Editing JTAG (section)

=== Storing firmware ===
JTAG allows [[Programmer (hardware)|device programmer hardware]] to transfer data into internal non-volatile device memory (e.g., [[Complex programmable logic device|CPLDs]]). Some device programmers serve a double purpose for programming as well as debugging the device. In the case of FPGAs, volatile memory devices can also be programmed via the JTAG port, normally during development work. In addition, internal monitoring capabilities (temperature, voltage, and current) may be accessible via the JTAG port.

JTAG programmers are also used to write software and data into [[flash memory]]. This is usually done using the same data bus access the CPU would use, and is sometimes handled by the CPU. In other cases the memory chips themselves have JTAG interfaces. Some modern debug architectures provide internal and external bus master access without needing to halt and take over a CPU. In the worst case, it is usually possible to drive external bus signals using the boundary scan facility.

As a practical matter, when developing an embedded system, emulating the instruction store is the fastest way to implement the ''debug cycle'' (edit, compile, download, test, and debug).{{citation needed|date=June 2015}} This is because the in-circuit emulator simulating an instruction store can be updated very quickly from the development host via, say, USB. Using a serial [[UART]] port and bootloader to upload firmware to Flash makes this debug cycle quite slow and possibly expensive in terms of tools; installing firmware into Flash (or SRAM instead of Flash) via JTAG is an intermediate solution between these extremes.