altos/lisp: Append a CRC to the saved image to validate on restore

The CRC is actually of the ROM bits, so we can tell if the restored
image relates to the currently running code.

Signed-off-by: Keith Packard <keithp@keithp.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/src/lisp/ao_lisp_make_const.c b/src/lisp/ao_lisp_make_const.c
index 0a8c9d07..6a29f402 100644
--- a/src/lisp/ao_lisp_make_const.c
+++ b/src/lisp/ao_lisp_make_const.c
@@ -86,6 +86,33 @@ is_atom(int offset)
 	return 0;
 }
 
+#define AO_FEC_CRC_INIT	0xffff
+
+static inline uint16_t
+ao_fec_crc_byte(uint8_t byte, uint16_t crc)
+{
+	uint8_t	bit;
+
+	for (bit = 0; bit < 8; bit++) {
+		if (((crc & 0x8000) >> 8) ^ (byte & 0x80))
+			crc = (crc << 1) ^ 0x8005;
+		else
+			crc = (crc << 1);
+		byte <<= 1;
+	}
+	return crc;
+}
+
+uint16_t
+ao_fec_crc(const uint8_t *bytes, uint8_t len)
+{
+	uint16_t	crc = AO_FEC_CRC_INIT;
+
+	while (len--)
+		crc = ao_fec_crc_byte(*bytes++, crc);
+	return crc;
+}
+
 int
 main(int argc, char **argv)
 {
@@ -126,6 +153,7 @@ main(int argc, char **argv)
 	printf("extern const uint8_t ao_lisp_const[AO_LISP_POOL_CONST] __attribute__((aligned(4)));\n");
 	printf("#define ao_builtin_atoms 0x%04x\n", ao_lisp_atom_poly(ao_lisp_atoms));
 	printf("#define ao_builtin_frame 0x%04x\n", ao_lisp_frame_poly(ao_lisp_frame_global));
+	printf("#define ao_lisp_const_checksum ((uint16_t) 0x%04x)\n", ao_fec_crc(ao_lisp_const, ao_lisp_top));
 
 	for (a = ao_lisp_atoms; a; a = ao_lisp_poly_atom(a->next)) {
 		char	*n = a->name, c;