THE COMPUTER SYSTEM



THE COMPUTER SYSTEM.

System Buses

System Buses

Interconnecting Basic Components

Computer Components

• The von Neumann architecture is based on three key concepts:

o Data and instructions are stored in a single read-write memory

o The contents of this memory are addressable by location, without regard to the type of

data contained there

o Execution occurs in a sequential fashion (unless explicitly modified) from one

instruction to the next

• Two approaches to programming

o hardwired programming - constructing a configuration of hardware logic components

to perform a particular set of arithmetic and logic operations on a set of data

o software - a sequence of codes or instructions, each of which supply the necessary

control signals to a general-purpose configuration of control and logic functions (which

may themselves be hardwired programs)

• Other components needed

o I/O Components - a means to:

♣ accept data and instructions in some form, and convert to an internal form of

signals

♣ report results

o Main memory

♣ distinguished from external storage/peripherals

♣ a place to temporarily store both:

♣ instructions - data interpreted as codes for generating control signals

♣ data - data upon which computations are performed

• Interactions among Computer Components

o Memory Address Register - specifies

address for next read or write

o Memory Buffer Register - contains

data to be written into or receives data

read from memory

o I/O address register - specifies a

particular I/O device

o I/O buffer register - used for

exchange

of data between an I/O module and

CPU (or memory)

o Memory module - a set of locations

♣ with sequentially numbered

addresses

♣ each holds a binary number that

can be either an instruction or

data

Computer Function

• Processing required for a single instruction is called an instruction cycle

• Simple POV (Point-Of-View): 2 steps

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o Fetch - CPU reads an instruction from a location in memory

♣ Program counter (PC) register keeps track of which instruction executes next

♣ Normally, CPU increments PC after each fetch

♣ Fetched instruction is loaded into the instruction register (IR)

o Execute - CPU executes the instruction

♣ May involve several operations

♣ May utilize previously changed state of CPU and (indirectly) other devices

♣ General categories:

♣ CPU-Memory: Data may be transferred from CPU to memory or vice-versa

♣ CPU-IO: Data may be transferred between CPU and an I/O module

♣ Data Processing: CPU may perform some arithmetic or logic operation on the

data

♣ Control: An instruction may specify that the sequence of execution be altered

• More complex instructions

o May combine these categories

o May perform more than one reference to memory

o May specify I/O operation instead of memory reference

o May specify an operation to be performed on a vector of numbers or a string of

characters

• Expanded execution cycle

o Instruction Address Calculation (iac) - determine the address of the next instruction

o Instruction Fetch (if)

o Instruction Operation Decoding (iod) - analyze op to determine op type and operands

o Operand Address Calculation (oac)

o Operand Fetch (of)

o Data Operation (do) - perform indicated op

o Operand Store (os) - write result into memory or out to I/O

• Interrupts

o Mechanism by which other modules may interrupt the normal processing of the CPU

o Classes

♣ Program - as a result of program execution

♣ Timer - generated by hardware timer

♣ I/O - to signal completion of I/O or error

♣ Hardware failure

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