types (e.g., 22-bit, FPP, CIS hardware), memory configurations, and disk controllers. Command-line switches like /CSR=x can facilitate booting on systems with non-standard controller addresses.
• Memory Management: SAV manages memory partitions, overlays, and common memory areas, performing checks to guarantee that the system image file is sufficiently large to hold all essential memory components.
• M-PLUS vs. RSX-11M Compatibility: The document details how RSX-11M V4.0 was updated to better align with M-PLUS's approach to handling system image bootstraps and transfer addresses, aiming for greater interoperability.

SAV has certain restrictions, such as not supporting systems with ECC errors in their boot blocks (as SAV does not perform ECC correction), which can lead to system halts. While robust, SAV is not entirely fault-tolerant and assumes a "reasonable" system environment during the saving process.