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The kernel had no clock: TimerISR was a reti stub, IEF_TIMER masked, and
IME was never enabled - the vectors were decorative. sys_sleep just polls
DIV deltas per-process; nothing counted globally.
Now: TAC runs the hardware timer at 16384 Hz with TMA=0, so TIMA overflows
at exactly 64 Hz; TimerISR increments a monotonic 32-bit wTicks (wraps
after ~2.1 years). Scheduling stays cooperative - the ISR is transparent.
Enabling IME in a kernel written for zero interrupts needs care wherever
SP points into memory whose bank is being switched (an IRQ pushes onto SP):
- read_block/write_block map the disk bank over the $A000 window that
holds the caller's stack -> di/ei around the transfer (~2ms, well under
the 15.6ms tick period, so no tick is ever lost)
- hSwitchTo switches SVBK + cart-RAM banks under the outgoing stack ->
di on entry, ei once the incoming stack is mapped
- fork already runs on KSTACK_TOP2 (fixed WRAM) - safe as-is
- term_putc's SVBK switch only remaps $Dxxx, stacks live in $Axxx/$Cxxx
SYS_UPTIME (36) copies the counter (4B LE, di/ei so the read can't tear)
to a user buffer; libc gticks(); usr/uptime.c formats 'up [Nd] H:MM:SS'.
uptime avoids SDCC long div/shift entirely: sm83.lib modules link into
their own areas that land in the $A000 RAM window (latent build.sh trap,
documented there) - bytewise >>6 plus bounded subtraction loops instead.
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Userland sources live in usr/ (fits the Unix theme better than 'c').
SDCC output .bin blobs land in build/usr/ with the other build artifacts
instead of littering the source dir; programs.asm INCBINs them from there.
Byte-identical ROM.
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- c/libc.c: shared C helpers linked into every program - putu (print decimal),
atou (parse decimal), argv_parse (tokenize getargs() into argc/argv).
- wc: count lines/words/chars of stdin. head [n]: first n lines (drains rest to
EOF). args: argc/argv demo.
- pid now uses libc putu; build.sh links libc.c; Makefile CBLOBS + libc dep.
- README: document the new tools + argv_parse.
- verified: 'args one two three' -> argc=3/argv[..]; wc '2 3 16'; head 2.
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- c/{gbos.h,crt0.s,libc.s,build.sh}: SDCC sm83 -> ROM-bank blob toolchain.
libc syscall wrappers save/restore BC/DE/HL around rst $30 (trap clobbers
them; SDCC expects them preserved).
- build via SDCC native asxxxx path (sdasgb/sdldgb), crt0 linked first so
_start is the $4000 entry; blob INCBIN'd into a ROM bank.
- chello.c: prints a message + getpid() -> runs as 'chello' shell command.
- Makefile: auto-build c/*.bin, track as a dep of programs.o; gitignore blobs.
- README: document the C toolchain + the SDCC --asm=rgbds codegen bug that
forced the native-toolchain approach.
- verified: '$ chello' -> 'hello from C on gbos!' / 'my pid is 2'.
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