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, 54 insertions, 0 deletions

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