Data Hacking

140 Part II — Software Hacks
This process isn’t as straightforward as using MeMap, but it does take you closer to the code of
the firmware and enables you to do more than you can with MeMap.
Before we go further, let’s have a quick tutorial on the basic data structures in computers and
look at how hex and bytes fit in.
A byte is composed of eight bits. A bit is the most basic unit of data in a computer. Bits can be
either on (1) or off (0). These are the basic units of the number system known as binary.For
example, if you have eight bits that are all on, you could represent the number 11111111. This
same number would be FF in hexadecimal because hexadecimal uses characters to count past 9:
A, B, C, D, E, and F.
Putting this all together, the binary notation for the largest number a byte can represent would
be 11111111, which is equal to 256 in decimal and equivalent to FF in hexadecimal.
Fortunately, you don’t really need to know that much about bits, bytes, or hexadecimal to modify
the GPS firmware.
If you want to experiment with binary, decimal, and hexadecimal numbers, fire up Windows
Calculator, which enables you to work with all three formats.
The text strings that appear on the screen of your Garmin eTrex or eMap are stored in the
firmware as text, which means that it is relatively easy for you to search for, find, and replace
these strings of text using a hex editor, as shown in the following steps:
1. Open the firmware file in your selected hex editor. This is usually done by selecting
File ➪ Open and searching for the appropriate file (see Figure 6-20).
Garmin eMap and eTrex firmware files have a .RGN file extension so they are quite easy
to find.
FIGURE 6-20: Search for the appropriate file.