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, 29 insertions, 1 deletions

diff --git a/include/gbos.inc b/include/gbos.inc
index 156876b..7c3f325 100644
--- a/include/gbos.inc
+++ b/include/gbos.inc
@@ -84,7 +84,32 @@ DEF SYS_GETPID  EQU 8
 DEF SYS_KILL    EQU 9
 DEF SYS_BRK     EQU 10
 DEF SYS_YIELD   EQU 11
-DEF SYS_MAX     EQU 12
+DEF SYS_GETB    EQU 12   ; read a byte from an open file  (B=fd -> A, CF=EOF)
+DEF SYS_PUTB    EQU 13   ; append a byte to an open file  (B=fd, E=byte)
+DEF SYS_LIST    EQU 14   ; list a directory slot          (B=slot, DE=namebuf)
+DEF SYS_REMOVE  EQU 15   ; delete a file                  (DE=name)
+DEF SYS_MAX     EQU 16
+; (SYS_OPEN=4 / SYS_CLOSE=5 are now implemented by the filesystem)
+
+; -----------------------------------------------------------------------------
+; RAM filesystem (lives in WRAMX $D000-$DFFF, 4 KiB; fixed in DMG mode).
+; 8 files x 512 bytes: name[8] (first byte 0 = free) + len[2] + data[502].
+; -----------------------------------------------------------------------------
+DEF FS_BASE      EQU $D000
+DEF FS_MAX_FILES EQU 8
+DEF FS_FILE_SIZE EQU 512
+DEF FS_NAME      EQU 0
+DEF FS_LEN       EQU 8
+DEF FS_DATA      EQU 10
+DEF FS_MAX_DATA  EQU 502
+
+; open-file table (kernel WRAM0): inuse, slot, mode, pos(2)
+DEF OF_MAX   EQU 4
+DEF OF_INUSE EQU 0
+DEF OF_SLOT  EQU 1
+DEF OF_MODE  EQU 2
+DEF OF_POS   EQU 3
+DEF OF_SIZE  EQU 5
 
 ; -----------------------------------------------------------------------------
 ; Program table indices (see programs.asm). exec() takes one of these in B.
@@ -102,5 +127,8 @@ DEF PROG_CAT    EQU 9
 DEF PROG_WC     EQU 10
 DEF PROG_HEAD   EQU 11
 DEF PROG_ARGS   EQU 12
+DEF PROG_LS     EQU 13
+DEF PROG_SAVE   EQU 14
+DEF PROG_RM     EQU 15
 
     ENDC