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, 11 insertions, 0 deletions

diff --git a/c/gbos.h b/c/gbos.h
index 47f75ad..d8c8eb3 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -19,4 +19,15 @@ unsigned char atou(const char *s);               /* parse decimal          */
 unsigned char argv_parse(char **argv, unsigned char maxv); /* argc/argv     */
 unsigned char hasflag(char **argv, unsigned char argc, char f);   /* -x flag  */
 char         *optval(char **argv, unsigned char argc, char opt);  /* -x VALUE */
+
+/* filesystem (c/libc.s -> kernel syscalls) */
+unsigned char open(const char *name, unsigned char mode);  /* 0=read 1=write */
+void          close(unsigned char fd);
+int           fgetc(unsigned char fd);          /* a byte, or -1 at EOF */
+void          fputc(unsigned char fd, char ch);
+unsigned char flist(unsigned char slot, char *namebuf); /* 1 if slot used */
+void          fremove(const char *name);
+#define O_READ  0
+#define O_WRITE 1
+#define NOFD    0xFF
 #endif