From ef88f0e530b898ebcb14f9e3dff1d58e83089ada Mon Sep 17 00:00:00 2001
From: user <user@clank>
Date: Thu, 16 Jul 2026 00:49:58 +0200
Subject: 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'.
---
 c/crt0.s | 11 +++++++++++
 1 file changed, 11 insertions(+)
 create mode 100644 c/crt0.s

(limited to 'c/crt0.s')

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$:	jr	1$
-- 
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