Advantages
Virtually all embedded systems have a hardware element and a software element, which are separate but tightly interdependent. The ICE allows the software element to be run and tested on the actual hardware on which it is to run, but still allows programmer conveniences to help isolate faulty code, such as "source-level debugging" (which shows the program the way the programmer wrote it) and single-stepping (which lets the programmer run the program step-by-step to find errors).
Most ICEs consist of an adaptor unit that sits between the ICE host computer and the system to be tested. A header and cable assembly connects the adaptor to a socket where the actual CPU or microcontroller mounts within the embedded system. Recent ICEs enable a programmer to access the on-chip debug circuit that is integrated into the CPU via JTAG or BDM (Background Debug Mode) in order to debug the software of an embedded system. These systems often use a standard version of the CPU chip, and can simply attach to a debug port on a production system. They are sometimes called in-circuit debuggers or ICDs, to distinguish the fact that they do not replicate the functionality of the CPU, but instead control an already existing, standard CPU. Since the CPU does not have to be replaced, they can operate on production units where the CPU is soldered in and cannot be replaced. An example is Microchip Technology's ICD, which interfaces with most recent PIC microcontrollers to debug software by attaching to the PIC's In-circuit programming/debugging port. On x86 Pentiums, a special 'probe mode' is used by ICEs to aid in debugging.
In the context of embedded systems, the ICE is not emulating hardware. Rather, it is providing direct debug access to the actual CPU. The system under test is under full control, allowing the developer to load, debug and test code directly.
Most host systems are ordinary commercial computers unrelated to the CPU used for development - for example, a Linux PC might be used to develop software for a system using a Freescale 68HC11 chip, which itself could not run Linux.
The programmer usually edits and compiles the embedded system's code on the host system, as well. The host system will have special compilers that produce executable code for the embedded system. These are called cross compilers or cross assemblers.
Read more about this topic: In-circuit Emulator
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