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, 12 insertions, 6 deletions

diff --git a/c/cat.c b/c/cat.c
index 4092778..dde58cd 100644
--- a/c/cat.c
+++ b/c/cat.c
@@ -1,10 +1,16 @@
 #include "gbos.h"
-/* cat: copy stdin to stdout until EOF (Ctrl-D / pipe end). */
+/* cat [file]: print a file, or copy stdin to stdout until EOF. */
 void main(void) {
-    char c;
-    for (;;) {
-        c = readc();
-        if (c == EOF) break;
-        putc(c);
+    char *argv[4];
+    unsigned char argc = argv_parse(argv, 4);
+    if (argc > 0) {
+        unsigned char fd = open(argv[0], O_READ);
+        int ch;
+        if (fd == NOFD) { puts("cat: no such file"); nl(); return; }
+        while ((ch = fgetc(fd)) >= 0) putc((char)ch);
+        close(fd);
+    } else {
+        char c;
+        for (;;) { c = readc(); if (c == EOF) break; putc(c); }
     }
 }