A1 CS - VON NEUMANN/ASSEMBLY LANGUAGE

CPU

central processing unit

The purpose of the CPU is to carry out the set of instructions given to the processor from a program

constantly process data and instructions in a fetch-decode-execute cycle

stored program concept

machine code instructions stored in main memory are fetched and executed serially by a processor

general purpose registers

additional registers in the CPU used for memory or data when needed, multipurpose, used for most instructions, used to hold results

specific purpose registers

used for certain instructions, specialised for specific task, hold status of a program

program counter

program counter holds the address of the next instruction to be executed; after content of PC is copied to MAR it is incremented by 1

MAR

Memory Address Register

holds address of current instruction

MDR

memory data register

holds data of the current instruction to be executed; memory buffer register

CIR

current instruction register

it recieves data from MDR; instructed and decoded and executed in this register

IX

index register

used in index addressing

used for loops, dumps, array index

status register

holds status of different processes, used by ALU to make decisions

control unit

• brain of computer

• maintain synchronisation of instructions w. the help of timing signals generated by system clock

• decoding and execution of instructions

• contains IAS, PC, CIR

ALU

arithmetic and logic unit

decision making is performed by this unit

contains accumulator register

memory unit

stores data permanently in secondary storage, fetches data from RAM towards CPU for execution

contains MAR and MDR

system clock

the system clock synchronises the entire fetch-execute cycle

overclocking

overclocking allows you to run your processor at higher clock speeds than originally intended

IAS/cache

Immediate Access Store, holds the data needed straightaway by the control unit

data bus

• bidirectional bus

• carries data/instructions

• bus width is important as it defines amount of data a bus can carry

address bus

• unidirectional

• carries addresses

control bus

• carries instructions to other components of computer

• bidirectional

• also used to carry timing signals

interrupts

signals generated from software or hardware to stop the process of processor for certain period of time

software interrupts

• signal to the processor initiated by software

• buffer is full

• divide by zero

• stack overflow

• user input

hardware interrupt

• printer out of paper

• user presses key on keyboard

• power failure

• hardware failure

how interrupt is executed

• at the end of FDE cycle, we check for interrupts

• if interrupt has higher priority:

• values of current instruction (PC along with other registers) are stored in STACK (memory)

• load interrupt in the PC w. the help of ISR

• interrupt is executed

• check for further interrupts, if no interrupt, then previous instructions are loaded back to PC

assembly language

low-level programming language that communicates directly with the computer’s hardware. directly deals with registers; execution is faster

addressing modes

how we (assembly language) accesses our own memory

opcode

represents a specific instruction for the processor to perform

operand

the value on which the instruction is performed

instruction list

set of instructions (opcode+operand) that a microprocessor can understand and execute

immediate addressing

• no memory is involved

• operand acts as a value that is copied to accumulator

direct addressing

operand holds an address, contents of given address copied to ACC

indirect addressing

content of given address acts as a second address, contents of second address is copied to ACC

index addressing

we create a new address by adding given address to the value stored in Index Register, contents of new address is copied to ACC

relative addressing

memory is related from base address

if base address is not given it is considered 0

base address is stored in a register called base register

macros

set of instructions that performs specific task, they are also called subroutines, they can be called multiple times, less chances of errors that are fully tested

directives

they are instructions for the assembler, they are not programming code, just provide info to the assembler (starting address, space for variable, external file included)