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'.

5 files changed, 103 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
diff --git a/c/chello.c b/c/chello.c
new file mode 100644
index 0000000..f570b61
--- /dev/null
+++ b/c/chello.c
@@ -0,0 +1,9 @@
+#include "gbos.h"
+void main(void) {
+    writes("hello from C on gbos!", 21);
+    nl();
+    writes("my pid is ", 10);
+    /* print pid as a digit */
+    { unsigned char c = getpid() + '0'; char d = c; writes(&d, 1); }
+    nl();
+}
diff --git a/c/crt0.s b/c/crt0.s
new file mode 100644
index 0000000..cde4b67
--- /dev/null
+++ b/c/crt0.s
@@ -0,0 +1,11 @@
+	.module crt0
+	.globl _main
+	;; gbos C entry. exec() maps our ROM bank and jumps to the start of _CODE
+	;; (linked at 0x4000). Call main(), then exit(0). exec() already set SP.
+	.area _CODE
+_start::
+	call	_main
+	ld	b, #0
+	ld	c, #0		; SYS_EXIT
+	rst	#0x30
+1$:	jr	1$
diff --git a/c/gbos.h b/c/gbos.h
new file mode 100644
index 0000000..9a09a5d
--- /dev/null
+++ b/c/gbos.h
@@ -0,0 +1,9 @@
+/* gbos userland C API (thin wrappers over rst $30 syscalls; see c/libc.asm) */
+#ifndef GBOS_H
+#define GBOS_H
+void          writes(const char *buf, unsigned char len); /* write to console */
+void          nl(void);                                   /* newline (CRLF)   */
+unsigned char readc(void);                                /* read one byte    */
+void          sexit(unsigned char code);                  /* exit(code)       */
+unsigned char getpid(void);
+#endif
diff --git a/c/libc.s b/c/libc.s
new file mode 100644
index 0000000..4fca5e3
--- /dev/null
+++ b/c/libc.s
@@ -0,0 +1,54 @@
+	.module libc
+	.globl _writes, _nl, _readc, _sexit, _getpid
+	.area _CODE
+	;; void writes(const char *buf /*DE*/, unsigned char len /*A*/)
+_writes:
+	push	hl
+	push	de
+	push	bc
+	ld	b, a
+	ld	c, #3		; SYS_WRITE
+	rst	#0x30
+	pop	bc
+	pop	de
+	pop	hl
+	ret
+	;; void nl(void) - CRLF straight to serial
+_nl:
+	ld	a, #0x0d
+	ld	(0xff01), a
+	ld	a, #0x81
+	ld	(0xff02), a
+	ld	a, #0x0a
+	ld	(0xff01), a
+	ld	a, #0x81
+	ld	(0xff02), a
+	ret
+	;; unsigned char readc(void) -> A
+_readc:
+	push	hl
+	push	de
+	push	bc
+	ld	c, #2		; SYS_READ
+	rst	#0x30
+	pop	bc
+	pop	de
+	pop	hl
+	ret
+	;; void sexit(unsigned char code /*A*/)
+_sexit:
+	ld	b, a
+	ld	c, #0		; SYS_EXIT
+	rst	#0x30
+	ret
+	;; unsigned char getpid(void) -> A
+_getpid:
+	push	hl
+	push	de
+	push	bc
+	ld	c, #8		; SYS_GETPID
+	rst	#0x30
+	pop	bc
+	pop	de
+	pop	hl
+	ret