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kernel-boot-and-syscalls: Further Learning Resources

Books

xv6 book — Chapters 3 (Page tables) and 4 (Traps and system calls)

  • The closest companion to ccc's kernel. Free PDF. Read alongside src/kernel/. Difficulty: Beginner-to-Intermediate.

Operating Systems: Three Easy Pieces — §6 ("Limited Direct Execution")

  • The conceptual model of why kernel privilege exists. Free online. Difficulty: Beginner.

Advanced Programming in the UNIX Environment (APUE) — Stevens & Rago, Chapter 8

  • Process API from the user side: fork, exec, wait, signals. The "what does the syscall look like to user programs" perspective. Difficulty: Intermediate.

Specifications

RISC-V Privileged ISA — §3 (Trap mechanism), §4 (Sv32 paging)

  • The mechanism s_trap_entry and vm.zig implement. Difficulty: Reference.

The RISC-V Linux ABI (psABI)

  • The argument-passing rules for _start and syscalls. Difficulty: Reference.

Comparable code

xv6-riscv: kernel/main.c

  • xv6's kmain analog. Read side-by-side with ccc/src/kernel/kmain.zig. Difficulty: Intermediate.

xv6-riscv: kernel/swtch.S

  • 14-instruction context switch. Almost line-identical to ccc's. Difficulty: Intermediate.

Linux: arch/riscv/kernel/entry.S

  • Production-grade trap entry. Lots more bookkeeping than ccc's — perf counters, ftrace, audit, etc. Difficulty: Advanced.

OpenSBI: lib/sbi/sbi_init.c

  • Real M-mode firmware boot. What boot.S is, but for production hardware. Difficulty: Advanced.

Lectures

MIT 6.S081 Lectures 4–7

  • Page tables, traps, system calls, scheduling. Free videos. Pair with the xv6 book.

Stanford CS 140 ("OS Concepts")

  • Pintos-based undergrad OS course. Different code but same concepts.

Articles

"How a system call works" — various authors

  • Linux kernel docs on adding a syscall. Even if you don't end up patching Linux, the walk-through illuminates the kernel's view. Difficulty: Intermediate.

"The Cost of Context Switching"

  • A classic blog post on real-hardware context-switch costs. ccc's emulator hides these costs, but reading this gives perspective on what production OSes optimize. Difficulty: Intermediate.

Tools

zig build run -- --trace kernel.elf | head -200

  • The cleanest way to see boot in action: every instruction from 0x80000000 to the first user-mode addi. The privilege column flips from [M] to [S] to [U] exactly where you'd expect.

scripts/qemu-diff-kernel.sh

  • Diffs ccc's --trace output for kernel.elf against qemu-system-riscv32. If a CSR write or trap routing differs from QEMU, this catches it. Requires QEMU.

When you're ready

Next: processes-fork-exec-wait — how the kernel manages multiple processes. We've shown one process; now we make N.