Extracting Flash Memory using JTAG

I previously mentioned dumping memory contents using SPI, with a BusPirate. Sometimes that’s not feasible – such as if the flash memory module is a little inaccessible and you’re not feeling like deconstructing the board just yet. An alternative is to pull memory over JTAG. I talked about accessing JTAG and interacting with a chip using OpenOCD previously, however this time around I’d like to go a step further. The board I’m using in this example is a Netgear DG834Gv1, which has an exposed JTAG, shown below on the bottom right of the board (in red):

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JTAGulating JTAG!

Discovering JTAG ports with the JTAGulator, and connecting to them with UM232H! What is JTAG? JTAG is short for Joint Test Action Group and generally refers to on-chip debugging interfaces that follow the IEEE 1149.x standard. The standard doesn’t mandate a certain connection – it just dictates a standard for communicating with chips in a device. It uses 5 pins: TCK, TMS, TDI, TDO and (options) TRST; which are (Test) Clock, Mode Select, Data In, Data Out, and Reset. It can be useful to hardware hackers in various ways, such as extracting device IDs, extracting firmware, overwriting memory.

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