[wesbiggs]

Problem

Once Ethernet is initialized (a cable is not needed), NativeEthernet runs the whole FNET stack from a 1 ms IntervalTimer (PIT) ISR (FNET_POLL_TIME in NativeEthernet.h). Measured with late-ISR counters in isrPHI2, about 39 phi2 interrupts per second came in late (>1.1 µs) even though the GPIO interrupt has the higher priority. Masking IRQ_ENET changed nothing; masking IRQ_PIT removed them.

Under Kernal Replace, late interrupts hurt most on the first read after a $01 write, where KernalCheck senses HIRAM before it drives the bus. Programs that switch $01 constantly got corrupt reads, NMIs or froze:

• My test RPG project game (keeps its map under the I/O area and the KERNAL)
• CCGMS with SwiftLink over TCP, running under a replaced KERNAL (its NMI handler writes $01)

Change

While Kernal Replace serves the KERNAL and the stack is up, IRQ_PIT is masked and FnetLoopPollService() (Teensy.ino, FLASHMEM) calls fnet_poll() from loop() every ms. Late phi2 interrupts drop to about one every 2 s. "Serves the KERNAL" means the Kernal Replace Special IO, or fKernRepl set under Fab04_GlobalKernalReplace.

• Starts at PreStart. loop() only sees the new handler ~0.4 s after a launch, and the ~15 late interrupts in that window were still enough to crash Quest for the King. InitHndlr_KERNALReplace_PreStart() therefore starts the loop poll at once (FnetLoopPollStart()), and SetUpMainMenuROM() ends it.
• AT commands. Under Fab04_GlobalKernalReplace, SwiftLink can run while the KERNAL is replaced. connect, DHCP and DNS block waiting for the stack, so ProcessATCommand() unmasks IRQ_PIT while an AT command runs. The wrapper is compiled only under that flag.
• TR+ only. Everything is guarded by Fab04_KernalReplace, so the plain TR build is unchanged (same link report as main).
• Scope. Other handlers are unchanged, plain SwiftLink on the stock KERNAL included: its late-interrupt rate hasn't been measured, so it isn't in this PR.
• Docs. docs/Architecture/Constraints.md describes the mechanism and its two consequences: TCP is serviced only as often as loop() comes round, and new blocking network calls that can run while the KERNAL is replaced need the PIT back.

Memory (link report, before → after)

	TR+ (--no-extensions)	TR
RAM1 free for local variables	85532 → 85532	unchanged
RAM1 padding	31800 → 31784	unchanged
RAM2 variables	17600 → 17600	unchanged
FLASH code	269004 → 269284	unchanged

Testing

• Hardware: real C64 (breadbin), TR+ (PCB v0.4), with Ethernet initialized.
  • RPG plays and loads/saves under Kernal Replace.
  • CCGMS with SwiftLink over TCP under a replaced KERNAL: no stalls or flow-control problems; a repeatable first-page receive glitch remains, cause not yet found.
  • Tested as part of the teensyrom-vdrive firmware build (the same mechanism, with VDrive's handlers in the list).
• Host: npm test passes; TR and TR+ firmware build, and TR+ also builds with Fab04_GlobalKernalReplace defined.
• No host test added: the change is interrupt masking and polling cadence, which the existing harnesses can't exercise meaningfully.

🤖 Generated with Claude Code