kernel: stat/fsstat/isatty syscalls, and O_APPEND

Four things userland had no way to ask for:

- SYS_STAT: type, link count, size and inode for a path. The trap needs HL
  for its dispatch table, so a call taking both a path and an output buffer
  passes a request block - the shape SYS_NET already established. usr/stat.h
  wraps it, header-only like sock.h, because the block must live in the
  caller's RAM.
- SYS_FSSTAT: blocks/inodes, total and free. The bitmaps are already cached
  write-through in fs_bmbuf, so this is the popcount boot_fs already prints.
- SYS_ISATTY: is this process's stdin the console? The shell needs it to tell
  an interactive session from a script.
- O_APPEND: open a file positioned at EOF. There is no seek syscall, so this
  is the only way to add to a file - without it nothing running ON the Game
  Boy can build a multi-line file, shell scripts included.

Together these back ls -l, df, and `>>`.

3 files changed, 61 insertions, 3 deletions

diff --git a/usr/gbos.h b/usr/gbos.h
index 5fd897a..014aea4 100644
--- a/usr/gbos.h
+++ b/usr/gbos.h
@@ -20,6 +20,9 @@ unsigned char pollin(void);             /* poll OSK for a typed char, or 0 */
 unsigned char pollcon(void);            /* poll console ring (link-injected
                                            input, filled by net pumps), or 0 */
 unsigned char sys_net(void *req);       /* socket layer trap (see sock.h)  */
+unsigned char sys_stat(void *req);      /* stat trap (see stat.h)          */
+unsigned char sys_fsstat(void *buf);    /* free blocks/inodes (see stat.h) */
+unsigned char isatty(void);             /* 1 if stdin is the console       */
 void          gsleep(unsigned char units); /* delay units*(1/64)s (raw trap)  */
 void          gticks(unsigned char buf[4]); /* 64 Hz uptime ticks, 32-bit LE  */
 void          msleep(unsigned int ms);     /* delay ~ms milliseconds          */
@@ -42,8 +45,9 @@ unsigned char mkdir(const char *path);
 unsigned char chdir(const char *path);
 unsigned char opendir(const char *path);
 void          poweroff(void);   /* clean emulator shutdown (saves) */
-#define O_READ  0
-#define O_WRITE 1
+#define O_READ   0
+#define O_WRITE  1
+#define O_APPEND 2 /* write at the end, keeping the existing contents */
 #define NOFD    0xFF
 #define EISDIR  0xFE     /* open() failed: path is a directory */
 
diff --git a/usr/libc.s b/usr/libc.s
index fbca836..de087b6 100644
--- a/usr/libc.s
+++ b/usr/libc.s
@@ -1,7 +1,7 @@
 	.module libc
 	.globl _writes, _putc, _puts, _nl, _strlen, _readc, _sexit, _getpid, _getargs
 	.globl _open, _close, _fgetc, _fputc, _flist, _fremove, _pipe, _ssend, _srecv, _srecv_nb, _pollin, _pollcon
-	.globl _sys_net, _gsleep, _gticks
+	.globl _sys_net, _gsleep, _gticks, _sys_stat, _sys_fsstat, _isatty
 	;; SDCC sm83 sdcccall(1): 1st arg -> A (byte) or DE (pointer); ret A / BC.
 	;; rst $30 (the trap) clobbers BC/DE/HL; SDCC treats them caller-saved, so
 	;; wrappers need not preserve them - but we compute cleanly regardless.
@@ -318,3 +318,21 @@ _opendir:
 _poweroff:
 	.db	0xED
 1$:	jr	1$
+
+	;; unsigned char sys_stat(void *req /*DE*/) -> A   (see fs.asm sys_stat)
+_sys_stat:
+	ld	c, #37			; SYS_STAT
+	rst	#0x30
+	ret
+
+	;; unsigned char sys_fsstat(void *buf /*DE*/) -> A
+_sys_fsstat:
+	ld	c, #38			; SYS_FSSTAT
+	rst	#0x30
+	ret
+
+	;; unsigned char isatty(void) -> A (1 = stdin is the console)
+_isatty:
+	ld	c, #39			; SYS_ISATTY
+	rst	#0x30
+	ret
diff --git a/usr/stat.h b/usr/stat.h
new file mode 100644
index 0000000..10acc54
--- /dev/null
+++ b/usr/stat.h
@@ -0,0 +1,36 @@
+#ifndef STAT_H
+#define STAT_H
+/* stat.h - file metadata and filesystem totals. Header-only (one copy per
+   program), like sock.h: the request block has to live in the caller's RAM.
+   The kernel trap needs HL for its dispatch table, so a call with both a path
+   and an output buffer passes a request block rather than two pointers. */
+#include "gbos.h"
+
+#define ST_FILE 1
+#define ST_DIR  2
+
+struct gstat {
+    unsigned char type; /* ST_FILE / ST_DIR */
+    unsigned char nlink;
+    unsigned int size; /* bytes */
+    unsigned char ino;
+};
+
+static unsigned char _sreq[7]; /* path ptr, then type/nlink/size/ino */
+
+/* -> 1 on success, 0 if the path does not exist */
+static unsigned char gstat(const char *path, struct gstat *st) {
+    _sreq[0] = (unsigned char)(unsigned int)path;
+    _sreq[1] = (unsigned char)(((unsigned int)path) >> 8);
+    if (sys_stat(_sreq) != 0) return 0;
+    st->type = _sreq[2];
+    st->nlink = _sreq[3];
+    st->size = (unsigned int)_sreq[4] | ((unsigned int)_sreq[5] << 8);
+    st->ino = _sreq[6];
+    return 1;
+}
+
+/* out[] = { blocks total, blocks free, inodes total, inodes free } */
+static void gfsstat(unsigned char *out) { sys_fsstat(out); }
+
+#endif