Michael Barr: Programming Embedded Systems in C and C++

mutual exclusion

A guarantee of exclusive access to a shared resource. In embedded systems , the shared resource is typically a block of memory, a global variable, or a set of registers . Mutual exclusion can be achieved with the use of a semaphore or mutex .

NVRAM

Nonvolatile Random-Access Memory. A type of RAM that retains its data even when the system is powered down. NVRAM frequently consists of an SRAM and a long-life battery.

OTP

See one-time programmable.

object code

A set of processor- readable opcodes and data. The output of compilers , assemblers , linkers , and locators are files containing object code.

object file

A file containing object code . The output of a compiler or assembler .

one-time programmable

Any programmable device, like a PROM , that can be programmed just once by the end user. However, this term is used almost exclusively to refer to microcontrollers that have on-chip PROM.

opcode

A sequence of bits that is recognized by the processor as one of the instructions in its instruction set.

operating system

A piece of software that makes multitasking possible. An operating system typically consists of a set of function calls, or software interrupts , and a periodic clock tick. The operating system is responsible for deciding which task should be using the processor at a given time and for controlling access to shared resources.

oscilloscope

A hardware debugging tool that allows you to view the voltage on one or more electrical lines. For example, you might use an oscilloscope to determine if a particular interrupt is currently asserted.

PROM

Programmable Read-Only Memory. A type of ROM that can be written (programmed) with a device programmer . These memory devices can be programmed only once, so they are sometimes referred to as write-once or one-time programmable devices.

parallel processing

The ability to apply two or more processors to a single computation.

peripheral

A piece of hardware other than the processor, usually memory or an I/O device . The peripheral can reside within the same chip as the processor, in which case it is called an internal peripheral.

physical address

The actual address that is placed on the address bus when accessing a memory location or register .

preemptive

A scheduler is said to be preemptive if it allows the running task to be suspended when a higher- priority task becomes ready. Non-preemptive schedulers are easier to implement but less appropriate for embedded systems.

priority

The relative importance of one task compared to another.

priority inversion

An unwanted software situation in which a high-priority task is delayed while waiting for access to a shared resource that is not even being used at the time. For all practical purposes, the priority of this task has been lowered during the delay period.

process

A word that is often confused with task or thread . The crucial distinction is that all of the tasks in a system share a common memory space. Processes, on the other hand, always have their own private memory space. Processes are common in multi-user systems but are rarely, if ever, found in embedded systems .

processor

A generic term that does not distinguish between microprocessor , microcontroller , and digital signal processor . I have purposefully used this term throughout the book because the actual processor type has very little impact on the type of embedded software development described here.

processor family

A set of related processors , usually successive generations from the same manufacturer. For example, Intel's 80x86 family began with the 8086 and now includes the 80186, 286, 386, 486, Pentium, and many others. The later models in a family are typically backwards-compatible with the ones that came before.

processor-independent

A piece of software that is independent of the processor on which it will be run. Most programs that can be written in a high-level language are processor-independent. Contrast with processor-specific .

processor-specific

A piece of software that is highly dependent on the processor on which it will be run. Such code must usually be written in assembly language . Contrast with processor-independent .

profiler

A software development tool that collects and reports execution statistics for your programs. These statistics include the number of calls to each subroutine and the total amount of time spent within each. This data can be used to learn which subroutines are the most critical and, therefore, demand the greatest code efficiency.

program counter

See instruction pointer.

RAM

Random-Access Memory. A broad classification of memory devices that includes all devices in which individual memory locations can be read or written as required.

RISC

Reduced Instruction Set Computer. Describes the architecture of a processor family . RISC processors generally feature fixed-length instructions, a load-store memory architecture, and a large number of general-purpose registers or register windows. The MIPS processor family is an excellent example. Contrast with CISC .

ROM

Read-Only Memory. A broad classification of memory devices that includes all devices in which the individual memory locations can be read but not written.

ROM emulator

A debugging tool that takes the place of — or emulates — the ROM on your target board. A ROM emulator acts very much like a debug monitor , except that it includes its own serial or network connection to the host .

ROM monitor

See debug monitor.

RTOS

Real-Time Operating System. An operating system designed specifically for use in real-time systems .

race condition

A situation in which the outcome of a program can be affected by the exact order in which the instructions are executed. Race conditions are only an issue where interrupts and/or preemption are possible and where critical sections exist.

real-time system

Any computer system, embedded or otherwise, that has deadlines . The following question can be used to identify real-time systems: is a late answer as bad as, or even worse than, a wrong answer? In other words, what happens if the computation doesn't finish in time? If nothing bad happens, it's not a real-time system. If someone dies or the mission fails, it's generally considered "hard" real-time, which is meant to imply that the system has "hard" deadlines. Everything in between is "soft" real-time.

recursive

Refers to software that calls itself. Recursion should generally be avoided in an embedded system because it frequently requires a large stack .