feat(riscv): bare-metal startup + linker script generators#89
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Track B3 — adds the runtime infrastructure that turns synth's RISC-V .text into a bootable firmware ELF. ## startup.rs — RiscVStartupGenerator Emits C source (with inline RISC-V asm) that: 1. Disables interrupts (mie, mip = 0) 2. Sets sp/gp/s11 (linear-memory base) from linker symbols 3. Installs `_trap_entry` into mtvec (direct mode) 4. Optionally enables the FPU via mstatus.FS = 1 5. Copies .data from flash to RAM 6. Zeroes .bss 7. Calls main(); spins via `wfi` if main returns The trap entry saves the AAPCS-style caller-saved set (ra, t0..t6, a0..a7), marshals mcause/mepc/mtval into a0/a1/a2, and dispatches to a weak C function `synth_trap_handler` that the firmware can override. `StartupConfig` exposes three knobs: external_irq_count (PLIC wiring), trap_returns (mret vs spin in default), enable_fpu (mstatus FPU enable). ## linker_script.rs — RiscVLinkerScriptGenerator Emits a GNU-`ld` script that: - Places `.text._reset` at the entry point so the reset vector is at the start of flash. - Aligns `.text._trap_entry` to 64 bytes (works for both mtvec direct and vectored modes — vectored requires 64-byte alignment). - Splits `.data` between flash (load address) and RAM (run address) so the startup copy loop has well-defined bounds. - Provides `_stack_top`, `_data_start`/`_data_end`/`_data_load`, `_bss_start`/`_bss_end`, `__linear_memory_base`/`__linear_memory_end`, and `__global_pointer$` (= `_data_start + 0x800`, the standard RV small-data anchor). - `LinkerScriptConfig` configures flash/ram origins, linear-memory size, and stack size. ## Memory model The wasm linear-memory base lives in **s11** — chosen because the RV psABI marks it callee-saved, so all selector-emitted load/store sequences (`add tmp, s11, addr; lw rd, offset(tmp)`) work without per-callee re-loading. The startup code pre-loads s11 from `__linear_memory_base`, which the linker script positions in RAM. ## Test summary * 16 new tests (8 in startup, 8 in linker_script). Total RISC-V tests now 98 (was 82). * All workspace tests still pass. * `cargo clippy --workspace --all-targets -- -D warnings` clean. ## Out of scope (B4) * CLI integration of `--link` for RISC-V (riscv64-unknown-elf-gcc invocation) * PLIC wiring (external_irq_count is exposed but not yet emitted) * Calculator demo running on Renode RV32 with full firmware * Kiln-builtins-style host bridge for RV32 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Track B3 — adds the runtime infrastructure that turns synth's RISC-V .text into a bootable firmware ELF. Stacked on #88.
What this adds
`startup.rs` — `RiscVStartupGenerator`
Emits C source (with inline RISC-V asm) that:
Trap entry saves caller-saved registers, marshals mcause/mepc/mtval into a0/a1/a2, dispatches to a weak `synth_trap_handler` (firmware-overridable).
`linker_script.rs` — `RiscVLinkerScriptGenerator`
Emits a GNU-`ld` script that:
Memory model
The wasm linear-memory base lives in s11 — chosen because the RV psABI marks it callee-saved, so all selector load/store sequences (`add tmp, s11, addr; lw rd, offset(tmp)`) work without per-callee re-loading.
Test summary
Out of scope (B4)
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