filesystem: a RAM FS (WRAMX) + ls/cat/save/rm; wc/head read files

- fs.asm: 8-slot RAM filesystem in WRAMX ($D000-$DFFF), name[8]+len[2]+data[502];
  syscalls open/close/getb/putb/list/remove, seeded with a 'readme' at boot.
- libc: open/close/fgetc/fputc/flist/fremove wrappers + O_READ/O_WRITE/NOFD.
- tools: ls, save (stdin->file, one line), rm; cat/wc/head now take a file arg.
- fix: syscall dispatch clobbers A, so putb takes its byte in E (was reading the
  handler's low address byte, 0xDC); ls NUL-terminates 8-char names.
- verified: save/cat/ls/rm cycle; wc readme -> '2 7 38'; head -n 1 readme.
- README: document the filesystem + the A-clobber ABI note.

1 files changed, 48 insertions, 0 deletions

diff --git a/c/libc.s b/c/libc.s
index 29031e9..a26042e 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -1,5 +1,6 @@
 	.module libc
 	.globl _writes, _putc, _puts, _nl, _strlen, _readc, _sexit, _getpid, _getargs
+	.globl _open, _close, _fgetc, _fputc, _flist, _fremove
 	;; SDCC sm83 sdcccall(1): 1st arg -> A (byte) or DE (pointer); ret A / BC.
 	;; rst $30 (the trap) clobbers BC/DE/HL; SDCC treats them caller-saved, so
 	;; wrappers need not preserve them - but we compute cleanly regardless.
@@ -84,3 +85,50 @@ _getpid:
 	ld	c, #8			; SYS_GETPID
 	rst	#0x30
 	ret
+
+	;; unsigned char open(const char *name /*DE*/, unsigned char mode /*A*/) -> A(fd)
+_open:
+	ld	b, a			; B = mode
+	ld	c, #4			; SYS_OPEN
+	rst	#0x30
+	ret
+
+	;; void close(unsigned char fd /*A*/)
+_close:
+	ld	b, a
+	ld	c, #5			; SYS_CLOSE
+	rst	#0x30
+	ret
+
+	;; int fgetc(unsigned char fd /*A*/) -> BC (byte, or -1 at EOF)
+_fgetc:
+	ld	b, a
+	ld	c, #12			; SYS_GETB
+	rst	#0x30
+	jr	c, 1$
+	ld	c, a
+	ld	b, #0
+	ret
+1$:	ld	bc, #0xffff
+	ret
+
+	;; void fputc(unsigned char fd /*A*/, char ch /*E*/)
+	;; byte goes to the kernel in E (the trap clobbers A during dispatch)
+_fputc:
+	ld	b, a			; B = fd  (ch stays in E)
+	ld	c, #13			; SYS_PUTB
+	rst	#0x30
+	ret
+
+	;; unsigned char flist(unsigned char slot /*A*/, char *namebuf /*DE*/) -> A
+_flist:
+	ld	b, a
+	ld	c, #14			; SYS_LIST
+	rst	#0x30
+	ret
+
+	;; void fremove(const char *name /*DE*/)
+_fremove:
+	ld	c, #15			; SYS_REMOVE
+	rst	#0x30
+	ret