Computer Organisation
COS2621
Chapter 1: Basic Concepts and Computer Evolution
Organization and Architecture
Computer architecture refers to those attributes of a system visible to a programmer or, put another way, those
attributes that have a direct impact on the logical execution of a program. A term that is often used
interchangeably with computer architecture is instruction set architecture (ISA). The ISA defines instruction
formats, instruction opcodes, registers, instruction and data memory; the effect of executed instructions on the
registers and memory; and an algorithm for controlling instruction execution. Computer organization refers to the
operational units and their interconnections that realize the architectural specifications.
In a class of computers called microcomputers, the relationship between architecture and organization is very
close. Changes in technology not only influence organization but also result in the introduction of more powerful
and more complex architectures. Generally, there is less of a requirement for generation- to- generation
compatibility for these smaller machines.
Structure and Function
A computer is a complex system; contemporary computers contain millions of elementary electronic components.
How, then, can one clearly describe them? The key is to recognize the hierarchical nature of most complex
systems, including the computer [SIMO96]. A hierarchical system is a set of interrelated subsystems, each of the
latter, in turn, hierarchical in structure until we reach some lowest level of elementary subsystem
The hierarchical nature of complex systems is essential to both their design and their description. The designer
need only deal with a particular level of the system at a time. At each level, the designer is concerned with
structure and function:
• Structure: The way in which the components are interrelated.
• Function: The operation of each individual component as part of the structure.
In terms of description, we have two choices: starting at the bottom and building up to a complete description, or
beginning with a top view and decomposing the system into its subparts. Evidence from a number of fields
suggests that the top- down approach is the clearest and most effective [WEIN75].
Function
Both the structure and functioning of a computer are, in essence, simple. In general terms, there are only four
basic functions that a computer can perform:
• Data processing:
o Data may take a wide variety of forms, and the range of processing requirements is broad.
• Data storage:
o Even if the computer is processing data on the fly (i.e., data come in and get processed, and the
results go out immediately), the computer must temporarily store at least those pieces of data
that are being worked on at any given moment.
• Data movement:
o The computer’s operating environment consists of devices that serve as either sources or
destinations of data.
• Control:
o Within the computer, a control unit manages the computer’s resources and orchestrates the
performance of its functional parts in response to instructions.
,Structure
We now look in a general way at the internal structure of a computer. We begin with a traditional computer with a
single processor that employs a microprogrammed control unit, then examine a typical multicore structure.
SIMPLE SINGLE-PROCESSOR COMPUTER
There are four main structural components:
• Central processing unit (CPU):
o Controls the operation of the computer and performs its data processing functions; often simply
referred to as processor.
• Main memory:
o Stores data.
• I/O:
o Moves data between the computer and its external environment.
• System interconnection:
o Some mechanism that provides for communication among CPU, main memory, and I/O. A
common example of system interconnection is by means of a system bus.
There may be one or more of each of the aforementioned components. Traditionally, there has been just a single
processor. In recent years, there has been increasing use of multiple processors in a single computer.
CPU major structural components are as follows:
• Control unit:
o Controls the operation of the CPU and hence the computer.
• Arithmetic and logic unit (ALU):
o Performs the computer’s data processing functions.
• Registers:
o Provides storage internal to the CPU.
• CPU interconnection:
o Some mechanism that provides for communication among the control unit, ALU, and registers.
,MULTICORE COMPUTER STRUCTURE
As was mentioned, contemporary computers generally have multiple processors. When these processors all reside
on a single chip, the term multicore computer is used, and each processing unit s called a core. To clarify the
terminology, this text will use the following definitions.
• Central processing unit (CPU):
o That portion of a computer that fetches and executes instructions. It consists of an ALU, a control
unit, and registers. In a system with a single processing unit, it is often simply referred to as a
processor.
• Core:
o An individual processing unit on a processor chip. A core may be equivalent in functionality to a
CPU on a single- CPU system. Other specialized processing units, such as one optimized for vector
and matrix operations, are also referred to as cores.
• Processor:
o A physical piece of silicon containing one or more cores. The processor is the computer
component that interprets and executes instructions. If a processor contains multiple cores, it is
referred to as a multicore processor.
Another prominent feature of contemporary computers is the use of multiple layers of memory, called cache
memory, between the processor and main memory. Cache memory is smaller and faster than main memory and is
used to speed up memory access, by placing in the cache data from main memory, that is likely to be used in the
near future.
Before describing this arrangement, we need to define some terms.
• A printed circuit board (PCB)
o Rigid, flat board that holds and interconnects chips and other electronic components.
• System board or Motherboard
o The main printed circuit board in a computer, while smaller ones that plug into the slots in the
main board are called expansion boards. The most prominent elements on the motherboard are
the chips.
• A chip
o Single piece of semiconducting material, typically silicon, upon which electronic circuits and logic
gates are fabricated. The resulting product is referred to as an integrated circuit.
, Next, we zoom in on the structure of a single core, which occupies a portion of the processor chip. In general
terms, the functional elements of a core are:
• Instruction logic:
o This includes the tasks involved in fetching instructions and decoding each instruction to
determine the instruction operation and the memory locations of any operands.
• Arithmetic and logic unit (ALU):
o Performs the operation specified by an instruction.
• Load/store logic:
o Manages the transfer of data to and from main memory via cache.
The main subareas within this core area are the following for figure 1.4/1.5:
• ISU (instruction sequence unit):
o Determines the sequence in which instructions are executed in what is referred to as a
superscalar architecture.
• IFU (instruction fetch unit):
o Logic for fetching instructions.
• IDU (instruction decode unit):
o The IDU is fed from the IFU buffers, and is responsible for the parsing and decoding of all
z/Architecture operation codes.
• LSU (load- Store unit):
o The LSU contains the 96-kB L1 data cache,1 and manages data traffic between the L2 data cache
and the functional execution units. It is responsible for handling all types of operand accesses of
all lengths, modes, and formats as defined in the z/Architecture.
• XU (translation unit):
o This unit translates logical addresses from instructions into physical addresses in main memory.
The XU also contains a translation lookaside buffer (TLB) used to speed up memory access.
• FXU (fixed- Point unit):
o The FXU executes fixed- point arithmetic operations.
• BFU (binary floating-Point unit):
o The BFU handles all binary and hexadecimal floating-point operations, as well as fixed point
multiplication operations.
• DFU (decimal floating- Point unit):
o The DFU handles both fixed- point and floating- point operations on numbers that are stored as
decimal digits.
• RU (recovery unit):
o The RU keeps a copy of the complete state of the system that includes all registers, collects
hardware fault signals, and manages the hardware recovery actions.
• COP (dedicated co- processor):
o The COP is responsible for data compression and encryption functions for each core.
• I-cache:
o This is a 64-kB L1 instruction cache, allowing the IFU to prefetch instructions before they are
needed.
• L2 control:
o This is the control logic that manages the traffic through the two L2 caches.
• Data-L2:
o A 1-MB L2 data cache for all memory traffic other than instructions.
• Instr-L2:
o A 1-MB L2 instruction cache.
