RiscyOS

A minimalist RISC-V multitasking kernel developed as a final project for the University of Manchester Microcontrollers (COMP22111) unit. RiscyOS demonstrates a full vertical slice of OS design from bare-metal boot and hardware initialisation to loading and executing user-space programs from a FAT32 filesystem.

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University of Manchester Project Context

Non-Conventional Toolchain

RiscyOS uses a bespoke assembly syntax tailored for the project's specific assembler (found in tools/) created by Dr James Garside.

• The Assembler: Unlike standard GNU Assembler (GAS), this dialect uses square brackets unlike conventional parentheses.
• Transpilation: A custom Python utility (tools/gas-to-jim/) is included to convert GAS RISC-V assembly patterns into the specific dialect required by the university's internal teaching tools.

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Core Design

Trap Handling & Context Switching

• Trap Frame Model: Machine-mode traps save all 31 general-purpose registers into a trap_frame structure. The kernel operates directly on this frame, and return values are written into the saved a0/a1 slots.
• Memory-Constrained Stack: To save RAM, the kernel utilizes a single global stack; there are no per-process kernel stacks.
• State Restoration: Context switching swaps the mepc, mstatus, and register sets stored directly in the Process Control Block (PCB).

Memory Management

• Slab Allocator: Fast, fixed-size object pools for frequent kernel structures (PCBs, file descriptors).
• Kernel Heap (kmalloc): A header-based allocator with coalescing for general kernel tasks.
• Base/Limit MMU: Implements process isolation via a lightweight custom design:
• IMMU: Base + Limit.
• DMMU: Base + Limit + Virtual Start.
• Translation: $physical = target - virtual_start + base$

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Process & Execution Model

ELF Loading Pipeline

1. Locate: Find file via FAT32 directory traversal.
2. Fetch: Read from SD card via SPI interrupt-driven state machine.
3. Parse: Validate ELF headers and segments.
4. Allocate: Map segments using the flat bitfield allocator.
5. Run: Read and copy segment contents and schedule the process.

Process Operations

• fork(): Shares the text segment (read-only) and duplicates the data segment.
• execve(): Overwrites current process state with a new ELF binary from disk.
• Scheduler: A PID-based round-robin scheduler. If no processes are READY, the kernel enters a kidle state to conserve cycles.

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Filesystem & I/O

• FAT32 Support: Implements cluster walking, sector-to-LBA mapping, and file read operations over SPI.
• Interrupt-Driven I/O: Serial (UART) and SD card operations are managed via non-blocking Event-Driven FSM queues.
• UART/TTY Layer: Provides a terminal interface with line-discipline support (handling backspace and newline echos).

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Hardware Notes

• Architecture: 32-bit RISC-V (RV32I).
• Storage: SD card connected via SPI.
• Implementation Note: Developed on physical hardware

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Status

Working:

• Multitasking & Context Switching.
• FAT32 Driver (Read-only) & SPI FSM.
• Async ELF loading and execution.
• Base/Limit memory isolation.
• Slab & Bitfield allocators.

In Progress:

• Process termination (exit) and Signal handling.
• Filesystem write support (mkdir, write).
• Shell environment (Internal commands like cd, ls).
• Enhanced QEMU parity for faster testing.

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Developed as part of the University of Manchester COMP22111 Microcontrollers Project.