ARCH-AVR requirements

Requirement: Manifest and tiny defaults ARCH-AVR-001

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

The AVR port shall declare 8-bit words, 16-bit pointers, byte stack alignment, a 37-byte context frame, no nesting, no zero-latency class, no compare-and-swap, no interrupt stack, no protection, no fault entry, and tickless limited to idle modes; the tiny defaults (table scheduler, 32-bit time, 8-bit notifications, no system work queue, no idle thread) shall be mandatory on it. Rationale: SPEC-012 §2; ADR-036. Status: Accepted 2026-10-07 Verification: Manifest schema check; configuration test refusing the isolated profile on AVR. Trace: SPEC-012 §2 Normative source.

Requirement: One frame layout ARCH-AVR-002

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

The thread-context switch, the interrupt epilogue switch, and the initial frame shall use the single 35-byte frame layout (37-byte call/context reserve) of SPEC-012 §4.1 (return address, r0, SREG, r1, r2..r31), so that an interrupt-exit switch is an exchange of stack pointers. Rationale: One switch path (PORT-004) and the cheapest possible P1 on a core without an interrupt stack. Status: Accepted 2026-10-07 Verification: Test: arch/avr/frame_layout, arch/avr/switch_from_isr_and_thread_interoperate. Trace: SPEC-012 §4 Normative source.

Requirement: Atomic launch and switch tail ARCH-AVR-003

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

The switch and launch sequences shall restore the incoming SREG and r0 before ret; M1 uses out SREG; pop r0; ret. A physical pending-interrupt test shall establish coherent context throughout that tail; a one-instruction enable delay shall not be used to claim the intervening pop is absent or the return necessarily precedes IRQ entry (A27). Rationale: The actual restore tail and pending IRQ entry must be validated together; the historical out/ret-only description did not match the assembly. Status: Accepted 2026-10-07 Verification: Test: arch/avr/launch_with_pending_interrupt. Trace: SPEC-012 §4.2, §4.3 Normative source.

Requirement: Interrupt prologue, epilogue, and reserve ARCH-AVR-004

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

EMB_ISR shall save the full frame with I forced set in the saved SREG, run the body with interrupts masked, and switch at exit when a reschedule is pending and the scheduler is unlocked; EMB_ISR_RAW shall be a signal handler that may not call the kernel; every thread stack shall include CONFIG_EMB_AVR_ISR_STACK_RESERVE (default 64 bytes), documented per release from supported compiler stack reports plus call-graph/disassembly accounting of naked routines; avr-gcc cannot report those routines with -fstack-usage. Rationale: SPEC-002 §6.2, §7.2 on this core. Status: Accepted 2026-10-07 Verification: Test: arch/avr/isr_frame, arch/avr/isr_reserve_check; documentation review. Trace: SPEC-012 §4.4, §5 Normative source.

Requirement: Timer1 modes and latency ARCH-AVR-005

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

The default tick shall be an exact 1 ms Timer1 CTC interrupt (prescaler 64, OCR1A = 249 at 16 MHz); tickless shall use Timer1 free-running at prescaler 256 with a 1.048 s MAX_INTERVAL; arming latency shall be measured at init; emb_arch_cycles shall be a prescaled 32-bit count with its resolution documented; deep sleep shall be EMB_ENOTSUP until a tick-loss policy exists. Rationale: SPEC-003 §13; KRN-TIM-036. Status: Accepted 2026-10-07 Verification: HIL test: tick exactness over one minute against the host clock; arch/avr/tickless_hop; latency measurement test. Trace: SPEC-012 §7 Normative source.

Requirement: Footprint budget ARCH-AVR-006

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

The tiny reference configuration on this port shall fit the T1 and T2 targets: kernel text at most 4 KB at -Os on EmbCC and avr-gcc, kernel static RAM at most 64 bytes, TCB at most 32 bytes; the footprint test shall enforce them from the map file. Rationale: R-003 §4. Status: Accepted 2026-10-07 Verification: Footprint test in CI on both compilers. Trace: SPEC-012 §8, §13 Normative source.

Requirement: Flash constants and linker outputs ARCH-AVR-007

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

Kernel tables shall use EMB_FLASH_CONST and its accessors so that avr-gcc places them in program memory while EmbCC builds still work; large tables shall be compiled out on tiny; the generator shall emit the embld option set and a GNU ld script for the board from one region model. Rationale: 09 §2 (const in RAM), §7 (no linker scripts on AVR); HW-006. Status: Accepted 2026-10-07 Verification: Footprint comparison EmbCC versus avr-gcc; generator test for both outputs. Trace: SPEC-012 §8 Normative source.

Requirement: Robustness without faults ARCH-AVR-008

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

Checked builds on this port shall enable the switch-out stack guard check, the interrupt-reserve check, and the software watchdog service; a kernel fault shall write the small crash record to .noinit and reset through the watchdog by default. Rationale: No fault hardware; the watchdog and the guards are the only safety nets. Status: Accepted 2026-10-07 Verification: Test: arch/avr/guard_detects_overflow, arch/avr/crash_record_survives_wdt_reset. Trace: SPEC-012 §8, §9; FLT-001, FLT-004 Normative source.

Requirement: Debug and emulation path ARCH-AVR-009

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

Symbolic debugging shall use the qualified avr-gcc reference build until the EmbCC RTOS debugging path is qualified; the EmbCC build shall be verified by the conformance suite under QEMU and on the arduino_uno board, and timing from emulation shall never be published. Rationale: EmbCC’s AVR DWARF and remote layout are now implemented (09 §7/§9, reviewed 2026-10-14), but tool capability does not replace RTOS target/session qualification (SIM-002, SIM-003). The original acceptance date below is design status, not evidence that those runs passed. Status: Accepted 2026-10-07 Verification: CI: QEMU AVR run of the conformance suite on both compilers; HIL run on the Uno. Trace: SPEC-012 §10, §12 Normative source.

Requirement: Board description ARCH-AVR-010

status: Accepted evidence: not evaluated design_source: requirements/ARCH-AVR.md

boards/arduino/arduino_uno.yaml shall describe the 16 MHz crystal, the USART0 console, the user LED, and the flash partitions leaving Optiboot’s 512 bytes reserved; board init shall configure only the console and the LED. Rationale: A new board on this SoC is a description file (01 §6). Status: Accepted 2026-10-07 Verification: Generator test; boot test on the board. Trace: SPEC-012 §11 Normative source.