Put serial_offset into PROGMEM

This saves a bit of RAM, at the cost of a bit of extra flash (more code
to load the values) in place. However, since this is the last variable
in the .data section, this avoids the overhead gcc generated for loading
data into the .data section, effectively saving 16 bytes of flash.

Author: matthijskooijman

bootloader.cpp

@@ -18,6 +18,7 @@
 #include <string.h>
 #include <avr/io.h>
 #include <avr/boot.h>
+#include <avr/pgmspace.h>
 #include <util/delay.h>
 #include <stdio.h>
 
@@ -183,7 +184,7 @@ cmd_result processCommand(uint8_t cmd, uint8_t *datain, uint8_t len, uint8_t *da
 			// These are offsets into the device signature imprint table, which
 			// store the parts of the serial number (lot number, wafer number, x/y
 			// coordinates).
-			static const uint8_t serial_offset[] = {0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x15, 0x16, 0x17};
+			static const uint8_t PROGMEM serial_offset[] = {0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x15, 0x16, 0x17};
 
 			if (len != 0)
 				return cmd_result(Status::INVALID_ARGUMENTS);
@@ -192,7 +193,7 @@ cmd_result processCommand(uint8_t cmd, uint8_t *datain, uint8_t len, uint8_t *da
 				return cmd_result(Status::NO_REPLY);
 
 			for (uint8_t i = 0; i < sizeof(serial_offset); ++i)
-				dataout[i] = boot_signature_byte_get(serial_offset[i]);
+				dataout[i] = boot_signature_byte_get(pgm_read_byte(&serial_offset[i]));
 			return cmd_ok(sizeof(serial_offset));
 		}
 		case Commands::START_APPLICATION: