嵌入式系统词汇表

A
ASIC
Application-Specific Integrated Circuit. A piece of custom-designed hardware
in a
chip.
address bus
A set of electrical lines connected to the processor and all of the peripher
als with
which it communicates. The address bus is used by the processor to select a
specific memory location or register within a particular peripheral. If the
address
bus contains n electrical lines, the processor can uniquely address up to 2^
n such
locations.
application software
Describes software modules specific to a particular embedded project. The
application software is unlikely to be reusable across embedded platforms, s
imply
because each embedded system has a different application.
assembler
A software development tool that translates human-readable assembly language

programs into machine-language instructions that the processor can understan
d
and execute.
assembly language
A human-readable form of a processor's instruction set. Most processor-speci
fic
functions must be written in assembly language.
B
BSP
See board support package.
binary semaphore
A type of semaphore with just two states. Also called a mutex.
board support package
Part of a software package that is processor or platform-dependent. Typicall
y,
sample source code for the board support package is provided by the package
developer. The sample code must be modified as necessary, compiled, and link
ed
with the remainder of the software package.
bond-out processor
A special version of a processor that has some of the internal signals broug
ht out
to external pins. A bond-out processor is most often found within an emulato
r and
is never intended to be used in a production system.
breakpoint
A location in a program at which execution is to be stopped and control of t
he
processor switched to the debugger. Mechanisms for creating and removing
breakpoints are provided by most debugging tools.
C
CISC
Complex Instruction Set Computer. Describes the architecture of a processor
family. CISC processors generally feature variable-length instructions, mult
iple
addressing formats, and contain only a small number of general-purpose regis
ters.
Intel's 80x86 family is the quintessential example of CISC. Contrast with RI
SC.
CPU
Central Processing Unit. The part of a processor that executes instructions.

compiler
A software development tool that translates high-level language programs int
o the
machine-language instructions that a particular processor can understand and

execute.
context
The current state of the processor's registers and flags.
context switch
The process of switching from one task to another in a multitasking operatin
g
system. A context switch involves saving the context of the running task and

restoring the previously-saved context of the other. The piece of code that
does
this is necessarily processor-specific.
counting semaphore
A type of semaphore that is used to track multiple resources of the same typ
e. An
attempt to take a counting semaphore is blocked only if all of the available


resources are in use. Contrast with binary semaphore.
critical section
A block of code that must be executed in sequence and without interruption t
o
guarantee correct operation of the software. See also race condition.
cross-compiler
A compiler that runs on a different platform than the one for which it produ
ces
object code. A cross-compiler runs on a host computer and produces object co
de
for the target.
D
DMA
Direct Memory Access. A technique for transferring data directly between two

peripherals (usually memory and an I/O device) with only minimal interventio
n by
the processor. DMA transfers are managed by a third peripheral called a DMA
controller.
DRAM
Dynamic Random-Access Memory. A type of RAM that maintains its contents only


as long as the data stored in the device is refreshed at regular intervals.
The
refresh cycles are usually performed by a peripheral called a DRAM controlle
r.
DSP
See digital signal processor.
data bus
A set of electrical lines connected to the processor and all of the peripher
als with
which it communicates. When the processor wants to read (write) the contents
of
a memory location or register within a particular peripheral, it sets the ad
dress
bus pins appropriately and receives (transmits) the contents on the data bus
.
deadline
The time at which a particular set of computations must be completed. See al
so
real-time system.
deadlock
An unwanted software situation in which an entire set of tasks is blocked, w
aiting
aiting
for an event that only a task within the same set can cause. If a deadlock o
ccurs,
the only solution is to reset the hardware. However, it is usually possible
to
prevent deadlocks altogether by following certain software design practices.

debug monitor
A piece of embedded software that has been designed specifically for use as
a
debugging tool. It usually resides in ROM and communicates with a debugger v
ia a
serial port or network connection. The debug monitor provides a set of primi
tive
commands to view and modify memory locations and registers, create and remov
e
breakpoints, and execute your program. The debugger combines these primitive
s
to fulfill higher-level requests like program download and single-step.
debugger
A software development tool used to test and debug embedded software. The
debugger runs on a host computer and connects to the target through a serial
port
port
or network connection. Using a debugger you can download software to the tar
get
for immediate execution. You can also set breakpoints and examine the conten
ts
of specific memory locations and registers.
device driver
A software module that hides the details of a particular peripheral and prov
ides a
high-level programming interface to it.
device programmer
A tool for programming non-volatile memories and other
electrically-programmable devices. Typically, the programmable device is ins
erted
into a socket on the device programmer and the contents of a memory buffer a
re
then transferred into it.
digital signal processor
A device that is similar to a microprocessor, except that the internal CPU h
as been
optimized for use in applications involving discrete-time signal processing.
In
addition to standard microprocessor instructions, DSPs usually support a set
of
complex instructions to perform common signal-processing computations quickl
y.
Common DSP families are TI's 320Cxx and Motorola's 5600x series.
E
EEPROM
Electrically Erasable, Programmable Read-Only Memory. (Pronounced
"Double-E"-PROM.) A type of ROM that can be erased electronically.
EPROM
Erasable, Programmable Read-Only Memory. A type of ROM that can be erased by

exposing it to ultraviolet light. Once erased, an EPROM can be reprogrammed
with
the help of a device programmer.
embedded system
A combination of computer hardware and software, and perhaps additional
mechanical or other parts, designed to perform a dedicated function. In some

cases, embedded systems are part of a larger system or product, as is the ca
se of
an anti-lock braking system in a car. Contrast with general-purpose computer
.
.
emulator
Short for In-Circuit Emulator (ICE). A debugging tool that takes the place
of-emulates-the processor on your target board. Emulators frequently
incorporate a special "bond-out" version of the target processor that allows
you to
observe and record its internal state as your program is executing.
executable
A file containing object code that is ready for execution on the target. All
that
remains is to place the object code into a ROM or download it via a debuggin
g tool.
F
firmware
Embedded software that is stored as object code within a ROM. This name is m
ost
common among the users of digital signal processors.
flash memory
A RAM-ROM hybrid that can be erased and rewritten under software control.
Such devices are divided into blocks, called sectors, that are individually-
erasable.
Flash memory is common in systems that require nonvolatile data storage at v
ery
ery
low cost. In some cases, a large flash memory may even be used instead of a
disk-drive.
G
general-purpose computer
A combination of computer hardware and software that serves as a
general-purpose computing platform. For example, a personal computer. Contra
st
with embedded system.
H
HLL
See high-level language.
heap
An area of memory that is used for dynamic memory allocation. Calls to mallo
c and
free and the C++ operators new and delete result in run-time manipulation of
the
heap.
high-level language
A language, such as C or C++, that is processor-independent. When programmin
g in
a high-level language, it is possible to concentrate on algorithms and appli
cations
cations
without worrying about the details of a particular processor.
host
A general-purpose computer that communicates with the target via a serial po
rt or
network connection. This term is usually used to distinguish the computer on
which
the debugger is running from the embedded system that is being developed.
I
ICE
In-Circuit Emulator. See emulator.
I/O
Input/Output. The interface between a processor and the world around it. The

simplest examples are switches (inputs) and LEDs (outputs).
I/O device
A piece of hardware that interfaces between the processor and the outside wo
rld.
Common examples are switches and LEDs, serial ports, and network controllers
.
I/O map
A table or diagram containing the name and address range of each peripheral
addressable by the processor within the I/O space. I/O maps are a helpful ai
d in
getting to know the target.
I/O space
A special memory region provided by some processors and generally reserved f
or
the attachment of I/O devices. Memory locations and registers within an I/O
space can be accessed only via special instructions. For example, processors
in the
80x86 family have special I/O space instructions called in and out. Contrast
with
memory space.
ISR
See interrupt service routine.
instruction pointer
A register in the processor that contains the address of the next instructio
n to
be executed. Also known as a program counter.
interrupt
An asynchronous electrical signal from a peripheral to the processor. When t
he
peripheral asserts this signal, we say that an interrupt occurs. When an int
errupt
errupt
occurs, the current state of the processor is saved and an interrupt service

routine is executed. When the interrupt service routine exits, control of th
e
processor is returned to whatever part of the software was previously runnin
g.
interrupt latency
The amount of time between the assertion of an interrupt and the start of th
e
associated interrupt service routine.
interrupt service routine
A piece of software executed in response to a particular interrupt.
interrupt type
A unique number associated with each interrupt.
interrupt vector
The address of an interrupt service routine.
interrupt vector table
A table containing interrupt vectors and indexed by interrupt type. This tab
le
contains the processor's mapping between interrupts and interrupt service
routines and must be initialized by the programmer.
intertask communication
A mechanism used by tasks and interrupt service routines to share informatio
n
and synchronize their access to shared resources. The most common building
blocks of intertask communication are semaphores and mutexes.
K
kernel
An essential part of any multitasking operating system, the kernel contains
just
the scheduler and context-switch routine.
L
linker
A software development tool that accepts one or more object files as input a
nd
outputs a relocatable program. The linker is thus run after all of the sourc
e files
have been compiled or assembled.
locator
A software development tool that assigns physical addresses to the relocatab
le
program produced by the linker. This is the last step in the preparation of
software for execution by an embedded system and the resulting file is calle
d an
d an
executable. In some cases, the locator's function may be hidden within the l
inker.
logic analyzer
A hardware debugging tool that can be used to capture the logic levels (0 or
1) of
dozens, or even hundreds, of electrical signals in real-time. Logic analyzer
s can be
quite helpful for debugging hardware problems and complex processor-peripher
al
interactions.
M
memory map
A table or diagram containing the name and address range of each peripheral
addressable by the processor within the memory space. Memory maps are a help
ful
aid in getting to know the target.
memory-mapped I/O
An increasingly common hardware design methodology in which I/O devices are
placed into the memory space rather than the I/O space. From the processor's

point of view, memory-mapped I/O devices look very much like memory devices.


memory space
A processor's standard address space. Contrast with I/O space.
microcontroller
A microcontroller is very similar to a microprocessor. The main difference i
s that
a microcontroller is designed specifically for use in embedded systems.
Microcontrollers typically include a CPU, memory (a small amount of RAM and/
or
ROM), and other peripherals on the same chip. Common examples are the 8051,
Intel's 80196, and Motorola's 68HCxx series.
microprocessor
A piece of silicon containing a general-purpose CPU. The most common example
s
are Intel's 80x86 and Motorola's 680x0 families.
monitor
In the context of this book, a debug monitor. However, there is a second mea
ning
for this word that is associated with intertask communication. In that conte
xt, a
monitor is a language-level synchronization feature.
multiprocessing
The use of more than one processor in a single computer system. So-called
"multiprocessor systems" usually have a common memory space through which th
e
processors can communicate and share data. In addition, some multiprocessor
systems support parallel processing.
multitasking
The execution of multiple software routines in pseudo-parallel. Each routine

represents a separate "thread of execution" and is referred to as a task. Th
e
operating system is responsible for simulating parallelism by parceling out
the
processor's time.
mutex
A data structure for mutual exclusion, also known as a binary semaphore. A m
utex
is basically just a multitasking-aware binary flag that can be used to synch
ronize
the activities of multiple tasks. As such, it can be used to protect critica
l sections
of the code from interruption and shared resources from simultaneous use.
mutual exclusion
A guarantee of exclusive access to a shared resource. In embedded systems, t
he
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 m
utex.
N
NVRAM
Non-Volatile 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.
O
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 th
e
end user. However, this term is used almost exclusively to refer to
microcontrollers with on-chip PROM.
opcode
A sequence of bits that is recognized by the processor as one of the instruc
tions
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 shou
ld be
using the processor at a given time and for controlling access to shared res
ources.
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.
P
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 devic
e.
The peripheral may reside within the same chip as the processor, in which ca
se 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 sus
pended
when a higher-priority task becomes ready. Non-preemptive schedulers are eas
ier
to implement but less appropriate for embedded systems.
polling
polling
A method of interfacing with hardware that involves repeatedly reading a sta
tus
register until the device has reached the awaited state. Device drivers are
either
polling or interrupt-driven, with the latter being more generally preferred.

priority
The relative importance of one task compared to another.
priority inversion
An unwanted software situation in which a high-priority task is delayed whil
e
waiting for access to a shared resource that is not even being used at the t
ime.
For all practical purposes, the priority of this task has been lowered durin
g the
delay period.
process
A word that is often confused with task or thread. The crucial distinction i
s 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, microcontro
ller,
and digital signal processor.
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 model
s 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 wi
ll be
run. Such code must usually be written in assembly language. Contrast with
processor-independent.
profiler
profiler
A software development tool that collects and reports execution statistics f
or
your programs. These statistics include the number of calls to each subrouti
ne and
the total amount of time spent within each. This data can be used to learn w
hich
subroutines are the most critical and, therefore, demand the greatest code
efficiency.
program counter
See instruction pointer.
R
RAM
Random-Access Memory. A broad classification of memory devices that includes

all devices in which individual memory locations may be read or written as r
equired.
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 and/or
register windows. The MIPS processor family is an excellent example. Contras
t
with CISC.
ROM
Read-Only Memory. A broad classification of memory devices that includes all

devices in which the individual memory locations may be read, but not writte
n.
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 us
e in
real-time systems.
race condition
A situation in which the outcome of a program may be affected by the exact o
rder
in which the instructions are executed. Race conditions are only an issue wh
ere
ere
interrupts and/or preemption are possible and where critical sections exist.

real-time system
Any computer system, embedded or otherwise, that has deadlines. The followin
g
question can be used to distinguish real-time systems from the rest: "Is a l
ate
answer as bad, or even worse, than a wrong answer?" In other words, what hap
pens
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 consi
dered
"hard" real-time, which is meant to imply that the system has "hard" deadlin
es.
Everything in between is "soft" real-time.
recursive
Said of software that calls itself. Recursion should generally be avoided in
an
embedded system, since it frequently requires a large stack.
reentrant
Said of software that can be executed multiple times simultaneously. A reent
rant
function can be safely called recursively or from multiple tasks. The key to
making
code reentrant is to ensure mutual exclusion whenever accessing global varia
bles
or shared registers.
register
A memory location that is part of a processor or an I/O device. In other wor
ds,
it's not normal memory. Generally, each bit or set of bits within the regist
er
controls some behavior of the larger device.
relocatable
A file containing object code that is almost ready for execution on the targ
et. The
final step is to use a locator to fix the remaining relocatable addresses wi
thin the
code. The result of that process is an executable.
reset address
The address from which the first instruction will be fetched after the proce
ssor
is powered on or reset.
reset code
A small piece of code that is placed at the reset address. The reset code is
usually
written in assembly language and may simply be the equivalent of "jump to th
e
startup code."
reset vector
See reset address.
S
SRAM
Static Random-Access Memory. A type of RAM that retains its contents as long
as
the system is powered on. Data stored in an SRAM is lost when the system is
powered down or reset.
scheduler
The part of an operating system that decides which task to run next. This de
cision
is based on the readiness of each task, their relative priorities, and the s
pecific
scheduling algorithm implemented.
semaphore
A data structure that is used for intertask synchronization. Semaphores are
usually provided by the operating system.
simulator
A debugging tool that runs on the host and pretends to be the target process
or. A
simulator can be used to test pieces of the software before the embedded
hardware is available. Unfortunately, attempts to simulate interactions with

complex peripherals are often more trouble than they are worth.
software interrupt
An interruption of a program that is initiated by a software instruction. So
ftware
interrupts are commonly used to implement breakpoints and operating system
entry points. Unlike true interrupts, they occur synchronously with respect
to
program execution. In other words, software interrupts always occur at the
beginning of an instruction execution cycle. Compare with trap.
stack
An area of memory that contains a last-in-first-out queue of storage for
parameters, automatic variables, return addresses, and other information tha
t
must be maintained across function calls. In multitasking situations, each t
ask
ask
generally has its own stack.
stack frame
An area of the stack associated with a particular function call.
startup code
A piece of assembly language code that prepares the way for software written
in a
high-level language. Most C/C++ cross-compilers come with startup code that
you
can modify, compile, and link with your embedded programs.
T
target
Another name for the embedded system. This term is usually used during softw
are
development, to distinguish the embedded system from the host with which it
communicates.
task
The central abstraction of an operating system. Each task must maintain its
own
copy of the instruction pointer and general-purpose registers. Unlike proces
ses,
tasks share a common memory space and must be careful to avoid overwriting e
ach
ach
other's code and data.
thread
Another name for a task. This name is more common in operating systems that
support processes. A task is simply a thread in a single-process system.
tracepoint
Like a breakpoint except that a counter is incremented rather than stopping
the
program. Tracepoints are not supported by all debugging tools.
trap
An interruption of a program that is triggered by the processor's own intern
al
hardware. For example, the processor might trap if an illegal opcode is foun
d
within the program. Compare with software interrupt.
V
volatile
A value that may change without the intervention of software is said to be v
olatile.
For example, values within the registers of some I/O devices may change in
response to external events. C's volatile keyword should be used to warn you
r
compiler about any pointers that point to such registers. This will ensure t
hat the
actual value is reread each time the data is used.
W
watchdog timer
A hardware timer that is periodically reset by software. If the software cra
shes
or hangs, the watchdog timer will expire, and the entire system will be rese
t
automatically.