C toolchain: run SDCC-compiled C programs as gbos processes

- 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'.

1 files changed, 20 insertions, 0 deletions

diff --git a/c/build.sh b/c/build.sh
new file mode 100755
index 0000000..6a2deab
--- /dev/null
+++ b/c/build.sh
@@ -0,0 +1,20 @@
+#!/bin/sh
+# Build a gbos C program into a self-contained 16 KiB ROM-bank image.
+# SDCC native toolchain; crt0 linked FIRST so _start is the $4000 entry.
+# usage: c/build.sh <prog.c> <out.bin>
+set -e
+SRC="$1"; OUT="$2"; B=build/c
+LIBDIR=/usr/share/sdcc/lib/sm83
+mkdir -p "$B"
+sdasgb -o "$B/crt0.rel" c/crt0.s
+sdasgb -o "$B/libc.rel" c/libc.s
+sdcc -msm83 -c -Ic "$SRC" -o "$B/prog.rel"
+# link: crt0 first (=> _start at 0x4000), then libc, program, and sm83 lib
+sdldgb -n -m -w -j -i "$B/prog.ihx" \
+       -b _CODE=0x4000 -b _DATA=0xa000 \
+       -k "$LIBDIR" -l sm83 \
+       "$B/crt0.rel" "$B/libc.rel" "$B/prog.rel"
+makebin -Z -p "$B/prog.ihx" "$B/prog.bin"
+dd if="$B/prog.bin" of="$OUT" bs=1024 skip=16 count=16 2>/dev/null
+echo "wrote $OUT ($(wc -c < "$OUT") bytes); entry:"
+grep -iE ' _start| _main' "$B/prog.map" | head -2