4CCS1CS1 - Computer Systems

KCL Yr1 Comp Sci

What is computer organisation?

the physical aspects of a computer, how it works on a physical level (e.g. circuit design, control signals, memory types)

What is Computer Architecture ?

the logical aspects of system implementation as seen by the programmer

At the most basic level, what does a computer consist of?

processor, memory, data transfer mechanism

What is a supercomputer?

a computer consisting of a larger memory and more powerful processors for large scale computations/calculation

What is the NEGATIVE of running supercomputers?

expensive

How many levels are within the Computer Level Heirarchy?

7

What do each of the levels within the Computer Level Hierarchy represent?

the different levels at which we can understand and study a computer system

What do the highest and lowest levels of the Computer Level Hierarchy mean?

highest = most abstract, lowest = least abstract (physical detail of computer)

What does Level 6 in the Computer level Hierarchy represent?

USER LEVEL = this is the most abstract level where users interact with the computer and it is the most detached from the physical processor

What does Level 5 in the Computer level Hierarchy represent?

HIGH-LEVEL LANGUAGE = the level used when writing in high level languages such as Java

What does Level 4 in the Computer level Hierarchy represent?

ASSEMBLY LANGUAGE = high level program instructions are translated into assembly language and then into machine level instructions

What does Level 3 in the Computer level Hierarchy represent?

SYSTEM SOFTWARE = controls the processes that are being executed and protects the system from damaging/hacks

What does Level 2 in the Computer level Hierarchy represent?

MACHINE = contains the instructions of the machine (aka Instruction Set Architecture), this is written in machine language so doesn’t need to be complied and can be executed immediately

What does Level 1 in the Computer level Hierarchy represent?

CONTROL = decodes and executes instructions

What does Level 0 in the Computer level Hierarchy represent?

DIGITAL LOGIC = where all the digital circuits and microchips are, which implements the mathematical logic of all the other levels

What are the types of control units?

microprogrammed, hardwired

What is a microprogram?

program written in low level language that’s implemented by the hardware to give more flexibility on the number of operations that can be implemented

What is a hardwired control unit?

hardware that directly executes machine instructions = makes them very fast

What is a negative of Hardwired control units?

they rely on physical processes so they’re typically used for specific operations that are completed over and over again

How to measure a computer system?

processor speed, memory size

what is the Von Neumann Bottleneck?

a single data path between the CPU and main memory

On top the the 3 components of a computer, what does VNM also specify?

capacity to carry out sequential instruction processing, a single data path between the CPU and main memory

What takes place in the Fetch phase of FDE cycle?

the control unit fetches the next instruction from memory and updates the program counter

What takes place in the decode phase of the FDE cycle?

the instruction is translated so it can be understood by the ALU

What takes place in the execute phase of the FDE cycle?

any necessary data is loaded into the CPU registers, the instruction is executed and results are stored in registers or memory

What are the advantages of the Von Neumann Architecture?

flexible use of memory // impact of VNM bottleneck can be reduced through caching and pipelining

What are the disadvantages of the Von Neumann Architecture?

the VNM bottleneck = fetch instruction and then wait to fetch data on the same path

What are the advantages of Non-Von Neumann Architecture?

no VNM bottleneck

What are the disadvantages of Non-Von Neumann Architecture?

rigid use of memory // complex bus structure

What’s the difference between radix notation and prefix notation?

radix - uses subscript : 10110₁₀
prefix = 0b10110

How many bits is a nybble?

4 bits

How does signed magnitude work?

The left most significant bit indicates the sign of the number

1 = NEGATIVE (11001 = -9)
0 = POSITIVE (01001 = 9)

Waht are the advantages of signed magnitude?

easy to understand, easy to convert

Waht are the disadvantages of signed magnitude?

difficult to perform arithmetic // 2 representations of 0 = 0000 or 1000

How does one’s complement work?

0 —> 1 // 1 —> 0 // MSB is a sign bit

What are the advantages of one’s complement?

implementation is easy allowing for simple circuits

What are the disadvantages of one’s complement?

still 2 representations of 0 = 0000 // 1111

What are the advantages of 2’s complement?

no bits with special meanings, only one representation of 0

What are the disadvantages of 2’s Complement?

none

Define Overflow.

when the result of a computation is too large to be stored in the computer’s memory in the format given

When is overflow detected in 2’s Complement addition?

When the carry in is not the same as the carry out

How is 0 represented in the IEEE-754 format?

0/1 00000000 00000000000000000000000 (all 0 and any sign bit value)

How is ±∞ represented in the IEEE-754 format?

0/1 11111111 00000000000000000000000 (the entire exponent is 1, entire significant is 0)

How is NaN (not a number) represented in the IEEE-754 format?

0/1 11111111 (any non-zero significand)

What is underflow in IEEE-754 arithmetic?

magnitude is too small to store in the format

What is catastrophic cancellation?

a lack of significant figures to represent the result of colliding large and small numbers so the small values are cancelled out

How can you calculate the relative error of a representation?

error = [(true value - represented value) / true value] * 100

On top of the 3 basic components of a computers, what are other features do modern digital computer systems rely on?

buses, clocks/interrupts

What are the main parts of the CPU

datapaths (buses) control unit

How are the ALU and registers connected

by a databus

What do registers do?

hold data that can be readily accessed by the CPU

What does the ALU do?

carries out the logical and arithmetic operations of the CPU

What does the control unit do?

sends signals to components of the CPU to perform certain operations

How does the control unit determine the action to be executed?

by checking the values held in the program counter and status register

What does computer memory consist of?

a linear array of addressable storage cells

What are the storage cells of memory that are of a regular, fixed sixe called?

word

What does it mean to be word-addressable?

means that the processor can identify a particular word when it would like to read from or write the memory

What is the main memory in modern computer systems composed of?

RAM

What are the 2 mapping types of IO devices

memory mapped, instruction mapped

Define memory mapped

when the device behaves like main memory from the point of view of the CPU

Define instruction mapped

When the CPU has specific instructions to control the device

What is a bus?

and array of wires that simultaneously convey a single bit along each line

How does the CPU share data with other system components?

data bus

What are the 2 most common buses?

point-to-point, multi-point

What is a point to point bus

connects 2 components together

What is a multipoint bus?

many components are able to access a bus that can connect to any other component that is connected to it

What are the 3 types of bus?

data, control, address

Waht is the role of the databus?

convey bits between one device to another

What is the role of control bus?

determines the direction of the flow of data and when a device can gain access to the bus

What is the role of the address bus?

determines the source or destination location of data

Which buses are bi-directional?

data, control

What does the clock do?

synchronises the activities of the CPU

What does the clock frequency determine?

the speed at which all operations are carried out

How is clock cycle time calculated?

time = 1 / freq

How is CPU time calculated?

What do interrupts do?

alter the normal flow of the execution of a program when an event of higher priority occurs

What are the specifications of MARIE?

binary 2’s complement, fixed word length, 4k words of word-addressable memory, 16-bit word size, 16-bit instructions, 16-bit ALU, seven registers

What are the 7 registers in MARIE?

Accumulator, Memory Address Register, Memory Buffer Register, Instruction register, Program Counter, Input register, output register

How is an instruction in MARIE split?

first 4 bits = OPCODE
last 12 bits = ADDRESS

What does M[X] mean in RTL?

indicates the actual data stored at memory location X

what does the arrow ← indicate?

the transfer of bytes to a register or memory location

How often does the computer system check for interrupts?

After executing each instruction

What are the steps of processing an interrupt?

1) save data stored in register into the memory
2) look up the ISR (Interrupt Service Routine) address in the interrupt table
3) place ISR address in the PC
4) execute instruction in ISR
5) restore data to registers from memory
6) return to FDE cycle

What does it mean for an interrupt to be maskable?

low priority interrupts that are ignored when another interrupt is running

What are non-maskable interrupts?

interrupts that must be processed to keep the system in a stable condition

What is the RTL for input?

AC ← IN

What do assemblers do?

Translates the mnemonic instructions that are comprehensible to humans into machine language so that is comprehensible for the machine

What is the output of an assembler?

object program file

What does the assemble do in each pass?

1st pass → assembles most of the program and creates a symbol table with memory references for all the symbols in the program

2nd pass → completes the instructions using the values in the symbol table

What does the Clear instruction do?

sets the contents of the AC to 0

What does SKIPCOND 000 mean?

Skip the next instruction if AC < 0

What does SKIPCOND 400 mean?

Skip the next instruction if AC = 0

What does SKIPCOND 800 mean?

Skip the next instruction if AC > 0

What must you consider when designing ISAs?

• instruction length

• number of operands per instruction

• type/ size/ location of operands

• memory organisation

• addressing modes

• types of operations available

• number of operations

What are the main measures when choosing an ISA?

• Main memory space

• instruction complexity

• instruction length in bits

• total number of instructions in the set

what does endianness refer to?

refers to the way in which data is stored in the system → the order of the bytes

What is meant by a little endian machine?

the LSB (byte) is followed by the MSB (byte)

What is meant by a big endian machine?

MSB is followed by LSB

What are the advantages of BIG ENDIAN?

• More natural

• sign of number can easily be seen by looking at the byte at the address offset 0

• strings / integers stored in same order