Sometimes a gray area exists when using processor type to differentiate between embedded and non-embedded systems. It is worth noting that, in large-scale, high-performance embedded systems, the choice between embedded processors and universal microprocessors is a difficult one.
In high-end embedded systems, system performance in a predefined context outweighs power consumption and cost. The choice of a high-end, general purpose processor is as good as the choice of a high-end, specialized embedded processor in some designs. Therefore, using processor type alone to classify embedded systems may result in wrong classifications.
1.1.6 Hardware and Software Co-Design Model
The hardware and software co-design model reemphasizes the fundamental characteristic of embedded systems-they are application-specific. An embedded system is usually built on custom hardware and software. Therefore, using this development model is both permissible and beneficial.
1.1.7 Cross-Platform Development
cross-platform development platformThe host system is the system on which the embedded software is developed. The target system is the embedded system under development.
The main software tool that makes cross-platform development possible is a cross compiler. A cross compiler is a compiler that runs on one type of processor architecture but produces object code for a different type of processor architecture. A cross compiler is used because the target system cannot host its own compiler. For example, the DIAB compiler from Wind River Systems is such a cross compiler. The DIAB compiler runs on the Microsoft Windows operating system (OS) on the IA-32 architecture and runs on various UNIX operating systems, such as the Solaris OS on the SPARC architecture. The compiler can produce object code for numerous processor types, such as Motorolas 68000, MIPS, and ARM. We discuss more cross-development tools in Chapter 2.
1.1.8 Software Storage and Upgradeability
The choice of software storage device has an impact on development. The process to reprogram an EPROM when small changes are made in the software can be tedious and time-consuming, and this occurrence is common during development. Removing an EPROM device from its socket can damage the EPROM; worse yet, the system itself can be damaged if careful handling
is not exercised.
The choice of the storage device can also have an impact on the overall cost of maintenance. Although PROM and EPROM devices are inexpensive, the cost can add up if a large volume of shipped systems is in the field. Upgrading an embedded system in these cases means shipping replacement PROM and EPROM chips. The embedded system can be upgraded without the need for chip replacement and can be upgraded dynamically over a network if flash memory or EEPROM is used as the code storage device (see the following sidebar).
Armed with the information presented in the previous sections, we can now attempt to answer the questions raised earlier. A personal computer is not an embedded system because it is built using a general-purpose processor and is built independently from the software that runs on it. The software applications developed for personal computers, which run operating systems such as FreeBSD or Windows, are developed natively (as opposed to cross-developed) on those operating systems. For the same reasons, an Apple iBook used only as a DVD player is used like an embedded system but is not an embedded system.
· Mask Programmed ROM - the memory content is programmed during the manufacturing process. Once programmed, the content cannot be changed. It cannot be reprogrammed.
· Field Programmable ROM (PROM) - the memory content can be custom-programmed one time. The memory content cannot change once programmed.
· Erasable Programmable ROM (EPROM) - an EPROM device can be custom-programmed, erased, and reprogrammed as often as required within its lifetime (hundreds or even thousands of times). The memory content is non-volatile once programmed. Traditional EPROM devices are erased by exposure to ultraviolet (UV) light. An EPROM device must be removed from its housing unit first. It is then reprogrammed using a special hardware device called an EPROM programmer.
· Electrically Erasable Programmable ROM (EEPROM or E2PROM) - modern EPROM devices are erased electrically and are thus called EEPROM. One important difference between an EPROM and an EEPROM device is that with the EEPROM device, memory content of a single byte can be selectively erased and reprogrammed. Therefore, with an EEPROM device, incremental changes can be made. Another difference is the EEPROM can be reprogrammed without a special programmer and can stay in the device while being reprogrammed. The versatility of byte-level programmability of the EEPROM comes at a price, however, as programming an EEPROM device is a slow process.
· Flash Memory - the flash memory is a variation of EEPROM, which allows for block-level (e.g., 512-byte) programmability that is much faster than EEPROM.