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, 19 insertions, 8 deletions

diff --git a/c/head.c b/c/head.c
index 952d4c5..e8bf846 100644
--- a/c/head.c
+++ b/c/head.c
@@ -1,17 +1,28 @@
 #include "gbos.h"
-/* head [-n N] [N]: print the first N lines of stdin (default 10). */
+/* head [-n N] [N] [file]: first N lines of a file or stdin (default 10). */
 void main(void) {
     char *argv[8];
     unsigned char argc = argv_parse(argv, 8);
     char *nv = optval(argv, argc, 'n');
-    unsigned char n = 10, lines = 0;
-    char c;
+    unsigned char n = 10, lines = 0, i, fd = NOFD;
+    char *fname = 0;
+    int ch;
     if (nv) n = atou(nv);
-    else if (argc > 0 && argv[0][0] >= '0' && argv[0][0] <= '9') n = atou(argv[0]);
+    for (i = 0; i < argc; i++) {
+        if (argv[i][0] == '-') continue;
+        if (nv == argv[i]) continue;
+        if (!nv && argv[i][0] >= '0' && argv[i][0] <= '9') { n = atou(argv[i]); continue; }
+        fname = argv[i]; break;
+    }
+    if (fname) {
+        fd = open(fname, O_READ);
+        if (fd == NOFD) { puts("head: no such file"); nl(); return; }
+    }
     for (;;) {
-        c = readc();
-        if (c == EOF) break;
-        if (lines < n) putc(c);
-        if (c == '\n') lines++;
+        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        else { ch = fgetc(fd); if (ch < 0) break; }
+        if (lines < n) putc((char)ch);
+        if (ch == '\n') lines++;
     }
+    if (fd != NOFD) close(fd);
 }