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Figure 1.1 Growth in processor performance since 1980s
Classes of Computers
The evolution of various classes of computers:
1960: Large Main frames (Millions of $ )
(Applications: Business Data processing, large Scientific computing)
1970: Minicomputers (Scientific laboratories, Time sharing concepts)
1980: Desktop Computers ( Ps) in the form of Personal computers and workstations.
(Larger Memory, more computing power, Replaced Time sharing systems)
1990: Emergence of Internet and WW, PDAs, emergence of high performance digital
consumer electronics
2000: Cell phones
These changes in computer use have led to three different computing classes each
characterized by different applications, requirements and computing technologies.
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Desktop computing
The first and still the largest market in dollar terms is desktop computing. Desktop
computing system cost range from $ 500 (low end) to $ 5000 (high-end configuration).
Throughout this range in price, the desktop market tends to drive to optimize price-
performance. The performance concerned is compute performance and graphics
performance. The combination of performance and price are the driving factors to the
customers and the computer designer. Hence, the newest, high performance and cost
effective processor often appears first in desktop computers.
Servers:
Servers provide large-scale and reliable computing and file services and are mainly
used in the large-scale enterprise computing and web based services. The three important
characteristics of servers are:
Dependability: Severs must operate 24x7 hours a week. Failure of server system
is far more catastrophic than a failure of desktop. Enterprise will lose revenue if
the server is unavailable.
Scalability: as the business grows, the server may have to provide more
functionality/ services. Thus ability to scale up the computing capacity, memory,
storage and I/O bandwidth is crucial.
Throughput: transactions completed per minute or web pages served per second
are crucial for servers.
Embedded Computers
Simple embedded microprocessors are seen in washing machines, printers,
network switches, handheld devices such as cell phones, smart cards video game devices
etc. embedded computers have the widest spread of processing power and cost. The
primary goal is often meeting the performance need at a minimum price rather than
achieving higher performance at a higher price. The other two characteristic requirements
are to minimize the memory and power.
In many embedded applications, the memory can be substantial portion of the
systems cost and it is very important to optimize the memory size in such cases. The
application is expected to fit totally in the memory on the processor chip or off chip
memory. The importance of memory size translates to an emphasis on code size which is
dictated by the application. Larger memory consumes more power. All these aspects are
considered while choosing or designing processor for the embedded applications.
Defining Computer Architecture
The computer designer has to ascertain the attributes that are important for a new
computer and design the system to maximize the performance while staying within cost,
power and availability constraints. The task has few important aspects such as Instruction
Set design, Functional organization, Logic design and implementation.
Instruction Set Architecture (ISA)
ISA refers to the actual programmer visible Instruction set. The ISA serves as
boundary between the software and hardware. The seven dimensions of the ISA are:
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i) Class of ISA: Nearly all ISAs today are classified as General-Purpose-
Register architectures. The operands are either Registers or Memory locations.
The two popular versions of this class are:
Register-Memory ISAs : ISA of 80x86, can access memory as part of many
instructions.
Load-Store ISA Eg. ISA of MIPS, can access memory only with Load or
Store instructions.
ii) Memory addressing: Byte addressing scheme is most widely used in all
desktop and server computers. Both 80x86 and MIPS use byte addressing.
Incase of MIPS the object must be aligned. An access to an object of s byte at
byte address A is aligned if A mod s =0. 80x86 does not require alignment.
Accesses are faster if operands are aligned.
ii) Addressing modes:
Specify the address of a M object apart from register and constant operands.
MIPS Addressing modes:
Register mode addressing
Immediate mode addressing
Displacement mode addressing
80x86 in addition to the above addressing modes supports the additional
modes of addressing:
i. Register Indirect
ii. Indexed
ii. Based with Scaled index
iv) Types and sizes of operands:
MIPS and x86 support:
8 bit (ASCII character), 16 bit(Unicode character)
32 bit (Integer/word)
64 bit (long integer/ Double word)
32 bit (IEE-754 floating point)
64 bit (Double precision floating point)
80x86 also supports 80 bit floating point operand.(extended double
precision)
v) Operations: The general category of operations are:
o Data Transfer
o Arithmetic operations
o Logic operations
o Control operations
o MIPS ISA: simple & easy to implement
o x86 ISA: richer & larger set of operations
vi) Control flow instructions:
All ISAs support:
Conditional & Unconditional Branches
Procedure Calls & Returns
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MIPS 80x86
Conditional Branches tests content of Register Condition code bits
Procedure Call JAL CALLF
Return Address in a R Stack in M
vii) Encoding an ISA
Fixed Length ISA Variable Length ISA
MIPS 32 Bit long 80x86 (1-18 bytes)
Simplifies decoding Takes less space
Number of Registers and number of Addressing modes have significant
impact on the length of instruction as the register field and addressing mode
field can appear many times in a single instruction.
Trends in Technology
The designer must be aware of the following rapid changes in implementation
technology.
Integrated Circuit (IC) Logic technology
Memory technology (semiconductor DRAM technology)
Storage or magnetic disk technology
Network technology
IC Logic technology:
Transistor density increases by about 35%per year. Increase in die size
corresponds to about 10% to 20% per year. The combined effect is a growth rate in
transistor count on a chip of about 40% to 55% per year.
Semiconductor DRAM technology: capacity increases by about 40% per year.
Storage Technology:
Before 1990: the storage density increased by about 30% per year.
After 1990: the storage density increased by about 60% per year.
Disks are still 50 to 100 times cheaper per bit than DRAM.
Network Technology:
Network performance depends both on the performance of the switches and on
the performance of the transmission system.
Although the technology improves continuously, the impact of these improvements can
be in discrete leaps.
Performance trends: Bandwidth or throughput is the total amount of work done in given
time. Latency or response time is the time between the start and the completion of an
event. (for eg. Millisecond for disk access)
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