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.

16 files changed, 697 insertions, 29 deletions

diff --git a/Makefile b/Makefile
index 857cc0a..8c38b3c 100644
--- a/Makefile
+++ b/Makefile
@@ -19,7 +19,8 @@ $(BUILD)/%.o: src/%.asm | $(BUILD)
 
 # C programs: compiled with SDCC into ROM-bank blobs and INCBIN'd by programs.asm
 CBLOBS := c/chello.bin c/echo.bin c/true.bin c/false.bin c/uname.bin \
-          c/pid.bin c/cat.bin c/wc.bin c/head.bin c/args.bin
+          c/pid.bin c/cat.bin c/wc.bin c/head.bin c/args.bin \
+          c/ls.bin c/save.bin c/rm.bin
 $(BUILD)/programs.o: $(CBLOBS)
 
 # C programs also depend on the shared libc sources
diff --git a/README.md b/README.md
index 84c5c72..85a01d2 100644
--- a/README.md
+++ b/README.md
@@ -214,15 +214,20 @@ unsigned char argc = argv_parse(argv, 8);   /* args one two -> argc=3 */
 | tool | what it does |
 |------|--------------|
 | `echo` | print its arguments (`echo hello world`) |
-| `cat`  | copy stdin to stdout until EOF (Ctrl-D / pipe end) |
-| `wc`   | count lines / words / chars of stdin |
-| `head` | print the first *n* lines of stdin (`head 5`, default 10) |
+| `cat`  | print a file (`cat readme`) or copy stdin to stdout |
+| `wc`   | count lines/words/chars of a file or stdin (`wc -l readme`) |
+| `head` | first *n* lines of a file or stdin (`head -n 5 readme`) |
+| `ls`   | list files |
+| `save` | read a line from stdin into a file (`save notes`) |
+| `rm`   | delete a file (`rm notes`) |
 | `args` | argc/argv demo (`args one two three`) |
 | `uname`| print the system name |
 | `pid`  | print the process's pid (decimal) |
 | `true` / `false` | exit 0 / 1 |
 | `chello` | the C "hello" demo |
 
+Options use `hasflag`/`optval` in libc: `wc -l`/`-w`/`-c`, `head -n N`.
+
 ```
 $ echo C tools on a Game Boy
 C tools on a Game Boy
@@ -247,6 +252,35 @@ so we use SDCC's native asxxxx path and INCBIN the blob instead. String literals
 with embedded control chars are also mangled in rgbds mode; C programs pass
 explicit lengths and emit newlines via `nl()`.
 
+## Filesystem
+
+A tiny **RAM filesystem** lives in **WRAMX (`$D000-$DFFF`, 4 KiB)** — always
+mapped in DMG mode and simultaneously accessible with a process's `$A000` data
+bank, so the kernel copies between file and process memory with no bounce
+buffer. 8 fixed slots of 512 bytes: `name[8]` + `len[2]` + `data[502]`. Seeded
+with a `readme` at boot.
+
+Syscalls: `open` (4), `close` (5), `getb` (12), `putb` (13, byte in `E`),
+`list` (14), `remove` (15). Libc: `open`/`close`/`fgetc`/`fputc`/`flist`/
+`fremove` (`c/libc.s`).
+
+```
+$ save notes
+gbos has a filesystem now
+$ cat notes
+gbos has a filesystem now
+$ wc notes
+1 5 26
+$ ls
+readme
+notes
+$ rm notes
+```
+
+> **ABI note:** the syscall trap indexes its jump table with `A`, so syscall
+> args go in `B`/`DE`/`E` (never `A`) and results come back in `A`. That's why
+> `putb` takes its byte in `E`.
+
 ## Roadmap
 
 - [x] `fork`: copy parent's 8 KiB RAM bank → free bank, child returns 0
@@ -259,7 +293,10 @@ explicit lengths and emit newlines via `nl()`.
 - [x] CLI tools in C: `echo`, `cat`, `uname`, `pid`, `true`, `false`
 - [x] Makefile builds all C programs (a `CBLOBS` list)
 - [x] `wc`, `head`, and an `argc/argv` demo (`args`) + `argv_parse` in libc
-- [ ] more tools (`rev`, `grep`), option parsing (`-n`), a RAM filesystem
+- [x] option parsing (`hasflag`/`optval`): `wc -l/-w/-c`, `head -n N`
+- [x] a RAM filesystem (WRAMX) + `ls`/`cat`/`save`/`rm`; `wc`/`head` read files
+- [ ] more tools (`rev`, `grep`, `tail`), shell redirection (`>`/`|`)
+- [ ] battery-backed files (move the FS to cart SRAM), directories
 - [ ] Preemptive scheduling: real context save in `TimerISR` → `hSwitchTo`
 - [ ] Use the `$D000-$DFFF` u-area (SVBK) for per-process kernel state / kstack
 - [ ] `brk`/heap allocator inside the task bank (heap up, stack down, collision = ENOMEM)
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); }
     }
 }
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
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);
 }
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
diff --git a/c/ls.c b/c/ls.c
new file mode 100644
index 0000000..f7e5161
--- /dev/null
+++ b/c/ls.c
@@ -0,0 +1,9 @@
+#include "gbos.h"
+/* ls: list files in the RAM filesystem. */
+void main(void) {
+    char name[9];
+    unsigned char i;
+    for (i = 0; i < 8; i++) {
+        if (flist(i, name)) { name[8] = 0; puts(name); nl(); }
+    }
+}
diff --git a/c/rm.c b/c/rm.c
new file mode 100644
index 0000000..25f3004
--- /dev/null
+++ b/c/rm.c
@@ -0,0 +1,7 @@
+#include "gbos.h"
+/* rm <name>: delete a file. */
+void main(void) {
+    char *argv[4];
+    unsigned char argc = argv_parse(argv, 4);
+    if (argc > 0) fremove(argv[0]);
+}
diff --git a/c/save.c b/c/save.c
new file mode 100644
index 0000000..dd2b8cd
--- /dev/null
+++ b/c/save.c
@@ -0,0 +1,19 @@
+#include "gbos.h"
+/* save <name>: read one line from stdin and store it as a file. */
+void main(void) {
+    char *argv[4];
+    unsigned char argc = argv_parse(argv, 4);
+    unsigned char fd;
+    char c;
+    if (argc < 1) { puts("usage: save <name>"); nl(); return; }
+    fd = open(argv[0], O_WRITE);
+    if (fd == NOFD) { puts("save: cannot create"); nl(); return; }
+    for (;;) {
+        c = readc();
+        if (c == EOF) break;
+        putc(c);            /* echo */
+        fputc(fd, c);
+        if (c == '\n') break;
+    }
+    close(fd);
+}
diff --git a/c/wc.c b/c/wc.c
index 86c3aed..424876d 100644
--- a/c/wc.c
+++ b/c/wc.c
@@ -1,5 +1,5 @@
 #include "gbos.h"
-/* wc [-l] [-w] [-c]: count lines, words, chars of stdin (default: all). */
+/* wc [-l][-w][-c] [file]: count lines/words/chars of a file or stdin. */
 void main(void) {
     char *argv[8];
     unsigned char argc = argv_parse(argv, 8);
@@ -7,17 +7,24 @@ void main(void) {
     unsigned char ww = hasflag(argv, argc, 'w');
     unsigned char wch = hasflag(argv, argc, 'c');
     unsigned int lines = 0, words = 0, chars = 0;
-    unsigned char inword = 0, first = 1;
-    char c;
+    unsigned char inword = 0, first = 1, i, fd = NOFD;
+    char *fname = 0;
+    int ch;
     if (!wl && !ww && !wch) { wl = ww = wch = 1; }
+    for (i = 0; i < argc; i++) if (argv[i][0] != '-') { fname = argv[i]; break; }
+    if (fname) {
+        fd = open(fname, O_READ);
+        if (fd == NOFD) { puts("wc: no such file"); nl(); return; }
+    }
     for (;;) {
-        c = readc();
-        if (c == EOF) break;
+        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        else { ch = fgetc(fd); if (ch < 0) break; }
         chars++;
-        if (c == '\n') lines++;
-        if (c == ' ' || c == '\n' || c == '\r' || c == '\t') inword = 0;
+        if (ch == '\n') lines++;
+        if (ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t') inword = 0;
         else if (!inword) { inword = 1; words++; }
     }
+    if (fd != NOFD) close(fd);
     if (wl)  { putu(lines); first = 0; }
     if (ww)  { if (!first) putc(' '); putu(words); first = 0; }
     if (wch) { if (!first) putc(' '); putu(chars); }
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
diff --git a/src/boot.asm b/src/boot.asm
index 73ea860..7c175ce 100644
--- a/src/boot.asm
+++ b/src/boot.asm
@@ -49,6 +49,7 @@ KernelInit:
     call ClearKernelRAM
     call CopyHramCode           ; move context-switch trampoline into HRAM
     call ProcInit               ; wipe process table
+    call fs_init                ; set up the RAM filesystem
 
     ; --- spawn the init task (pid 1); it fork()s the rest ---
     ld hl, TaskInit
diff --git a/src/fs.asm b/src/fs.asm
new file mode 100644
index 0000000..692fb45
--- /dev/null
+++ b/src/fs.asm
@@ -0,0 +1,451 @@
+; =============================================================================
+; fs.asm - a tiny RAM filesystem in WRAMX ($D000-$DFFF, 4 KiB).
+;
+; 8 fixed slots of 512 bytes: name[8] (first byte 0 = free) + len[2] + data[502].
+; Slot i base = $D000 + i*512  ->  high = $D0 + i*2, low = $00.
+; Because WRAMX is always mapped (DMG) and the caller's buffers live in the
+; mapped $A000 window, the kernel can copy between them directly.
+; =============================================================================
+INCLUDE "include/gbos.inc"
+
+SECTION "fs", ROM0
+
+; -----------------------------------------------------------------------------
+; FsSlotBase - in: A = slot (0..7)  out: HL = slot base.  (A clobbered)
+; -----------------------------------------------------------------------------
+FsSlotBase::
+    add a                       ; slot*2
+    add HIGH(FS_BASE)           ; + $D0
+    ld h, a
+    ld l, 0
+    ret
+
+; -----------------------------------------------------------------------------
+; FsNameEq - compare the <=8-byte NUL-terminated names at DE and HL.
+;   out: Z set if equal.  preserves DE, HL.
+; -----------------------------------------------------------------------------
+FsNameEq::
+    push de
+    push hl
+    ld b, 8
+.cmp
+    ld a, [de]
+    cp [hl]
+    jr nz, .ne
+    or a                        ; equal byte; if NUL, strings ended equal
+    jr z, .eq
+    inc de
+    inc hl
+    dec b
+    jr nz, .cmp
+.eq
+    pop hl
+    pop de
+    xor a                       ; Z set
+    ret
+.ne
+    pop hl
+    pop de
+    ld a, 1
+    and a                       ; Z clear
+    ret
+
+; -----------------------------------------------------------------------------
+; FsFind - in: DE = name  out: A = slot (0..7) or $FF.
+; -----------------------------------------------------------------------------
+FsFind::
+    ld c, 0
+.l
+    ld a, c
+    call FsSlotBase             ; HL = base
+    ld a, [hl]                  ; name[0]
+    or a
+    jr z, .next                 ; free slot
+    call FsNameEq               ; DE vs HL
+    jr z, .found
+.next
+    inc c
+    ld a, c
+    cp FS_MAX_FILES
+    jr c, .l
+    ld a, $FF
+    ret
+.found
+    ld a, c
+    ret
+
+; -----------------------------------------------------------------------------
+; FsAllocSlot - out: A = free slot or $FF.
+; -----------------------------------------------------------------------------
+FsAllocSlot::
+    ld c, 0
+.l
+    ld a, c
+    call FsSlotBase
+    ld a, [hl]
+    or a
+    jr z, .found
+    inc c
+    ld a, c
+    cp FS_MAX_FILES
+    jr c, .l
+    ld a, $FF
+    ret
+.found
+    ld a, c
+    ret
+
+; -----------------------------------------------------------------------------
+; FsSetName - copy the name at DE into slot wFsSlot (<=8 bytes, incl. NUL).
+; -----------------------------------------------------------------------------
+FsSetName::
+    ld a, [wFsSlot]
+    call FsSlotBase             ; HL = base (= name)
+    push de
+    ld b, 8
+.cp
+    ld a, [de]
+    ld [hl+], a
+    or a
+    jr z, .done
+    inc de
+    dec b
+    jr nz, .cp
+.done
+    pop de
+    ret
+
+; -----------------------------------------------------------------------------
+; OFPtr - in: A = fd  out: HL = &open-file entry.  preserves DE.
+; -----------------------------------------------------------------------------
+OFPtr::
+    push de
+    ld h, 0
+    ld l, a
+    ld d, h
+    ld e, l                     ; DE = fd
+    add hl, hl                  ; 2
+    add hl, hl                  ; 4
+    add hl, de                  ; 5*fd
+    ld de, wOFTable
+    add hl, de
+    pop de
+    ret
+
+; =============================================================================
+; sys_open(DE = name, B = mode 0=read/1=write) -> A = fd or $FF
+; =============================================================================
+sys_open::
+    ld a, b
+    ld [wFsMode], a
+    call FsFind                 ; A = slot or $FF (DE = name)
+    ld [wFsSlot], a
+    ld a, [wFsMode]
+    or a
+    jr nz, .write
+    ; --- read mode: must exist ---
+    ld a, [wFsSlot]
+    cp $FF
+    jr z, .fail
+    jr .alloc
+.write
+    ; --- write mode: create or truncate ---
+    ld a, [wFsSlot]
+    cp $FF
+    jr nz, .trunc
+    call FsAllocSlot
+    cp $FF
+    jr z, .fail
+    ld [wFsSlot], a
+    call FsSetName              ; DE still = name
+.trunc
+    ld a, [wFsSlot]
+    call FsSlotBase
+    ld a, l
+    add FS_LEN
+    ld l, a                     ; HL = &len
+    xor a
+    ld [hl+], a
+    ld [hl], a                  ; len = 0
+.alloc
+    ; --- allocate an open-file entry ---
+    ld c, 0
+.of
+    ld a, c
+    call OFPtr                  ; HL = &OF[c]
+    ld a, [hl]
+    or a
+    jr z, .gotof
+    inc c
+    ld a, c
+    cp OF_MAX
+    jr c, .of
+.fail
+    ld a, $FF
+    ret
+.gotof
+    ld a, 1
+    ld [hl+], a                 ; inuse
+    ld a, [wFsSlot]
+    ld [hl+], a                 ; slot
+    ld a, [wFsMode]
+    ld [hl+], a                 ; mode
+    xor a
+    ld [hl+], a                 ; pos lo
+    ld [hl], a                  ; pos hi
+    ld a, c
+    ret
+
+; =============================================================================
+; sys_close(B = fd)
+; =============================================================================
+sys_close::
+    ld a, b
+    cp OF_MAX
+    jr nc, .done
+    call OFPtr
+    xor a
+    ld [hl], a                  ; inuse = 0
+.done
+    xor a
+    ret
+
+; =============================================================================
+; sys_getb(B = fd) -> A = byte, CF set on EOF
+; =============================================================================
+sys_getb::
+    ld a, b
+    cp OF_MAX
+    jr nc, .eof
+    call OFPtr                  ; HL = OF entry
+    ld a, [hl]
+    or a
+    jr z, .eof
+    ld a, l
+    ld [wFsOFPtr], a
+    ld a, h
+    ld [wFsOFPtr+1], a
+    inc hl                      ; OF_SLOT
+    ld a, [hl]
+    ld [wFsSlot], a
+    inc hl                      ; OF_MODE
+    inc hl                      ; OF_POS lo
+    ld a, [hl+]
+    ld e, a
+    ld a, [hl]
+    ld d, a                     ; DE = pos
+    ; file length -> BC
+    ld a, [wFsSlot]
+    call FsSlotBase             ; HL = base
+    ld a, l
+    add FS_LEN
+    ld l, a
+    ld a, [hl+]
+    ld c, a
+    ld a, [hl]
+    ld b, a                     ; BC = len
+    ; pos >= len ?  EOF
+    ld a, e
+    sub c
+    ld a, d
+    sbc b
+    jr nc, .eof
+    ; addr = base + FS_DATA + pos
+    ld a, [wFsSlot]
+    call FsSlotBase             ; HL = base (low = 0)
+    ld a, FS_DATA
+    add e
+    ld l, a
+    ld a, h
+    adc d
+    ld h, a                     ; HL = &data[pos]
+    ld a, [hl]
+    ld [wFsByte], a
+    inc de                      ; pos++
+    ; store pos back
+    ld a, [wFsOFPtr]
+    add OF_POS
+    ld l, a
+    ld a, [wFsOFPtr+1]
+    adc 0
+    ld h, a
+    ld a, e
+    ld [hl+], a
+    ld a, d
+    ld [hl], a
+    ld a, [wFsByte]
+    and a                       ; CF = 0 (success)
+    ret
+.eof
+    scf
+    ret
+
+; =============================================================================
+; sys_putb(B = fd, E = byte)   -- byte is in E, not A: the syscall dispatch
+; clobbers A (it indexes the jump table with it).
+; =============================================================================
+sys_putb::
+    ld a, e
+    ld [wFsByte], a
+    ld a, b
+    cp OF_MAX
+    jr nc, .done
+    call OFPtr
+    ld a, [hl]
+    or a
+    jr z, .done
+    ld a, l
+    ld [wFsOFPtr], a
+    ld a, h
+    ld [wFsOFPtr+1], a
+    inc hl
+    ld a, [hl]
+    ld [wFsSlot], a
+    inc hl
+    inc hl                      ; OF_POS lo
+    ld a, [hl+]
+    ld e, a
+    ld a, [hl]
+    ld d, a                     ; DE = pos
+    ; pos >= FS_MAX_DATA ? full
+    ld a, e
+    sub LOW(FS_MAX_DATA)
+    ld a, d
+    sbc HIGH(FS_MAX_DATA)
+    jr nc, .done
+    ; addr = base + FS_DATA + pos
+    ld a, [wFsSlot]
+    call FsSlotBase
+    ld a, FS_DATA
+    add e
+    ld l, a
+    ld a, h
+    adc d
+    ld h, a
+    ld a, [wFsByte]
+    ld [hl], a                  ; write byte
+    inc de                      ; pos++
+    ; len = pos  (sequential append)
+    ld a, [wFsSlot]
+    call FsSlotBase
+    ld a, l
+    add FS_LEN
+    ld l, a
+    ld a, e
+    ld [hl+], a
+    ld a, d
+    ld [hl], a
+    ; store pos back
+    ld a, [wFsOFPtr]
+    add OF_POS
+    ld l, a
+    ld a, [wFsOFPtr+1]
+    adc 0
+    ld h, a
+    ld a, e
+    ld [hl+], a
+    ld a, d
+    ld [hl], a
+.done
+    xor a
+    ret
+
+; =============================================================================
+; sys_list(B = slot, DE = namebuf[8]) -> A = 1 if used (name copied), else 0
+; =============================================================================
+sys_list::
+    ld a, b
+    cp FS_MAX_FILES
+    jr nc, .no
+    call FsSlotBase             ; HL = base (name)
+    ld a, [hl]
+    or a
+    jr z, .no
+    ld b, 8
+.cp
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec b
+    jr nz, .cp
+    ld a, 1
+    ret
+.no
+    xor a
+    ret
+
+; =============================================================================
+; sys_remove(DE = name) -> A = 0 ok / $FF not found
+; =============================================================================
+sys_remove::
+    call FsFind
+    cp $FF
+    jr z, .no
+    call FsSlotBase
+    xor a
+    ld [hl], a                  ; name[0] = 0 -> free
+    xor a
+    ret
+.no
+    ld a, $FF
+    ret
+
+; =============================================================================
+; fs_init - mark all slots free, clear the OF table, seed a "readme" file.
+; =============================================================================
+fs_init::
+    ld c, 0
+.clr
+    ld a, c
+    call FsSlotBase
+    xor a
+    ld [hl], a                  ; name[0] = 0
+    inc c
+    ld a, c
+    cp FS_MAX_FILES
+    jr c, .clr
+    ; zero the open-file table
+    ld hl, wOFTable
+    ld b, OF_MAX * OF_SIZE
+    xor a
+.zof
+    ld [hl+], a
+    dec b
+    jr nz, .zof
+    ; seed slot 0 = "readme"
+    xor a
+    ld [wFsSlot], a
+    ld de, .rname
+    call FsSetName
+    ; copy content -> data
+    xor a
+    call FsSlotBase             ; HL = base
+    ld a, FS_DATA
+    ld l, a                     ; HL = &data (base low = 0)
+    ld de, .rtext
+    ld bc, .rtext_end - .rtext
+.ccpy
+    ld a, [de]
+    ld [hl+], a
+    inc de
+    dec bc
+    ld a, b
+    or c
+    jr nz, .ccpy
+    ; set len
+    xor a
+    call FsSlotBase
+    ld a, l
+    add FS_LEN
+    ld l, a
+    ld a, LOW(.rtext_end - .rtext)
+    ld [hl+], a
+    ld a, HIGH(.rtext_end - .rtext)
+    ld [hl], a
+    ret
+.rname
+    db "readme", 0
+.rtext
+    db "welcome to gbos!", $0D, $0A
+    db "files live in RAM.", $0D, $0A
+.rtext_end
diff --git a/src/kdata.asm b/src/kdata.asm
index 5b91030..9aca466 100644
--- a/src/kdata.asm
+++ b/src/kdata.asm
@@ -38,3 +38,10 @@ wExitParentPid:: DS 1
 wWaitMyPid::     DS 1
 wWaitRetPid::    DS 1
 wWaitRetCode::   DS 1
+
+; filesystem: open-file table + scratch
+wOFTable::   DS OF_MAX * OF_SIZE
+wFsMode::    DS 1
+wFsSlot::    DS 1
+wFsByte::    DS 1
+wFsOFPtr::   DS 2
diff --git a/src/programs.asm b/src/programs.asm
index a3a6db2..161ee2f 100644
--- a/src/programs.asm
+++ b/src/programs.asm
@@ -79,6 +79,15 @@ ProgHead:
 SECTION "prog_args",   ROMX[$4000], BANK[14]
 ProgArgs:
     INCBIN "c/args.bin"
+SECTION "prog_ls",     ROMX[$4000], BANK[15]
+ProgLs:
+    INCBIN "c/ls.bin"
+SECTION "prog_save",   ROMX[$4000], BANK[16]
+ProgSave:
+    INCBIN "c/save.bin"
+SECTION "prog_rm",     ROMX[$4000], BANK[17]
+ProgRm:
+    INCBIN "c/rm.bin"
 
 ; -----------------------------------------------------------------------------
 ; PROG_SH (bank 3) - a tiny shell.
@@ -217,6 +226,12 @@ ProgSh:
     db PROG_HEAD
     db "args", 0
     db PROG_ARGS
+    db "ls", 0
+    db PROG_LS
+    db "save", 0
+    db PROG_SAVE
+    db "rm", 0
+    db PROG_RM
     db 0                        ; terminator
 
 ; -----------------------------------------------------------------------------
@@ -277,3 +292,9 @@ ProgramTable::
     dw ProgHead
     db LOW(BANK(ProgArgs)), HIGH(BANK(ProgArgs))
     dw ProgArgs
+    db LOW(BANK(ProgLs)), HIGH(BANK(ProgLs))
+    dw ProgLs
+    db LOW(BANK(ProgSave)), HIGH(BANK(ProgSave))
+    dw ProgSave
+    db LOW(BANK(ProgRm)), HIGH(BANK(ProgRm))
+    dw ProgRm
diff --git a/src/syscall.asm b/src/syscall.asm
index bf75865..5ed3d6f 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -38,14 +38,18 @@ SyscallTable:
     dw sys_fork                 ; 1  FORK
     dw sys_read                 ; 2  READ
     dw sys_write                ; 3  WRITE
-    dw sys_nosys                ; 4  OPEN  (TODO)
-    dw sys_nosys                ; 5  CLOSE (TODO)
+    dw sys_open                 ; 4  OPEN
+    dw sys_close                ; 5  CLOSE
     dw sys_exec                 ; 6  EXEC
     dw sys_wait                 ; 7  WAIT
     dw sys_getpid               ; 8  GETPID
     dw sys_nosys                ; 9  KILL  (TODO)
     dw sys_nosys                ; 10 BRK   (TODO)
     dw sys_yield                ; 11 YIELD
+    dw sys_getb                 ; 12 GETB
+    dw sys_putb                 ; 13 PUTB
+    dw sys_list                 ; 14 LIST
+    dw sys_remove               ; 15 REMOVE
 
 ; -----------------------------------------------------------------------------
 sys_nosys: